What are alkenes and how do they differ from alkanes in Organic Chemistry?

Alkenes are a class of hydrocarbons that contain at least one carbon-carbon double bond, which distinguishes them from alkanes that only have single bonds. This double bond makes alkenes unsaturated hydrocarbons, allowing them to participate in additional chemical reactions. In the Edexcel IGCSE Chemistry curriculum, understanding the structural differences and reactivity of alkenes compared to alkanes is essential.

How are alkenes named according to IUPAC nomenclature?

In IUPAC nomenclature, alkenes are named by identifying the longest carbon chain containing the double bond and changing the suffix from '-ane' to '-ene'. The position of the double bond is indicated by the lowest possible number assigned to the first carbon of the double bond. For example, but-1-ene indicates a four-carbon chain with a double bond starting at the first carbon. This systematic naming is crucial for Edexcel IGCSE Chemistry students to master.

What is the general formula for alkenes and how can it be used?

The general formula for alkenes is CnH2n, where 'n' represents the number of carbon atoms. This formula helps in determining the molecular formula of any alkene and is a key concept in the Edexcel IGCSE Chemistry syllabus. For instance, if an alkene has 4 carbon atoms, its molecular formula would be C4H8.

What are some common reactions involving alkenes?

Alkenes commonly undergo addition reactions due to their carbon-carbon double bond. These reactions include hydrogenation (adding hydrogen), halogenation (adding halogens like chlorine or bromine), and hydration (adding water). Understanding these reactions is important for Edexcel IGCSE Chemistry students as they illustrate the reactivity of alkenes compared to alkanes.

Why are alkenes considered more reactive than alkanes?

Alkenes are more reactive than alkanes because of the presence of the carbon-carbon double bond, which is a region of high electron density. This makes alkenes more susceptible to attack by electrophiles, leading to various addition reactions. This concept is a fundamental part of the Edexcel IGCSE Chemistry curriculum, helping students understand the behavior of different organic compounds.

Why is "Listing 'methene' as the first alkene" a common mistake in Alkenes?

Why it happens: Following the pattern 'meth-, eth-, prop-'. How to avoid it: There is NO methene — you need at least 2 carbons for a C=C double bond. The smallest alkene is ETHENE (C₂H₄, n = 2). Methane (CH₄) is the smallest alkane. Source: 4CH1 Examiner Reports 2022-2024

Why is "Writing 'turns clear' instead of 'decolourised' for the bromine water test" a common mistake in Alkenes?

Why it happens: Colloquial use of 'clear'. How to avoid it: 'Clear' means transparent (you can see through both orange bromine water AND pure water). The colour change is from ORANGE-BROWN to COLOURLESS. Use 'decolourised' or 'turns from orange to colourless'. Source: 4CH1 Examiner Reports 2023-2024

Why is "Confusing addition (alkenes) with substitution (alkanes)" a common mistake in Alkenes?

Why it happens: Both involve halogens and an alkene/alkane. How to avoid it: Addition (alkene): the WHOLE small molecule joins the product, no atoms lost — e.g. CH₂=CH₂ + Br₂ → CH₂BrCH₂Br. Substitution (alkane): one atom is REPLACED by another — e.g. CH₃CH₃ + Br₂ → CH₃CH₂Br + HBr. Look for: (a) product complexity (1 product = addition; 2 products = substitution); (b) conditions (UV = substitution; no UV = addition). Source: 4CH1 Examiner Reports 2022-2024

Why is "Forgetting position numbers when naming alkene products" a common mistake in Alkenes?

Why it happens: For ethene only one position is possible. How to avoid it: Always include '1,2-' in dihaloalkane names (e.g. 1,2-dibromopropane). The halogen atoms go on the carbons that USED TO HAVE the double bond. For propene + Br₂, the product is 1,2-dibromopropane (NOT 2,3- which would be the same compound viewed from the other end — convention is to give the LOWEST possible locants).

Why is "Mixing up the conditions for hydration vs hydrogenation" a common mistake in Alkenes?

Why it happens: Both are addition reactions. How to avoid it: Hydrogenation (alkene + H₂ → alkane): **Ni catalyst, 150 °C**, slightly elevated P. Hydration (alkene + H₂O → alcohol): **H₃PO₄ catalyst, 300 °C, 60 atm**. Mnemonic: 'H₂ uses Nickel 150'; 'H₂O uses Phosphoric 300/60'. Source: 4CH1 Examiner Reports 2023-2024

Why is "Saying fermentation produces 'pure ethanol' or is 'faster than hydration'" a common mistake in Alkenes?

Why it happens: Confusing the two ethanol routes. How to avoid it: Fermentation: SLOW (days), BATCH, gives DILUTE ethanol (~ 15%) that needs distillation, uses RENEWABLE sugars (carbon-neutral). Hydration of ethene: FAST (minutes), CONTINUOUS, gives PURE ethanol (~ 95%), uses NON-RENEWABLE crude oil. Each has clear pros/cons — don't blur them.

Organic ChemistryPearson Edexcel IGCSE Chemistry 4CH1 Higher

Alkenes

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Alkenes — Pearson Edexcel International GCSE Chemistry 4CH1 Study Notes (2026 onwards, Spec Issue 4)

Unsaturated hydrocarbons with one C=C double bond. General formula CₙH₂ₙ. First three: ethene C₂H₄, propene C₃H₆, butene C₄H₈. More reactive than alkanes due to electron-rich C=C. Bromine water test: decolourised → unsaturation detected. FOUR addition reactions: with H₂ (Ni, 150 °C → alkane / margarine); with Br₂ (1,2-dibromoalkane); with H₂O (H₃PO₄, 300 °C, 60 atm → alcohol / industrial ethanol); with HBr (haloalkane).

At a glance

Alkene = unsaturated hydrocarbon, general formula CₙH₂ₙ (n ≥ 2).
First three: ethene C₂H₄, propene C₃H₆, butene C₄H₈ (positional isomers but-1-ene, but-2-ene).
All contain ONE C=C double bond. Carbons sp² hybridised; bond angle ~120°; planar around C=C.
MORE reactive than alkanes because the π bond of C=C is electron-rich, weaker, exposed.
Test for unsaturation: bromine water DECOLOURISED (orange → colourless). Alkanes don't react.
Addition reactions (the key chemistry — small molecule adds across C=C):
• H₂ + Ni 150 °C → alkane (HYDROGENATION → margarine from vegetable oils).
• Br₂ → 1,2-dibromoalkane (bromine water test).
• H₂O + H₃PO₄ 300 °C 60 atm → alcohol (HYDRATION → industrial ETHANOL from ethene).
• HBr → haloalkane.
No methene — smallest alkene is ethene (need at least 2 carbons for C=C).

What you’ll learn

Mapped to the Cambridge IGCSE 4CH1 syllabus (2026 onwards).

4.22 — Recall that alkenes are unsaturated hydrocarbons with the general formula CₙH₂ₙ and contain one C=C double bond.
4.23 — Name the first three alkenes (ethene, propene, butene) and write their structural formulae.
4.24 — Understand that alkenes are MORE REACTIVE than alkanes because of the C=C double bond.
4.25 — Write balanced equations for the ADDITION REACTIONS of alkenes with: hydrogen (Ni catalyst, 150 °C), bromine (room T), water/steam (H₃PO₄ catalyst, 300 °C, 60 atm), hydrogen bromide.
4.26P — (Paper 2) Describe the industrial manufacture of ethanol from ethene by hydration: catalysts, temperature, pressure, reversibility, and recycling of unreacted gases. Compare with fermentation.

Alkene structure and the C=C double bond (spec 4.22, 4.23, 4.24)

CₙH₂ₙ. One C=C. Ethene, propene, butene. sp² planar around C=C.

Alkenes are UNSATURATED HYDROCARBONS containing ONE C=C double bond per molecule.

General formula: $C_n H_{2n}$ (where n ≥ 2 — at least 2 carbons are needed for the double bond).

This is 2 fewer H atoms per molecule than the corresponding alkane (which is CₙH₂ₙ₊₂). The 'missing' 2 H atoms are because the C=C double bond uses up 2 H atoms' worth of bonding capacity.

First three alkenes.

n	Name	Molecular formula	Structure
2	Ethene	C₂H₄	CH₂=CH₂
3	Propene	C₃H₆	CH₂=CHCH₃
4	Butene	C₄H₈	CH₂=CHCH₂CH₃ (but-1-ene) or CH₃CH=CHCH₃ (but-2-ene)

NO 'methene' — you need at least 2 carbons for the C=C. The smallest alkene is ETHENE (n = 2).

Displayed structural formula of ethene.

H        H
 \      /
  C == C
 /      \
H        H

Ethene has a C=C double bond (one σ + one weaker π bond); the molecule is planar with ~120° bond angles.

Ethene is a PLANAR molecule (all 6 atoms in the same plane). The two carbons share TWO bonds (a double bond):

One σ bond (head-on overlap of orbitals — the same type as in alkanes).
One π bond (sideways overlap above and below the molecular plane — only in double/triple bonds).

Each carbon is sp² hybridised: 3 single bonds in a flat trigonal arrangement, with the π bond perpendicular. Bond angle ~ 120° (NOT 109.5° as in alkanes).

Bond strengths.

Bond	Energy (kJ/mol)
C–C single bond	348
C=C double bond (total: σ + π)	612
C=C π bond alone	~ 264

The π bond is significantly WEAKER than the σ bond — it can be broken (during addition reactions) while leaving the σ bond intact.

Positional isomerism in butene.

For C₄H₈, the double bond can be in different positions:

But-1-ene: C=C between C1 and C2. CH₂=CHCH₂CH₃.
But-2-ene: C=C between C2 and C3. CH₃CH=CHCH₃.

These are POSITIONAL isomers — same molecular formula, different position of the C=C. Different compounds with slightly different boiling points and chemical behaviour.

(Plus cyclobutane and methylenecyclopropane — also C₄H₈ but in ring forms, not relevant to 4CH1.)

Why alkenes are MORE REACTIVE than alkanes.

Three reasons related to the C=C double bond:

The π bond is WEAKER than a single bond (~ 264 vs ~ 348 kJ/mol). Easier to break.
The π bond is EXPOSED — the electron density of the π bond sticks out above and below the plane of the molecule (unlike σ bonds, which are inside the bond axis). Reagents can approach and attack the π electrons directly.
The π bond is ELECTRON-RICH — high electron density between the two carbons. This makes it electron-donating ('nucleophilic' towards an electrophile). Electrophiles like Br₂ are attracted to the C=C and react with it.

In contrast, alkanes have only single C–C and C–H bonds → no exposed electron-rich region → no easy target for reagents → unreactive at room T.

The 'extra' bond in alkenes is what makes them undergo a whole class of reactions (additions) that alkanes cannot.

Industrial origin.

Alkenes are NOT naturally present in crude oil — the C=C bond is too reactive to survive geological time. Alkenes are produced INDUSTRIALLY by CRACKING long-chain alkanes (see Topic 4.2 — Crude Oil):

Catalytic cracking: long alkane → shorter alkane + alkene.
Example: C₁₀H₂₂ → C₈H₁₈ + C₂H₄.

So ethene (C₂H₄) — the most-produced organic chemical in the world (~ 150 million tonnes/year) — comes from cracking.

Uses of alkenes — the plastics industry.

Alkenes are the MONOMERS for most common plastics:

Ethene → polyethene (poly(ethene), polythene) — bags, bottles, films.
Propene → polypropene — containers, fibres, car parts.
Styrene (an alkene with a benzene ring) → polystyrene.
Chloroethene (vinyl chloride) → PVC.

Without alkenes (and the cracking that makes them), there would be no plastics industry.

Alkenes: unsaturated, general formula CₙH₂ₙ (n ≥ 2).
First three: ethene C₂H₄, propene C₃H₆, butene C₄H₈.
One C=C double bond per molecule (σ + π).
sp² carbons, planar around C=C, bond angle ~120°.
MORE REACTIVE than alkanes: π bond weaker + exposed + electron-rich.
Made by cracking long alkanes from crude oil; not naturally in oil.
Used to make plastics (polyethene, polypropene).
Positional isomers possible from butene onwards (but-1-ene vs but-2-ene).

See the full worked example for alkenes →

Bromine water test for unsaturation (spec 4.25)

Orange/brown bromine water DECOLOURISED by alkenes → addition across C=C.

The bromine water test is the standard chemical test in 4CH1 (and most chemistry syllabuses) for the presence of a C=C double bond (unsaturation).

Procedure. Add a few cm³ of BROMINE WATER (orange/brown solution of Br₂ in water) to a small sample of the hydrocarbon. Shake.

Observation with an ALKENE.

Bromine water is DECOLOURISED — turns from ORANGE-BROWN to COLOURLESS within seconds.

The Br₂ is consumed by an addition reaction across the C=C double bond. The product is a 1,2-dibromoalkane (colourless).

$\text{CH}_2{=}\text{CH}_2 + \text{Br}_2 \rightarrow \text{CH}_2\text{Br}{-}\text{CH}_2\text{Br}$

(For ethene; 1,2-dibromoethane is the product.)

Observation with an ALKANE.

Bromine water REMAINS ORANGE-BROWN. No reaction occurs at room T without UV light.

(Alkanes CAN react with Br₂ via substitution — but only in UV light, which isn't present in the test conditions.)

An alkene decolourises orange bromine water; an alkane leaves it unchanged.

Mark-scheme keyword: DECOLOURISED.

Use the word 'DECOLOURISED' for the colour change with an alkene. NOT 'turns clear' — clear means transparent, and both orange bromine water AND pure water are transparent. The change is from a coloured solution to a colourless one → 'decolourised' OR 'turns from orange to colourless'.

Mechanism — what happens at the molecular level.

The C=C π bond is electron-rich and attracts the polarisable Br–Br molecule.
As Br₂ approaches the C=C, the Br atoms polarise: the nearer Br becomes slightly δ+, the farther Br slightly δ−.
The π bond breaks; the σ bond stays. One C of the former C=C bonds to one Br atom; the other C bonds to the other Br atom.
The C=C becomes a C–C single bond. The product is a 1,2-dibromoalkane.

No HBr is produced (this is the KEY difference from alkane + Br₂ substitution).

Why alkanes don't react.

Alkanes have ONLY single C–C and C–H bonds. There is NO C=C for Br₂ to ADD across. The single bonds are strong and there's no electron-rich site for Br₂ to attack at room T. Without UV light (to start a free-radical substitution), no reaction occurs.

Test products.

The product of the bromine water test is technically a MIXTURE — bromine water has small amounts of Br₂, HBr, HOBr, and OBr⁻ in equilibrium. The main product with an alkene is the dibromoalkane (mainly), plus a bit of a bromohydrin (CH₂BrCH₂OH from Br + OH adding). Both are colourless → the test still works visually.

Distinguishing alkanes vs alkenes — exam question.

A typical 4CH1 Paper 1 question gives you two unknown colourless hydrocarbons (e.g. hexane and hex-1-ene) and asks you to design a test to identify them.

Expected answer:

Test: add BROMINE WATER (orange-brown).
Alkene → DECOLOURISED → 'this is the alkene' (hex-1-ene).
Alkane → NO change (remains orange-brown) → 'this is the alkane' (hexane).
Equation for the reaction with hex-1-ene: C₆H₁₂ + Br₂ → C₆H₁₂Br₂ (1,2-dibromohexane).

Quantitative version (A-level).

By titrating an alkene with a known concentration of Br₂, you can MEASURE the number of C=C bonds per molecule. Each C=C consumes one Br₂. More moles of Br₂ used per mole of compound → more C=C bonds (e.g. polyunsaturated oils consume more Br₂ than monounsaturated fats).

Real-life application — testing vegetable oils.

Vegetable oils (sunflower, olive, soybean) contain long-chain fatty acids with MULTIPLE C=C bonds per molecule (polyunsaturated). They decolourise bromine water QUICKLY (many C=C per molecule × Br₂ → many additions).

Animal fats (saturated, no C=C) don't decolourise bromine water.

Margarine (partially hydrogenated vegetable oil) has fewer C=C bonds than the original oil → decolourises bromine water more slowly. The 'degree of unsaturation' of an oil can be measured by how much bromine water it consumes (the 'iodine value' — a similar test using I₂).

Test: add orange/brown bromine water to hydrocarbon.
Alkene → DECOLOURISED (orange → colourless). Alkane → NO change.
Mark-scheme word: DECOLOURISED (not 'turns clear').
Mechanism: Br₂ adds across C=C → C=C becomes C–C; both Br atoms join the product.
Product: 1,2-dibromoalkane (e.g. ethene + Br₂ → 1,2-dibromoethane).
Test exploits the difference between addition (alkenes) and substitution (alkanes, needs UV).
Used commercially to measure the degree of unsaturation in oils and fats.

See the full worked example for alkenes →

Four addition reactions of alkenes (spec 4.25)

H₂ (Ni 150 °C → alkane). + Br₂ (→ dibromide). + H₂O (H₃PO₄ 300/60 → alcohol). + HBr (→ haloalkane).

Alkenes undergo a characteristic family of reactions called ADDITION REACTIONS. A small molecule (H₂, Br₂, H₂O, HBr, etc.) adds ACROSS the C=C double bond. The C=C becomes a C–C single bond; one atom (or fragment) of the small molecule attaches to each of the former double-bond carbons.

General pattern.

$\text{alkene} + X{-}Y \rightarrow \text{single-bonded product with X on one C and Y on the other}$

No atoms are lost — all atoms of both reactants end up in the product.

In addition the π bond breaks: the C=C becomes a C−C single bond and each carbon gains one new atom.

The four addition reactions in 4CH1.

(1) Hydrogenation: alkene + H₂ → alkane.

Conditions: NICKEL catalyst; ~150 °C; slightly elevated pressure.
Equation: $\text{CH}_2{=}\text{CH}_2 + \text{H}_2 \xrightarrow{\text{Ni, 150 °C}} \text{CH}_3{-}\text{CH}_3$
Product: ethane (the saturated alkane).
Industrial use: converting LIQUID vegetable oils (unsaturated, multiple C=C per molecule) into SOLID margarine. The H₂ saturates the C=C bonds → straightens the chains → tight packing → higher mp → solid at room T.

Why margarine? Hydrogenated oils have a SOLID texture (like butter) but are made from cheaper plant oils. Partial hydrogenation gives 'soft' margarine; full hydrogenation gives 'hard' margarine. The degree is controlled by the H₂ exposure time.

(Modern concern: partial hydrogenation can produce 'trans-fats' which have negative health effects. Many manufacturers now use interesterification or other methods.)

(2) Halogenation: alkene + Br₂ → 1,2-dibromoalkane.

Conditions: none — happens RAPIDLY at room T with no catalyst.
Equation: $\text{CH}_2{=}\text{CH}_2 + \text{Br}_2 \rightarrow \text{CH}_2\text{Br}{-}\text{CH}_2\text{Br}$
Product: 1,2-dibromoethane (colourless). The orange Br₂ is consumed.
Test: bromine water decolourised → indicates C=C present. THE standard test for unsaturation.

Same reaction works with Cl₂: ethene + Cl₂ → 1,2-dichloroethane (used industrially to make PVC monomer chloroethene).

I₂ reacts more slowly (less reactive halogen) but the principle is the same.

(3) Hydration: alkene + H₂O → alcohol.

Conditions: PHOSPHORIC ACID (H₃PO₄) catalyst; 300 °C; 60 atm. Water as steam.
Equation (REVERSIBLE!): $\text{CH}_2{=}\text{CH}_2 + \text{H}_2\text{O} \rightleftharpoons \text{CH}_3{-}\text{CH}_2\text{OH}$
Product: ethanol (the corresponding alcohol).
Industrial use: the main route to industrial ethanol. Only ~ 5% conversion per pass at equilibrium; unreacted gases recycled → overall conversion ~ 97%.

This is the INDUSTRIAL method for making ethanol — the alternative is FERMENTATION of sugars by yeast (renewable, slow, batch process).

(4) Hydrohalogenation: alkene + HBr → haloalkane.

Conditions: room T; HBr gas (or HCl, HI) bubbled through the alkene in solution.
Equation: $\text{CH}_2{=}\text{CH}_2 + \text{HBr} \rightarrow \text{CH}_3{-}\text{CH}_2\text{Br}$
Product: bromoethane (a haloalkane).

For non-symmetric alkenes (e.g. propene CH₃CH=CH₂), the H and Br can add either way. The major product follows Markovnikov's rule: H goes to the C with more H atoms; Br goes to the C with fewer H atoms. So propene + HBr → mainly 2-bromopropane (CH₃CHBrCH₃), not 1-bromopropane. (Markovnikov is A-level — at IGCSE you typically just write one product, often the Markovnikov product.)

Summary table of addition reactions.

Reaction	Reagent	Conditions	Product type	Example product
Hydrogenation	H₂	Ni catalyst, 150 °C	Alkane	Ethane (from ethene)
Halogenation	Br₂ (or Cl₂)	Room T	Dihaloalkane	1,2-Dibromoethane
Hydration	H₂O (steam)	H₃PO₄, 300 °C, 60 atm	Alcohol	Ethanol
Hydrohalogenation	HBr (or HCl)	Room T	Haloalkane	Bromoethane

Polymerisation — a special addition reaction (Topic 4.8, MR 5).

Many alkene molecules can JOIN TOGETHER, each linking to the next via the C=C double bond, to form a long-chain POLYMER:

$n \text{CH}_2{=}\text{CH}_2 \rightarrow {-}(\text{CH}_2{-}\text{CH}_2{-})_n$

This is the basis of the plastics industry: poly(ethene) from ethene; poly(propene) from propene; PVC from chloroethene. We'll cover this in MR 5.

Why these reactions all work.

Each follows the same principle:

The C=C π bond is electron-rich and polarisable.
The reagent (Br₂, HBr, etc.) is polar / polarisable → attracted to the π electrons.
The π bond breaks; the σ bond stays.
Two new single bonds form to the carbon atoms — completing the addition.

The C=C double bond is THE 'functional group' that defines alkene chemistry. Same mechanism, different reagent.

Addition: small molecule adds across C=C → C–C single bond.
- H₂ (Ni, 150 °C) → alkane. Used to make margarine from oils.
- Br₂ (no conditions needed) → 1,2-dibromoalkane. Bromine water test.
- H₂O (H₃PO₄, 300 °C, 60 atm) → alcohol. Industrial ethanol from ethene.
- HBr (room T) → haloalkane.
No atoms lost in addition — both atoms of the small molecule join the product.
C=C functional group is the source of reactivity (electron-rich, weaker π bond, exposed).

See the full worked example for alkenes →

Industrial ethanol from ethene (Paper 2, spec 4.26P)

CH₂=CH₂ + H₂O ⇌ CH₃CH₂OH. H₃PO₄ catalyst, 300 °C, 60 atm. Recycled.

Ethanol is one of the most important industrial chemicals — used as a solvent, fuel additive, chemical feedstock, sanitiser, and (after distillation) as alcoholic drinks. There are TWO main industrial methods:

Method 1 — Hydration of ethene (the 'petrochemical' route).

Reaction. Ethene (from cracking crude oil) reacts with steam over a phosphoric acid catalyst:

$\text{CH}_2{=}\text{CH}_2(\text{g}) + \text{H}_2\text{O(g)} \rightleftharpoons \text{CH}_3\text{CH}_2\text{OH(g)}$

The water adds across the C=C: one C gets an H, the other gets an OH.

Conditions:

Catalyst: phosphoric acid (H₃PO₄) — typically adsorbed on silica.
Temperature: ~ 300 °C.
Pressure: ~ 60 atm.
Reagents: ethene + steam pumped through the reactor.

The reaction is reversible (note the ⇌). Only ~ 5% conversion per pass. Why these moderate conditions?

The forward reaction is exothermic. By Le Chatelier:
- LOW T would shift equilibrium right (more ethanol) — but rate too slow.
- HIGH T would shift equilibrium left — less ethanol.
- 300 °C compromise: useful rate + acceptable yield.
Forward reaction has 2 mol gas → 1 mol gas. By Le Chatelier:
- HIGH P would shift right (more ethanol) — but expensive equipment + safety.
- 60 atm compromise.

Recycling unreacted gases.

After one pass through the reactor, the mixture (containing ethanol + unreacted ethene + steam) is COOLED:

Ethanol condenses at ~ 78 °C and is collected.
Unreacted ethene + steam remain as gas → returned to the reactor.

Over many passes, OVERALL conversion reaches ~ 97%. This is a CONTINUOUS process (24/7 operation).

Method 2 — Fermentation of sugars (the 'biological' route).

Reaction. Glucose is converted to ethanol by yeast enzymes:

$\text{C}_6\text{H}_{12}\text{O}_6 \xrightarrow{\text{yeast}} 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2$

Conditions:

Catalyst: yeast (a living organism — its enzymes catalyse the reaction).
Temperature: 25-35 °C (yeast killed above ~ 40 °C).
Pressure: atmospheric.
Anaerobic (no oxygen — yeast switches to ethanol production when O₂ is limited).
Substrate: glucose, sucrose, fructose — typically from sugar cane, sugar beet, corn, or grape juice.

Yield: yeast dies when ethanol concentration reaches ~ 14-15% (alcohol toxic to the yeast). So fermentation gives DILUTE ethanol (~ 14%). Further concentration requires DISTILLATION (boil mixture; ethanol bp 78 °C; water bp 100 °C; ethanol vapour condensed separately).

Comparison: hydration vs fermentation.

Feature	Hydration of ethene	Fermentation
Source	Ethene (from crude oil)	Glucose (from sugar cane / corn / fruits)
Renewable?	NO (fossil fuel)	YES (plant-based)
Speed	Fast (minutes per pass)	Slow (days/weeks)
Mode	CONTINUOUS	BATCH
Conditions	300 °C, 60 atm, H₃PO₄	25-35 °C, atm P, yeast
Energy required	HIGH (high T + P)	LOW (near room T)
Concentration of product	HIGH (~ 95% pure ethanol)	LOW (~ 14% ethanol — needs distillation)
Yield	High (after recycling)	Moderate (yeast killed at ~ 15%)
Carbon footprint	High (fossil fuel)	Low (carbon-neutral plant CO₂)

Advantages of hydration:

Produces VERY PURE ethanol.
Fast — continuous process, high throughput.
High overall conversion via recycling.
Predictable, easy to control.

Disadvantages of hydration:

Requires NON-RENEWABLE crude oil (source of ethene).
High energy cost (300 °C, 60 atm — expensive equipment + electricity).
Capital-intensive (need cracker + hydration reactor + recycling).

Advantages of fermentation:

Uses RENEWABLE sugars from plants.
CARBON-NEUTRAL: CO₂ released = CO₂ absorbed by the plant when growing.
Low energy required → cheap to operate.
Can be done at small scale (home brewing, small breweries, biofuel co-ops).

Disadvantages of fermentation:

SLOW (days to weeks).
BATCH process (must restart between fermentations).
Dilute product → expensive distillation step.
Uses food crops (could feed people instead of cars).
Requires arable land (deforestation pressure in some regions).

Modern context.

Both methods are widely used:

Hydration of ethene dominates the manufacture of pure ethanol for industrial chemicals (solvent, antiseptic, chemical feedstock).
Fermentation is the source of alcoholic beverages (beer, wine, spirits) and increasingly of BIOETHANOL fuel (mixed with petrol; ~ 30% of Brazilian motor fuel is ethanol from sugar-cane fermentation).

The choice between methods depends on the application, the cost of feedstocks, the energy mix in the region, and environmental policy. As we move away from fossil fuels, fermentation will likely become more dominant.

Uses of ethanol:

Solvent — for paints, dyes, perfumes, pharmaceuticals.
Fuel — neat ethanol or blended with petrol (E10, E85 in some countries).
Sanitiser — hand sanitiser (60-95% ethanol denatures bacteria/viruses).
Chemical feedstock — to make ethanal, ethanoic acid, ethyl esters.
Beverages — beer (~ 5%), wine (~ 12%), spirits (~ 40%).

Industrial ethanol from ethene: CH₂=CH₂ + H₂O ⇌ CH₃CH₂OH.
Conditions: H₃PO₄ catalyst, 300 °C, 60 atm.
Reaction REVERSIBLE — ~ 5% conversion per pass; unreacted gases recycled → ~ 97% overall.
Continuous process, high purity (~ 95% ethanol), uses fossil-fuel ethene.
Alternative: fermentation of sugars by yeast (slow, batch, dilute product, renewable).
Hydration advantages: fast, pure, continuous. Disadvantages: non-renewable, high energy.
Fermentation advantages: renewable, low energy, carbon-neutral. Disadvantages: slow, batch, dilute.
Both used: hydration for industrial ethanol; fermentation for drinks + bioethanol fuel.

See the full worked example for alkenes →

Alkene vs alkane — key differences (spec 4.24)

C=C makes alkenes more reactive: addition vs substitution; bromine water test.

A frequent 4CH1 exam question: compare alkanes and alkenes. Here's a side-by-side summary plus deeper reasoning.

Comparison table.

Feature	Alkane	Alkene
General formula	CₙH₂ₙ₊₂	CₙH₂ₙ (n ≥ 2)
Bonds	All single (C–C and C–H)	One C=C + others single
Saturation	SATURATED	UNSATURATED
Smallest member	Methane (CH₄)	Ethene (C₂H₄) — no 'methene'
Functional group	(none)	C=C double bond
Reactivity	RELATIVELY UNREACTIVE	MORE REACTIVE
Combustion	Yes — CO₂ + H₂O	Yes — CO₂ + H₂O
Substitution with halogens	Yes — slow, UV needed	Rare (addition takes precedence)
Addition with halogens	NO	YES — fast, room T
Addition with H₂	NO	YES — Ni catalyst, 150 °C
Addition with H₂O	NO	YES — H₃PO₄, 300 °C, 60 atm
Bromine water test	NO change (orange)	DECOLOURISED (orange → colourless)
Polymerisation	NO	YES — makes plastics
Examples	methane, ethane, hexane	ethene, propene, hex-1-ene
State (small members)	Gas (C1-C4)	Gas (C2-C4)

Why alkenes are more reactive — the C=C double bond.

This is the central insight. The double bond between two carbons consists of TWO bonds:

A σ (sigma) bond — similar to a single bond, head-on orbital overlap. Strong (~ 348 kJ/mol).
A π (pi) bond — sideways overlap of p orbitals, above and below the molecular plane. WEAKER (~ 264 kJ/mol).

The π bond has THREE key features that make it reactive:

Weaker: easier to break than a σ bond.
Exposed: sticks out above/below the molecule, where reagents can attack.
Electron-rich: high electron density between the two carbons.

These features mean that small molecules (especially polarisable ones like Br₂ or polar ones like HBr) are ATTRACTED to the C=C and react with it. The π bond breaks; two new single bonds form to the carbon atoms. This is the addition reaction.

Alkanes have NO π bond → no exposed electron-rich region → no addition reactions → unreactive at room T.

Test for unsaturation — practical implications.

The bromine water test (orange → colourless for alkenes; no change for alkanes) is the standard 4CH1 way to detect a C=C bond. Use it:

To distinguish two unknown hydrocarbons (one alkane, one alkene).
To detect 'unsaturation' in oils and fats (vegetable oils are unsaturated, animal fats are usually saturated).
To check the progress of hydrogenation (the more H₂ added, the less Br₂ consumed).

Why this matters industrially.

The DIFFERENT reactivity of alkanes and alkenes is what allows the modern petrochemical industry to function:

Alkanes are the FUELS — stable, safe to store, predictable combustion.
Alkenes are the FEEDSTOCKS — reactive, easily converted to thousands of useful chemicals.
Cracking takes long-chain alkanes (often surplus) and converts some C–C bonds to make ALKENES (the most valuable products).

If alkenes were as unreactive as alkanes, we couldn't make plastics, industrial ethanol, antifreeze, paints, detergents, pharmaceuticals — modern chemistry depends on alkene reactivity.

Mnemonic for the comparison.

alkAne → 'A' for 'All single bonds, And unreactive'.
alkEne → 'E' for 'Has an Extra bond (the π bond), and Eagerly reacts'.

Both have alk- (because both are 'alkene-related' carbon chains); the suffix tells you which.

Bonus — alkyne (not 4CH1).

For completeness: ALKYNES have C≡C triple bonds; even more reactive than alkenes. Example: ethyne (acetylene, HC≡CH) — used in oxyacetylene welding. Not in 4CH1 syllabus.

Key difference: alkenes have C=C double bond; alkanes have only single bonds.
C=C π bond is weaker, exposed, electron-rich → reactive site.
Alkenes undergo ADDITION reactions (Br₂, H₂, H₂O, HBr). Alkanes do NOT.
Alkanes only react via combustion (with O₂) and substitution with halogens in UV.
Bromine water test: alkene DECOLOURISES; alkane no change.
Alkenes polymerise to plastics; alkanes do not.
Industrial implication: alkanes are fuels (stable); alkenes are feedstocks (reactive).

See the full worked example for alkenes →

Memorise this

Verbatim phrases and definitions Cambridge mark schemes credit.

Alkene general formula: CₙH₂ₙ (n ≥ 2).
First three alkenes: ethene C₂H₄, propene C₃H₆, butene C₄H₈.
Bromine water test: 'orange-brown bromine water DECOLOURISED by alkene; alkane → NO change'. Use the word 'decolourised'.
Hydrogenation conditions: Ni catalyst, ~150 °C. Used to make margarine.
Hydration conditions: H₃PO₄ catalyst, 300 °C, 60 atm. Reversible → recycle. Industrial ethanol.
Addition reaction: 'small molecule adds across C=C → C=C becomes C–C; both atoms of small molecule join the product'.
Why alkenes more reactive: 'the π bond of C=C is weaker, more exposed, and electron-rich than the σ bonds of an alkane'.
Reasons alkenes don't exist in crude oil naturally: 'too reactive — they would have reacted with other species over geological time'.

How it’s examined

Alkenes appear on every 4CH1 paper, typically worth 6-10 marks per session. Paper 1 (4CH1/1C) items: (a) define alkenes / give general formula / name first members (3-4 marks); (b) describe the bromine water test (4-5 marks); (c) write equations for addition reactions with conditions (4-5 marks per reaction). Paper 2 (4CH1/2C) items: deeper industrial ethanol manufacture — hydration of ethene (5-6 marks, comparing with fermentation); margarine production via hydrogenation. The bromine water test is the SINGLE most common alkene exam question — practise the wording (always 'decolourised', not 'turns clear') and the equation (C₂H₄ + Br₂ → C₂H₄Br₂).

Sources: Pearson Edexcel International GCSE Chemistry 4CH1 specification — Issue 4, September 2024 (valid for 2026 onwards); Pearson Edexcel 4CH1 Examiner Reports 2022-2024; 4CH1/1C May/Jun 2024 question paper and mark scheme; 4CH1/1C January 2024 question paper and mark scheme; 4CH1/2C May/Jun 2024 question paper and mark scheme. Last reviewed 2026-05-12.

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Step-by-step worked examples — Alkenes

Step-by-step solutions to past-paper-style questions on alkenes, written exactly the way a tutor would explain them at the board.

1Define the alkene homologous series and give first three members (4 marks, Paper 1)
Core• Adapted from 4CH1/1C May/Jun 2024 Q2• alkenes, general formula, spec-4.22, spec-4.23
Question
(a) State the general formula of the alkene homologous series. (b) Name and give the molecular formula of the first THREE members of the alkene series. (c) Draw the displayed (full structural) formula of ethene. (d) State ONE difference between the chemistry of alkanes and alkenes. (4 marks)
Step-by-step solution
1. Step 1
  (a) General formula (1 M). Alkenes have ONE C=C double bond. General formula:
  $C_n H_{2n}$
2. Step 2
  (b) First three alkenes (1 A). Note: there is NO 'methene' (CH₂ doesn't exist as a stable compound — at least 2 carbons are needed for the C=C double bond). So the smallest alkene is ethene (n = 2).
  
  n Name Molecular formula
  2 Ethene C₂H₄
  3 Propene C₃H₆
  4 Butene C₄H₈ (positional isomers: but-1-ene, but-2-ene)
3. Step 3
  (c) Displayed formula of ethene (1 A).
  
  H H \ / C == C / \ H H
  
  Ethene (C₂H₄): the two carbon atoms are joined by a DOUBLE BOND (C=C). Each carbon has 2 hydrogens. The molecule is planar (flat) — all 6 atoms lie in the same plane. Bond angle ≈ 120° around each C (sp² hybridised).
4. Step 4
  (d) One key chemistry difference (1 B). Alkenes UNDERGO ADDITION REACTIONS across the C=C double bond — e.g. Br₂ adds across it to give a 1,2-dibromoalkane → bromine water DECOLOURISES instantly. Alkanes do NOT undergo addition reactions (no C=C) → bromine water stays orange. This is the key test to distinguish them. Other differences: alkenes can be polymerised (alkanes cannot); alkenes are more reactive overall.
Answer
(a) CₙH₂ₙ. (b) Ethene C₂H₄; propene C₃H₆; butene C₄H₈. (c) Ethene displayed: H₂C=CH₂ (with all 4 H atoms shown and the C=C double bond explicit, planar, 120° angles). (d) Alkenes undergo addition reactions across the C=C (e.g. decolourise bromine water) — alkanes do not.
Examiner tip
Edexcel 4-mark mark scheme: 1 for general formula CₙH₂ₙ; 1 for naming all three alkenes correctly; 1 for displayed formula of ethene with double bond shown explicitly; 1 for any valid chemistry difference. Common error: drawing 'methene CH₂' (doesn't exist — needs 2 C for the double bond). Smallest alkene = ethene (n = 2).
2Test for unsaturation — bromine water (5 marks, Paper 1)
Core• Adapted from 4CH1/1C Jan 2024 Q4• bromine water, addition, spec-4.25, spec-4.6
Question
A student is given two colourless liquids, hex-1-ene and hexane. (a) Describe in detail the test that distinguishes them, including the observation expected for each. (b) Write the equation for the reaction between hex-1-ene (C₆H₁₂) and bromine, and name the product. (c) Explain why this is described as an 'addition reaction'. (5 marks)
Step-by-step solution
1. Step 1
  (a) Procedure (1 M). Add a few drops of BROMINE WATER (orange/brown solution of Br₂ in water) to a small sample of each liquid in a test tube. Shake gently. Observe the colour of the bromine water in each tube.
2. Step 2
  (a) Observation with hex-1-ene (alkene) (1 A). Bromine water is DECOLOURISED — turns from ORANGE-BROWN to COLOURLESS. This is rapid and dramatic. The Br₂ has been consumed by addition across the C=C double bond.
3. Step 3
  (a) Observation with hexane (alkane) (1 A). Bromine water remains ORANGE-BROWN — no colour change. Hexane and bromine water are immiscible; the orange colour stays in the aqueous layer. No reaction occurs.
4. Step 4
  (b) Equation + product name (1 B). Br₂ adds across the double bond. Each carbon of the former C=C gains one Br atom.
  $\text{C}_6\text{H}_{12} + \text{Br}_2 \rightarrow \text{C}_6\text{H}_{12}\text{Br}_2$
5. Step 5
  (b) Naming the product. Hex-1-ene (CH₂=CHCH₂CH₂CH₂CH₃) + Br₂ → CH₂BrCHBrCH₂CH₂CH₂CH₃ = 1,2-dibromohexane. The '1,2-' shows the Br atoms are on adjacent carbons (where the C=C used to be).
6. Step 6
  (c) Why 'addition' (1 B). An ADDITION REACTION is one where TWO molecules COMBINE into ONE — the small molecule (Br₂) is ADDED ACROSS the double bond. The C=C becomes a C–C single bond, and a Br atom is added to each of the two former double-bonded carbons. NO atoms leave — both Br atoms (and all atoms of the alkene) end up in the product. This contrasts with substitution (alkane + halogen, where one H is LOST as HCl).
Answer
(a) Test: add BROMINE WATER. Hex-1-ene: DECOLOURISED (orange → colourless) → alkene present. Hexane: NO change (remains orange) → saturated alkane. (b) C₆H₁₂ + Br₂ → C₆H₁₂Br₂ = 1,2-dibromohexane. (c) Addition reaction: Br₂ joins ACROSS the C=C double bond. Both atoms of Br₂ are added (one to each carbon of the former double bond); no atoms are lost; the C=C becomes a single bond.
Examiner tip
Edexcel 5-mark mark scheme: 1 for bromine water as the test reagent; 1 each for the two observations (decolourised vs no change); 1 for balanced equation; 1 for explaining addition (both atoms of Br₂ join the product, no atoms lost). Mark-scheme keyword: 'decolourised' (not 'turns clear' which is colloquial).
3Hydrogenation of ethene + industrial application (5 marks, Paper 1)
Core• Adapted from 4CH1/1C May/Jun 2024 Q5• hydrogenation, margarine, addition, spec-4.25
Question
(a) Write a balanced equation for the reaction between ethene and hydrogen. (b) State the conditions used. (c) Describe the industrial use of hydrogenation in the food industry. (5 marks)
Step-by-step solution
1. Step 1
  (a) Equation for ethene + H₂ (1 M). H₂ adds across the C=C of ethene → ethane.
  $\text{CH}_2{=}\text{CH}_2 + \text{H}_2 \rightarrow \text{CH}_3\text{CH}_3$
2. Step 2
  (a) Product (1 A). The product is ethane (C₂H₆) — the corresponding alkane. The C=C double bond becomes a C–C single bond; each former C of the C=C gains one new H atom (total 2 H added).
3. Step 3
  (b) Conditions (1 A). Two conditions: • Nickel catalyst (Ni, finely divided — gives high surface area). • Temperature ~ 150 °C (warm — not too hot, otherwise the H₂ would split off again). • Pressure is typically slightly elevated but not extreme (~ a few atm).
4. Step 4
  (c) Industrial use — margarine production (1 B). Vegetable oils (e.g. sunflower oil, soybean oil, rapeseed oil) contain long-chain UNSATURATED hydrocarbons (multiple C=C bonds per molecule). They are LIQUID at room temperature because the C=C bonds force kinks in the molecules → loose packing → low intermolecular forces → low mp.
5. Step 5
  (c) Why hydrogenation makes margarine (1 B). Bubbling H₂ through vegetable oil over a Ni catalyst at ~ 150 °C ADDS H₂ across the C=C double bonds → SATURATES them → the molecules become straighter → pack tightly → stronger intermolecular forces → SOLID at room T → MARGARINE (or solid cooking fat). The amount of hydrogenation is controlled to get the desired consistency (soft margarine = partially hydrogenated; hard margarine = fully hydrogenated).
  
  Note: partial hydrogenation can produce 'trans fats' which have negative health effects, so modern margarines often use interesterification or other methods instead.
Answer
(a) CH₂=CH₂ + H₂ → CH₃CH₃ (ethene + hydrogen → ethane). (b) Conditions: NICKEL catalyst, ~150 °C, slightly elevated pressure. (c) Industrial use — making MARGARINE: liquid vegetable oils contain unsaturated C=C bonds (kinked molecules → loose packing → liquid). Bubbling H₂ over Ni catalyst at 150 °C adds H₂ across the double bonds → straightens the molecules → tight packing → solid at room T → margarine.
Examiner tip
Edexcel 5-mark mark scheme: 1 for balanced equation; 1 for product = ethane; 1 for Ni catalyst + ~150 °C; 1 for margarine production / saturation of oils; 1 for explaining why hydrogenation makes a solid (saturation → tight packing → higher mp). Mark scheme accepts 'hydrogenation' as the name of the reaction.

n	Name	Molecular formula
2	Ethene	C₂H₄
3	Propene	C₃H₆
4	Butene	C₄H₈ (positional isomers: but-1-ene, but-2-ene)

4Industrial ethanol from ethene — hydration (6 marks, Paper 1)

Core• Adapted from 4CH1/1C Jan 2024 Q6• hydration, ethanol, industrial, spec-4.25

Question

Ethanol can be manufactured industrially from ethene by reaction with steam. (a) Write the balanced equation. (b) State the THREE conditions required (temperature, pressure, catalyst). (c) State whether the reaction is reversible and what this means for the yield. (d) Compare this industrial method with the fermentation method for making ethanol — list ONE advantage and ONE disadvantage of each. (6 marks)

Step-by-step solution

Step 1
(a) Equation (1 M). Ethene + steam (water) → ethanol. The water adds across the C=C: one C of the C=C gains an H, the other gains an OH.
$\text{CH}_2{=}\text{CH}_2 + \text{H}_2\text{O} \rightleftharpoons \text{CH}_3\text{CH}_2\text{OH}$
Step 2
(b) Conditions (1 A). Three conditions: • Catalyst: concentrated PHOSPHORIC ACID (H₃PO₄) — typically adsorbed onto a silica support. • Temperature: ~ 300 °C. • Pressure: ~ 60 atm.
Step 3
(c) Reversible — implication for yield (1 A). The reaction is REVERSIBLE (note the ⇌). It does NOT go to completion in one pass — only about 5% of the ethene is converted to ethanol per pass. The unreacted ethene + water is RECYCLED back into the reactor. Over many passes, overall conversion is ~ 97%.
Step 4
(d) Industrial hydration — advantages and disadvantages (1 B). • Advantages: produces VERY pure ethanol; CONTINUOUS process (24/7 operation); FAST (minutes per pass); high overall conversion via recycling. • Disadvantages: requires NON-RENEWABLE crude oil (source of ethene); HIGH ENERGY costs (high T + P); expensive equipment.
Step 5
(d) Fermentation — advantages and disadvantages (1 B). Fermentation: $\text{C}_6\text{H}_{12}\text{O}_6 \xrightarrow{\text{yeast}} 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2$ (anaerobic, 25-35 °C, weeks). • Advantages: uses RENEWABLE sugars (sugar cane, sugar beet, corn); low T + atmospheric P → low energy cost; CARBON-NEUTRAL (CO₂ released = CO₂ absorbed by the plant when growing). • Disadvantages: SLOW (days to weeks); BATCH process (must restart between fermentations); produces DILUTE ethanol (~ 12-15%) → requires distillation to concentrate; uses up FOOD CROPS.

Step 6

(d) Summary table (1 B).

Feature	Hydration of ethene	Fermentation
Source	Crude oil (non-renewable)	Sugar (renewable)
Speed	Fast (minutes)	Slow (days)
Mode	Continuous	Batch
Conditions	300 °C, 60 atm, H₃PO₄	25-35 °C, atm P, yeast
Energy	High	Low
Purity	High (pure ethanol)	Low (~ 15%, requires distillation)
Sustainability	Poor (fossil fuel)	Better (carbon-neutral)

Answer

(a) C₂H₄ + H₂O ⇌ C₂H₅OH. (b) Conditions: H₃PO₄ catalyst, 300 °C, 60 atm. (c) Reaction is REVERSIBLE — only ~5% conversion per pass; unreacted gases are RECYCLED. (d) Hydration: fast, continuous, pure ethanol BUT high energy + non-renewable. Fermentation: renewable, low energy, BUT slow, batch, dilute product.

Examiner tip

Edexcel 6-mark mark scheme: 1 for balanced equation (with ⇌); 1 for the three conditions; 1 for noting reversible / recycling; 1 each for one advantage of each method; 1 for one disadvantage of each (or a clear comparison). Mark-scheme keyword: 'phosphoric acid catalyst', '300 °C', '60 atm'.

5Propene + bromine — equation and naming (4 marks, Paper 1)
Core• Adapted from 4CH1/1C May/Jun 2024 Q3• addition, bromine, naming, spec-4.25
Question
Propene (CH₂=CHCH₃) reacts with bromine at room temperature. (a) Write the displayed equation for the reaction (showing all bonds). (b) Name the product. (c) State the EXPECTED OBSERVATION when bromine WATER is added to propene. (4 marks)
Step-by-step solution
1. Step 1
  (a) Displayed equation (1 M). Br₂ adds across the C=C double bond of propene. Each carbon of the former C=C gets one Br atom. The C=C becomes a C–C single bond.
  
  H H H H | | | | C==C-CH₃ + Br₂ → Br-C-C-CH₃ | | | | H H H Br
  
  (Showing the addition: the Br atoms add to the carbons that had the double bond.)
2. Step 2
  (a) Condensed equation (1 A).
  $\text{CH}_2{=}\text{CH}{-}\text{CH}_3 + \text{Br}_2 \rightarrow \text{CH}_2\text{Br}{-}\text{CHBr}{-}\text{CH}_3$
3. Step 3
  (b) Name of product (1 A). The product has Br atoms on C1 and C2 of a 3-C propane chain → 1,2-dibromopropane. Note the '1,2-' indicates the position of the two Br atoms.
4. Step 4
  (c) Observation with bromine water (1 B). Propene is an alkene with a C=C double bond. Bromine water (orange/brown Br₂(aq)) is DECOLOURISED — turns from ORANGE-BROWN to COLOURLESS. The Br₂ has been consumed by addition across the C=C. (The minor product with bromine WATER specifically also contains some −OH from the water, but the key observable is just the decolourisation.)
Answer
(a) CH₂=CH-CH₃ + Br₂ → CH₂Br-CHBr-CH₃. (b) 1,2-dibromopropane. (c) Orange-brown bromine water → COLOURLESS (decolourised).
Examiner tip
Edexcel 4-mark mark scheme: 1 for balanced addition equation; 1 for displayed formula (or condensed with structure clear); 1 for correct product name with positions (1,2-dibromopropane — '1,2-' essential); 1 for decolourised observation. Common error: getting the position of substitution wrong — Br atoms always go on the two carbons that USED TO BE the double bond.

Model Answers — Alkenes

High-scoring sample answers for alkenes on the Pearson Edexcel IGCSE 4CH1 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.

Question 1

4CH1/1C-style — alkene series overview6 marks

Describe the alkene homologous series. Include the general formula, name and structure of the first THREE members, explain why they are described as 'unsaturated', and compare their reactivity with the alkanes. (6 marks)

Model answer

General formula. $C_n H_{2n}$ where n ≥ 2 (you need at least 2 C atoms for a C=C bond).

Alkenes have 2 FEWER H atoms than alkanes of the same chain length, because the C=C double bond uses up 2 H atoms' worth of bonding capacity. e.g. ethane C₂H₆ vs ethene C₂H₄ → ethene has 2 fewer H.

First three alkenes.

n	Name	Molecular formula	Structure
2	Ethene	C₂H₄	CH₂=CH₂
3	Propene	C₃H₆	CH₂=CHCH₃
4	Butene	C₄H₈	CH₂=CHCH₂CH₃ (but-1-ene) or CH₃CH=CHCH₃ (but-2-ene)

Displayed formula of ethene (smallest alkene).

H        H
 \      /
  C == C
 /      \
H        H

Ethene is a PLANAR molecule (all 6 atoms in the same plane). Each carbon is sp² hybridised with bond angles of ~ 120° (NOT 109.5° as in alkanes). The C=C double bond consists of one strong sigma (σ) bond + one weaker pi (π) bond.

Why alkenes are 'unsaturated'.

The term 'unsaturated' means the molecule could ACCEPT more atoms — specifically, it could add 2 more hydrogens to convert the C=C into a C–C, making the molecule saturated. In contrast, alkanes are already 'saturated' (full of H atoms) and can't accept any more without breaking C–C or C–H bonds.

The 'extra bond' of the C=C double bond is what gives alkenes their characteristic ADDITION reactions: small molecules (Br₂, H₂, H₂O, HBr) can ADD ACROSS the double bond, converting the C=C into a C–C single bond. The product gains 2 atoms (or one diatomic molecule).

Comparison of reactivity with alkanes.

Feature	Alkanes	Alkenes
Saturation	Saturated	Unsaturated
Reactivity	Low (relatively unreactive)	Higher (more reactive)
Addition reactions	NO	YES — across C=C
Substitution with halogens	Yes — slow, UV needed	Rare (addition occurs first)
Combustion	Yes — CO₂ + H₂O	Yes — CO₂ + H₂O
Bromine water	NO change	DECOLOURISED
Polymerisation	NO	YES — to plastics

Why alkenes are more reactive.

The C=C double bond contains a 'π bond' — the second bond on top of the σ bond. This π bond is:

WEAKER than a single σ bond (~ 264 kJ/mol vs ~ 348 kJ/mol for C–C single).
EXPOSED — sticks out above and below the plane of the molecule, where it can be attacked by other species.
ELECTRON-RICH — high density of electrons between the two carbons.

This makes the C=C double bond a 'site of reactivity' or 'functional group' — small molecules can attack it easily. In contrast, alkanes have only strong, non-polar single bonds with no electron-rich sites.

Industrial importance.

Alkenes are NOT naturally present in crude oil; they are produced by CRACKING long-chain alkanes (see Topic 4.2). They are extremely valuable as feedstocks for the petrochemical industry:

Ethene → poly(ethene) plastic, ethanol, ethylene glycol (antifreeze), poly(vinyl chloride).
Propene → poly(propene) plastic, propan-2-ol, propylene oxide.
Butenes → synthetic rubber, butan-1-ol, gasoline blending.

Ethene is the world's most-produced organic chemical (~ 150 million tonnes per year).

Why this scores

Edexcel 6-mark mark scheme: 1 for general formula CₙH₂ₙ; 1 for naming all three with formulae; 1 for displayed formula of ethene with C=C explicit; 1 for explaining 'unsaturated' (could add more atoms); 1 for stating alkenes more reactive than alkanes; 1 for explaining WHY (C=C bond is weak/exposed/electron-rich → undergoes addition).

Question 2

4CH1/2C-style — alkene addition reactions6 marks

Alkenes undergo addition reactions with hydrogen, bromine, and water. For each reaction, give: (i) the conditions needed, (ii) the balanced equation using ethene as the alkene, (iii) the name of the product. (6 marks)

Model answer

General principle of addition. A small molecule (H₂, Br₂, H₂O, HBr, Cl₂) adds across the C=C double bond of an alkene. The C=C becomes a C–C single bond; one atom (or fragment) of the small molecule attaches to each of the two former double-bond carbons. The product has TWO new bonds and NO atoms are lost.

Reaction 1: Addition of hydrogen (hydrogenation).

Conditions:

Catalyst: NICKEL (finely divided to give high surface area).
Temperature: ~ 150 °C.
Pressure: slightly elevated (~ a few atm).

Equation:

$\text{CH}_2{=}\text{CH}_2 + \text{H}_2 \xrightarrow{\text{Ni, 150 °C}} \text{CH}_3{-}\text{CH}_3$

Product: ethane (the corresponding alkane).

Industrial significance. Hydrogenation converts liquid vegetable oils (UNSATURATED — multiple C=C per molecule) into solid margarine (SATURATED). The straightened molecules can pack tightly → higher mp → solid at room T. Partial hydrogenation is controlled by adjusting time and H₂ supply.

Reaction 2: Addition of bromine.

Conditions:

No catalyst needed.
Room temperature.
Br₂ in solution (e.g. bromine water, or Br₂ in CCl₄ at A-level).
The reaction is rapid and quantitative.

Equation:

$\text{CH}_2{=}\text{CH}_2 + \text{Br}_2 \rightarrow \text{CH}_2\text{Br}{-}\text{CH}_2\text{Br}$

Product: 1,2-dibromoethane (a colourless liquid).

Observation. Bromine water is ORANGE/BROWN; it is DECOLOURISED (turns colourless) when shaken with ethene. This is the standard chemical TEST for unsaturation (a C=C double bond).

Other halogens behave similarly: Cl₂ adds to give 1,2-dichloroethane (CH₂Cl–CH₂Cl); I₂ also adds (less reactive).

Reaction 3: Addition of water (hydration).

Conditions:

Catalyst: CONCENTRATED PHOSPHORIC ACID (H₃PO₄), often supported on silica.
Temperature: ~ 300 °C.
Pressure: ~ 60 atm.
Reagent: water as STEAM (high T).

Equation:

$\text{CH}_2{=}\text{CH}_2 + \text{H}_2\text{O} \rightleftharpoons \text{CH}_3{-}\text{CH}_2{-}\text{OH}$

(Reversible! Only ~ 5% conversion per pass; unreacted gas recycled.)

Product: ethanol (CH₃CH₂OH or C₂H₅OH).

Industrial significance. This is the main INDUSTRIAL route to ethanol — produces the millions of tonnes used annually as solvent, fuel additive, and chemical feedstock. The alternative is fermentation (slower, batch, uses food crops, but renewable).

Reaction 4 (bonus — addition of HBr).

Conditions: room temperature; HBr (gas) bubbled through the alkene in solution.

Equation:

$\text{CH}_2{=}\text{CH}_2 + \text{HBr} \rightarrow \text{CH}_3{-}\text{CH}_2\text{Br}$

Product: bromoethane (an example of a haloalkane).

For non-symmetrical alkenes (e.g. propene), the HBr can add either way: CH₃CH=CH₂ + HBr → CH₃CH(Br)CH₃ (2-bromopropane, the major product by Markovnikov's rule — H to C with most H atoms, Br to C with fewer H atoms) or CH₃CH₂CH₂Br (1-bromopropane, minor product). (Markovnikov rules are A-level, not 4CH1 — but worth knowing.)

Side-by-side summary.

Reaction	Reagent	Conditions	Product
Hydrogenation	H₂	Ni catalyst, 150 °C	Ethane
Halogenation	Br₂	None / room T	1,2-Dibromoethane
Hydration	H₂O	H₃PO₄, 300 °C, 60 atm	Ethanol
Hydrohalogenation	HBr	None / room T	Bromoethane

In all four cases, the alkene becomes a saturated compound; the C=C becomes C–C; 2 new bonds are formed.

Why this scores

Edexcel 6-mark mark scheme: 2 marks per reaction (1 for conditions + balanced equation, 1 for product name). Three reactions for full 6 marks. Mark scheme keywords: 'Ni catalyst 150 °C' for hydrogenation; 'H₃PO₄ catalyst 300 °C 60 atm' for hydration. Industrial significance is a bonus mark.

Question 3

4CH1/2C-style — industrial ethanol manufacture5 marks

Describe how ethanol is manufactured industrially from ethene. Include the conditions, the equation, the role of recycling, and ONE advantage and ONE disadvantage of this method compared with the fermentation of sugars. (5 marks)

Model answer

The reaction.

Ethanol is made industrially by hydration of ethene — the addition of water (steam) across the C=C double bond of ethene:

$\text{CH}_2{=}\text{CH}_2(\text{g}) + \text{H}_2\text{O(g)} \rightleftharpoons \text{CH}_3\text{CH}_2\text{OH(g)}$

A water molecule splits across the double bond: one H goes to one carbon, one OH to the other.

Conditions.

Catalyst: concentrated phosphoric acid (H₃PO₄) — typically adsorbed onto a solid silica support (giving a 'silica-phosphoric acid' catalyst).
Temperature: ~ 300 °C.
Pressure: ~ 60 atmospheres.
Reagents: ethene + steam (water vapour at high T), passed through a fixed-bed reactor containing the catalyst.

Why the reaction is reversible — and the role of recycling.

The ⇌ in the equation indicates a reversible reaction. The forward (hydration) is EXOTHERMIC; at 300 °C, equilibrium gives only ~ 5% conversion of ethene to ethanol per pass. Why are these moderate conditions chosen?

Higher T would shift equilibrium BACKWARDS (Le Chatelier — exothermic forward reaction).
Lower T would give better yield but the rate would be too slow.
Higher P would shift forward (2 mol gas → 1 mol gas) but the equipment cost is prohibitive.

So the design accepts low single-pass conversion but RECYCLES the unreacted gases. The reactor outlet is cooled; ethanol (bp 78 °C) condenses out and is collected. Unreacted ethene + steam are returned to the reactor. Over many passes the overall conversion reaches ~ 97%.

Comparison with fermentation.

Fermentation: $\text{C}_6\text{H}_{12}\text{O}_6 \xrightarrow{\text{yeast}} 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2$ . Glucose (from sugar cane, sugar beet, corn) is converted to ethanol by yeast under anaerobic conditions at 25-35 °C for several days.

Feature	Hydration of ethene	Fermentation
Source	Ethene (from cracking crude oil)	Glucose (from sugar cane / corn)
Renewable?	NO (fossil fuel)	YES (plant-based)
Speed	Fast (minutes/hours per pass)	Slow (days/weeks)
Mode	CONTINUOUS	BATCH
Conditions	300 °C, 60 atm, H₃PO₄ catalyst	25-35 °C, atm P, yeast
Energy required	High (high T + P)	Low (near room T)
Concentration of product	HIGH (~ 95% ethanol)	LOW (~ 12-15% ethanol — needs distillation)
Yield	High (after recycling)	Moderate (yeast killed at ~ 15% EtOH)

One advantage of hydration. The product is very PURE (~ 95% ethanol) and the process is CONTINUOUS — better for very large-scale industrial production. Faster throughput (minutes per pass vs days for fermentation).

One disadvantage of hydration. Requires NON-RENEWABLE ethene (from crude oil — a finite fossil resource). Also high energy cost (300 °C and 60 atm).

One advantage of fermentation. Uses RENEWABLE sugars from plants → 'carbon-neutral' (CO₂ released = CO₂ absorbed by the plant when growing). Low energy required → cheap operating costs.

One disadvantage of fermentation. SLOW (days to weeks) and BATCH (must stop and restart between batches). Dilute product needs distillation to concentrate. Uses up FOOD CROPS that could feed people.

Modern context. Both methods are used commercially. Industrial chemicals (solvents, ethanol for sanitisers, fuel-grade ethanol for chemicals industry) tend to come from hydration. Alcoholic drinks, biofuels (bioethanol), and 'green' ethanol from biomass come from fermentation. In countries like Brazil, sugar-cane-based fermentation produces millions of tonnes of bioethanol annually.

Why this scores

Edexcel 5-mark mark scheme: 1 for equation (with ⇌); 1 for the three conditions (H₃PO₄, 300 °C, 60 atm); 1 for recycling unreacted gases; 1 for one advantage of hydration; 1 for one disadvantage. Mark scheme keyword phrases: 'phosphoric acid catalyst', 'unreacted gases recycled', 'continuous process'.

Question 4

4CH1/1C-style — alkane vs alkene with bromine5 marks

Compare the reactions of an alkane (ethane) and an alkene (ethene) with bromine. Include the type of reaction, conditions, equation, and product for each. Explain why the two reactions are different. (5 marks)

Model answer

Ethane + bromine (SUBSTITUTION).

Type of reaction: SUBSTITUTION — a Br atom REPLACES a H atom on the ethane. The other H atom is lost as HBr.

Conditions: ULTRAVIOLET LIGHT (or strong sunlight) — essential to break the Br–Br bond and start the reaction. Room temperature is fine.

Equation:

$\text{CH}_3{-}\text{CH}_3 + \text{Br}_2 \xrightarrow{\text{UV}} \text{CH}_3{-}\text{CH}_2\text{Br} + \text{HBr}$

Product: bromoethane (CH₃CH₂Br). Plus HBr (hydrogen bromide gas).

Observation with bromine WATER: NO change — bromine water STAYS ORANGE-BROWN. (Bromine water test conditions don't include UV light.) The bromine and the ethane are also IMMISCIBLE, so they separate into two layers.

Why the reaction works: UV photons break the Br–Br bond to give reactive Br atoms (free radicals). These attack the alkane's C–H bonds (substitution). Once substituted, the H atom and one Br atom form HBr; the other Br atom takes the position of the H atom on the carbon.

Ethene + bromine (ADDITION).

Type of reaction: ADDITION — Br₂ ADDS ACROSS the C=C double bond. NO atoms are lost — both Br atoms end up in the product.

Conditions: No UV light needed. Room temperature. Just mix the reagents (Br₂(aq) for the test, or Br₂ in solution for the prep).

Equation:

$\text{CH}_2{=}\text{CH}_2 + \text{Br}_2 \rightarrow \text{CH}_2\text{Br}{-}\text{CH}_2\text{Br}$

Product: 1,2-dibromoethane (CH₂BrCH₂Br) — a colourless liquid.

Observation with bromine WATER: Bromine water is RAPIDLY DECOLOURISED — turns from ORANGE-BROWN to COLOURLESS within seconds. This is the standard test for unsaturation.

Why the reaction works: The C=C double bond is electron-rich (has a π bond on top of the σ bond). Br₂ is attracted to this electron-rich region. The π bond breaks easily; one Br adds to each of the two carbons of the former C=C. Single bonds form; the C=C becomes C–C.

Why the two reactions are DIFFERENT.

The key difference is the presence (or absence) of the C=C double bond:

Ethane has only single C–C and C–H bonds → no electron-rich site → unreactive towards Br₂ at room T → only reacts via substitution under harsh conditions (UV). The C–H bond is strong (~ 412 kJ/mol) → hard to break.
Ethene has a C=C double bond with an exposed π bond → ELECTRON-RICH → reactive towards Br₂ → undergoes addition very easily at room T. The π bond is weaker (~ 264 kJ/mol) → breaks easily during addition.

So the SAME reagent (Br₂) gives DIFFERENT reactions: substitution with alkanes (requires UV); addition with alkenes (fast at room T). The Br₂ doesn't 'care' chemically — it just reacts with the most accessible bond. The C=C π bond is far more accessible than a strong C–H bond.

Summary table.

Feature	Ethane + Br₂	Ethene + Br₂
Type	Substitution	Addition
Conditions	UV light required	No UV, room T
Speed	Slow	Fast
H lost?	Yes (as HBr)	No
Atoms added	1 Br (replaces 1 H)	2 Br (across C=C)
Bromine water test	NO change	DECOLOURISED
Product	CH₃CH₂Br + HBr	CH₂BrCH₂Br

This difference is the practical basis of the bromine-water test for unsaturation in 4CH1.

Why this scores

Edexcel 5-mark mark scheme: 1 each for the two equations; 1 for naming both products; 1 for stating UV requirement for alkane; 1 for explaining why (C=C is reactive, C–H is not). Mark scheme keywords: 'substitution' for alkane; 'addition' for alkene. Use these terms explicitly.

Key Definitions and Keywords — Alkenes

Definitions to memorise and the exact keywords mark schemes credit for alkenes answers — sharpened from recent examiner reports for the 2026 Pearson Edexcel IGCSE 4CH1 sitting.

Alkene
Examiner keyword
An UNSATURATED hydrocarbon containing AT LEAST ONE C=C double bond. General formula CₙH₂ₙ (n ≥ 2). Examples: ethene (C₂H₄), propene (C₃H₆), butene (C₄H₈). More reactive than alkanes due to the C=C double bond.
Unsaturated (alkene context)
Examiner keyword
Contains at least one C=C double bond (or C≡C triple bond). Could accept more atoms via addition reactions. Decolourises bromine water (the C=C reacts with Br₂).
C=C double bond
Examiner keyword
A double covalent bond between two carbon atoms — one σ bond + one π bond. The π bond is weaker and more exposed than the σ bond → site of reactivity for addition reactions. Bond angle around each C ≈ 120° (sp² hybridised). All atoms in the immediate vicinity are coplanar.
Ethene (C₂H₄)
Examiner keyword
The simplest alkene. Planar molecule with 120° bond angles. Made industrially by CRACKING of long-chain alkanes from crude oil. Used to make poly(ethene), ethanol (by hydration), and many other industrial products. The world's most-produced organic chemical.
Addition reaction
Examiner keyword
A reaction in which a small molecule (e.g. Br₂, H₂, H₂O, HBr) ADDS ACROSS the C=C double bond of an alkene. The C=C becomes a C–C single bond; one atom (or fragment) of the small molecule attaches to each of the two former double-bond carbons. No atoms are lost.
Hydrogenation
Examiner keyword
Addition of H₂ across the C=C double bond of an alkene to give the corresponding alkane. Conditions: NICKEL catalyst, ~150 °C. Industrial use: converting liquid vegetable oils (unsaturated) into solid MARGARINE (saturated).
Bromination of an alkene
Examiner keyword
Addition of Br₂ across the C=C double bond of an alkene → 1,2-dibromoalkane. Occurs RAPIDLY at room temperature without a catalyst. Used as the test for unsaturation (bromine water decolourised).
Hydration (of an alkene)
Examiner keyword
Addition of water (steam) across the C=C double bond of an alkene to give an alcohol. Conditions: PHOSPHORIC ACID (H₃PO₄) catalyst, 300 °C, 60 atm. Industrial use: making ethanol from ethene (CH₂=CH₂ + H₂O ⇌ CH₃CH₂OH).
Bromine water test (for C=C double bond)
Examiner keyword
Standard test for unsaturation. Add orange/brown bromine water to the hydrocarbon. Alkene DECOLOURISES it (Br₂ adds across the C=C). Alkane does NOT change the colour (no C=C → no addition).
Margarine production
Examiner keyword
Industrial process: pass H₂ gas through liquid vegetable oils (unsaturated) over a NICKEL catalyst at ~ 150 °C. The H₂ adds across the C=C bonds → saturates the molecules → straightens the chains → tight packing → SOLID at room T → MARGARINE. Partial hydrogenation controls the consistency.
Industrial ethanol manufacture
Examiner keyword
Hydration of ethene with steam: CH₂=CH₂ + H₂O ⇌ CH₃CH₂OH. Conditions: H₃PO₄ catalyst, 300 °C, 60 atm. Reaction is reversible — only ~5% conversion per pass; unreacted gases are RECYCLED. Continuous process; high-purity ethanol. Used as solvent, fuel, chemical feedstock.

Common Mistakes and Misconceptions — Alkenes

The traps other students keep falling into on alkenes questions — taken from recent Pearson Edexcel IGCSE 4CH1 examiner reports and mark schemes — and how to avoid them.

✕Listing 'methene' as the first alkene
4CH1 Examiner Reports 2022-2024
Why it happens
Following the pattern 'meth-, eth-, prop-'.
How to avoid it
There is NO methene — you need at least 2 carbons for a C=C double bond. The smallest alkene is ETHENE (C₂H₄, n = 2). Methane (CH₄) is the smallest alkane.
✕Writing 'turns clear' instead of 'decolourised' for the bromine water test
4CH1 Examiner Reports 2023-2024
Why it happens
Colloquial use of 'clear'.
How to avoid it
'Clear' means transparent (you can see through both orange bromine water AND pure water). The colour change is from ORANGE-BROWN to COLOURLESS. Use 'decolourised' or 'turns from orange to colourless'.
✕Confusing addition (alkenes) with substitution (alkanes)
4CH1 Examiner Reports 2022-2024
Why it happens
Both involve halogens and an alkene/alkane.
How to avoid it
Addition (alkene): the WHOLE small molecule joins the product, no atoms lost — e.g. CH₂=CH₂ + Br₂ → CH₂BrCH₂Br. Substitution (alkane): one atom is REPLACED by another — e.g. CH₃CH₃ + Br₂ → CH₃CH₂Br + HBr. Look for: (a) product complexity (1 product = addition; 2 products = substitution); (b) conditions (UV = substitution; no UV = addition).
✕Forgetting position numbers when naming alkene products
Why it happens
For ethene only one position is possible.
How to avoid it
Always include '1,2-' in dihaloalkane names (e.g. 1,2-dibromopropane). The halogen atoms go on the carbons that USED TO HAVE the double bond. For propene + Br₂, the product is 1,2-dibromopropane (NOT 2,3- which would be the same compound viewed from the other end — convention is to give the LOWEST possible locants).
✕Mixing up the conditions for hydration vs hydrogenation
4CH1 Examiner Reports 2023-2024
Why it happens
Both are addition reactions.
How to avoid it
Hydrogenation (alkene + H₂ → alkane): Ni catalyst, 150 °C, slightly elevated P. Hydration (alkene + H₂O → alcohol): H₃PO₄ catalyst, 300 °C, 60 atm. Mnemonic: 'H₂ uses Nickel 150'; 'H₂O uses Phosphoric 300/60'.
✕Saying fermentation produces 'pure ethanol' or is 'faster than hydration'
Why it happens
Confusing the two ethanol routes.
How to avoid it
Fermentation: SLOW (days), BATCH, gives DILUTE ethanol (~ 15%) that needs distillation, uses RENEWABLE sugars (carbon-neutral). Hydration of ethene: FAST (minutes), CONTINUOUS, gives PURE ethanol (~ 95%), uses NON-RENEWABLE crude oil. Each has clear pros/cons — don't blur them.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Video lesson

Short walkthrough of the concepts students most often get stuck on.

Alkenes — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Alkenes

Short Study Notes in the form of Slides

Short Study Notes — Alkenes

Detailed Notes

What you’ll learn

How it’s examined

1Define the alkene homologous series and give first three members (4 marks, Paper 1)

2Test for unsaturation — bromine water (5 marks, Paper 1)

3Hydrogenation of ethene + industrial application (5 marks, Paper 1)

4Industrial ethanol from ethene — hydration (6 marks, Paper 1)

5Propene + bromine — equation and naming (4 marks, Paper 1)

Alkene

Unsaturated (alkene context)

C=C double bond

Ethene (C₂H₄)

Addition reaction

Hydrogenation

Bromination of an alkene

Hydration (of an alkene)

Bromine water test (for C=C double bond)

Margarine production

Industrial ethanol manufacture

✕Listing 'methene' as the first alkene

✕Writing 'turns clear' instead of 'decolourised' for the bromine water test

✕Confusing addition (alkenes) with substitution (alkanes)

✕Forgetting position numbers when naming alkene products

✕Mixing up the conditions for hydration vs hydrogenation

✕Saying fermentation produces 'pure ethanol' or is 'faster than hydration'

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Alkenes — frequently asked questions