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 more reactive than alkanes. In the Cambridge IGCSE Chemistry curriculum, understanding the structure and reactivity of alkenes is crucial for studying organic reactions.

How are alkenes named according to the IUPAC nomenclature system?

In the IUPAC nomenclature system, alkenes are named by identifying the longest carbon chain that contains the double bond, and the chain is numbered from the end nearest the double bond. The suffix '-ene' is used to indicate the presence of a double bond. For example, ethene (C2H4) is the simplest alkene. This naming convention is an important part of the Cambridge IGCSE Chemistry syllabus.

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

The general formula for alkenes is CnH2n, where 'n' is the number of carbon atoms. This formula helps in identifying alkenes and differentiating them from other hydrocarbons like alkanes (CnH2n+2). Recognizing this formula is essential for students studying Organic Chemistry in the Cambridge IGCSE curriculum.

What are some common reactions involving alkenes that are studied in the Cambridge IGCSE Chemistry course?

Alkenes undergo several key reactions due to their double bonds, including addition reactions such as hydrogenation, halogenation, and polymerization. These reactions are significant in the Cambridge IGCSE Chemistry course as they illustrate the reactivity of alkenes and their role in industrial processes.

Why are alkenes considered unsaturated hydrocarbons in Organic Chemistry?

Alkenes are considered unsaturated hydrocarbons because they contain at least one carbon-carbon double bond, which means they have fewer hydrogen atoms compared to alkanes with the same number of carbon atoms. This unsaturation allows alkenes to participate in addition reactions, a concept that is important in the Cambridge IGCSE Chemistry curriculum.

Why is "Saying alkenes undergo substitution" a common mistake in Alkenes?

Why it happens: Confusing with alkanes. How to avoid it: Alkenes: ADDITION (across the C=C). Alkanes: SUBSTITUTION (with UV). Source: 0620/42 — recurring

Why is "Saying alkene + bromine goes from colourless → orange" a common mistake in Alkenes?

Why it happens: Inverting. How to avoid it: ORANGE → colourless (decolourised). The bromine is consumed.

Why is "Forgetting catalysts/conditions in hydration or hydrogenation" a common mistake in Alkenes?

Why it happens: Speed. How to avoid it: Hydration: H_3PO_4, 300°C, 60 atm. Hydrogenation: Ni, ~150°C.

Organic ChemistryCambridge IGCSE Chemistry 0620 Extended

Alkenes

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Alkenes — Cambridge IGCSE 0620 Chemistry Extended (2026)

Unsaturated hydrocarbons with a C=C double bond. General formula $\text{C}_{n}\text{H}_{2n}$ . The double bond makes alkenes reactive — addition reactions across the double bond. Plus the bromine water test.

What you’ll learn

Mapped to the Cambridge IGCSE 0620 syllabus (2026-2028).

13.5 — Describe alkenes as unsaturated hydrocarbons with general formula $\text{C}_{n}\text{H}_{2n}$ .
13.5 — Describe the test for an alkene using bromine water.
13.5 — Describe addition reactions of alkenes with Br₂, H₂, H₂O.
13.5 — Describe cracking as a source of alkenes.

Structure and the double bond

C=C double bond. Two of the four bonds on each carbon go to the same partner.

Unsaturated. Each alkene contains at least one C=C double bond. Carbons in the double bond have only TWO other bonds each (vs four for alkanes). They can ADD more atoms across the double bond — unlike alkanes which are "saturated" (full).

General formula: $\text{C}_{n}\text{H}_{2n}$ (two fewer hydrogens than alkanes).

First three alkenes.

Name	Formula	Structural
Ethene	$\text{C}_{2}\text{H}_{4}$	$\text{CH}_{2}=\text{CH}_{2}$
Propene	$\text{C}_{3}\text{H}_{6}$	$\text{CH}_{2}=\text{CHCH}_{3}$
Butene (1- or 2-)	$\text{C}_{4}\text{H}_{8}$	$\text{CH}_{2}=\text{CHCH}_{2}\text{CH}_{3}$ (but-1-ene)

The C=C double bond is the reactive site of every alkene.

Note. No "methene" — there's only one carbon to bond to itself, can't form a C=C. Alkenes start at 2 carbons.

Why are alkenes more reactive than alkanes? The C=C double bond consists of one strong $\sigma$ bond + one weaker $\pi$ bond (Cambridge IGCSE doesn't go this deep). The $\pi$ bond breaks easily during addition reactions, allowing new atoms to add to the carbons.

Unsaturated; C=C double bond.
$\text{C}_{n}\text{H}_{2n}$ .
First three: ethene, propene, butene.
More reactive than alkanes.

Test for unsaturation — bromine water

Add bromine water to an unknown. ORANGE/BROWN → COLOURLESS = alkene present.

The test. Add a few drops of orange/yellow-brown bromine water to a sample.

Result.

Alkene present: bromine water RAPIDLY DECOLOURISES (turns colourless).
Alkane present: bromine water STAYS orange/brown — no reaction (under normal conditions; substitution needs UV).

An alkene decolourises bromine water from orange-brown to colourless; an alkane does not.

Why? The alkene undergoes addition with Br₂: $\text{CH}_{2}=\text{CH}_{2} + \text{Br}_{2} \to \text{CH}_{2}\text{Br}\text{CH}_{2}\text{Br}.$

The product 1,2-dibromoethane is colourless → solution clears.

Worked qualitative. A student is given two unknown liquids labelled A and B, both colourless. Add a few drops of bromine water to each. A turns the bromine colourless quickly; B keeps the orange colour. A is unsaturated (alkene-like); B is saturated (alkane-like).

Cambridge tip. Always state BOTH the colour change AND the conclusion explicitly. "The bromine water decolourises FROM ORANGE TO COLOURLESS, indicating an alkene."

Bromine water test for alkenes.
Decolourises (orange → colourless) with alkene.
Stays orange with alkane.
Net reaction: addition of Br₂ across C=C.

Addition reactions

Alkene + small molecule → saturated product. Three big examples: H₂, Br₂, H₂O.

General. A small molecule X-Y adds across the C=C double bond. Each carbon gains a new bond. $\text{C}=\text{C} + \text{X-Y} \to \text{C(X)-C(Y)}.$

1. Addition of hydrogen (hydrogenation). Catalyst: nickel. Conditions: heat, $\sim 200\degree\text{C}$ . $\text{CH}_{2}=\text{CH}_{2} + \text{H}_{2} \xrightarrow{\text{Ni}} \text{CH}_{3}\text{CH}_{3}.$

Industrial use: turning vegetable oil (unsaturated) into margarine (more saturated, solid at room T).

2. Addition of halogens. No catalyst needed at room T (with Br₂ in solution, the bromine water test). $\text{CH}_{2}=\text{CH}_{2} + \text{Br}_{2} \to \text{CH}_{2}\text{Br}\text{CH}_{2}\text{Br}.$

3. Addition of water (hydration). Catalyst: phosphoric or sulfuric acid. Conditions: high T ( $\sim 300\degree\text{C}$ ) and pressure ( $\sim 60\,\text{atm}$ ). $\text{CH}_{2}=\text{CH}_{2} + \text{H}_{2}\text{O} \xrightarrow{\text{H}_{3}\text{PO}_{4}} \text{CH}_{3}\text{CH}_{2}\text{OH}.$

Industrial use: making ethanol for industrial / fuel use (NOT food/drink — for those, fermentation is used).

Cambridge tip. Memorise three additions and their conditions:

H₂: Ni catalyst, $200\degree\text{C}$ .
Br₂: no catalyst, room T.
H₂O: H₃PO₄ catalyst (or H₂SO₄), high T and P.

Addition: alkene + small molecule → saturated.
- H₂ (Ni cat) → alkane.
- Br₂ → dibromoalkane.
- H₂O (H₃PO₄ cat) → alcohol.

Cracking and polymerisation

Cracking: break long alkanes into short alkanes + alkenes. Alkenes feed polymerisation.

Cracking. Heat long-chain alkanes (from heavy crude-oil fractions) at high T ( $\sim 600-700\degree\text{C}$ ) with a catalyst (e.g. Al₂O₃, SiO₂, or zeolite). $\text{C}_{12}\text{H}_{26} \xrightarrow{\text{cat, heat}} \text{C}_{8}\text{H}_{18} + \text{C}_{4}\text{H}_{8}.$

(One short alkane + one short alkene from each long-chain.)

Why crack?

Long-chain alkanes are LESS USEFUL (high b.p., bad for petrol engines).
Short-chain alkanes are MORE USEFUL (gasoline range).
Alkenes are needed for the petrochemical industry (plastics, ethanol, etc.).

Polymerisation. Alkenes can join end-to-end to form polymers — long chains of repeating units. $n\text{CH}_{2}=\text{CH}_{2} \to (\text{-CH}_{2}\text{-CH}_{2}\text{-})_{n}.$

This is poly(ethene), the most-produced plastic. Conditions: high T, high P, catalyst.

See the Polymers study note for full details on polymerisation, plastics, and biodegradability.

Worked. What useful products come from cracking the dodecane?

Octane (gasoline component).
Butene (alkene → polymerisation feedstock).

Cracking: long → short + alkene.
$\text{C}_{12}\text{H}_{26} \to \text{C}_{8}\text{H}_{18} + \text{C}_{4}\text{H}_{8}$ .
Catalyst (Al₂O₃ etc.), high T.
Alkenes feed polymerisation.

How it’s examined

Alkenes appear every Paper 4 (6-8 marks): bromine test, addition equations, cracking. Examiner reports flag students writing the cracking products as TWO alkanes (forgetting that one product must be an alkene to balance H counts).

Sources: Cambridge IGCSE Chemistry 0620 syllabus 2026-2028 (13.5); 0620/42 Oct/Nov 2024 — Q24 (alkenes, addition); 0620 Examiner Reports 2022-2024. Last reviewed 2026-05-07.

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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.

1Test for an alkene
Core• test
Question
Describe the bromine-water test and state the result for an alkane and an alkene.
Step-by-step solution
1. Step 1
  Add bromine water (orange).
2. Step 2
  Alkane: NO change — stays orange.
3. Step 3
  Alkene: bromine adds across C=C → solution DECOLOURISES (orange → colourless).
Answer
Alkene: orange → colourless. Alkane: no change.
2Addition of bromine to ethene
Extended• Adapted from 0620/42 May/Jun 2024 Q15• addition
Question
Write the equation for ethene + bromine.
Step-by-step solution
1. Step 1
  C=C breaks; Br adds to each carbon.
  $\text{CH}_{2}\!=\!\text{CH}_{2} + \text{Br}_{2} \to \text{CH}_{2}\text{BrCH}_{2}\text{Br}$
Answer
Ethene + Br₂ → 1,2-dibromoethane.
3Hydration of ethene to ethanol
Extended• hydration
Question
Write the equation for the hydration of ethene to make ethanol, with conditions.
Step-by-step solution
1. Step 1
  Ethene + steam, $\text{H}_{3}\text{PO}_{4}$ catalyst, $\sim 300\degree\text{C}$ , $60\,\text{atm}$ .
  $\text{CH}_{2}\!=\!\text{CH}_{2} + \text{H}_{2}\text{O(g)} \to \text{C}_{2}\text{H}_{5}\text{OH}$
Answer
$\text{C}_{2}\text{H}_{4} + \text{H}_{2}\text{O} \to \text{C}_{2}\text{H}_{5}\text{OH}$ (steam, $\text{H}_{3}\text{PO}_{4}$ , 300°C, 60 atm).
4Hydrogenation of ethene
Extended• hydrogenation
Question
Write the equation for ethene + hydrogen with a nickel catalyst.
Step-by-step solution
1. Step 1
  Adds H across C=C with Ni catalyst.
  $\text{C}_{2}\text{H}_{4} + \text{H}_{2} \xrightarrow{\text{Ni}} \text{C}_{2}\text{H}_{6}$
Answer
$\text{C}_{2}\text{H}_{4} + \text{H}_{2} \to \text{C}_{2}\text{H}_{6}$ (Ni catalyst).

Key Formulae — Alkenes

The formulae you need to memorise for alkenes on the Cambridge IGCSE 0620 paper, with every variable defined in plain English and a note on when to use it.

Alkene general formula
$\text{C}_{n}\text{H}_{2n}$
When to use
One C=C bond.

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 0620 sitting.

Alkene
Examiner keyword
Unsaturated hydrocarbon with one C=C double bond. General formula $\text{C}_{n}\text{H}_{2n}$ .
Addition reaction
Examiner keyword
Two molecules combine to give one product; typical for alkenes (the C=C opens up).
Bromine-water test
Examiner keyword
Standard test for unsaturation: bromine water turns from orange to colourless if a C=C is present.

Common Mistakes and Misconceptions — Alkenes

The traps other students keep falling into on alkenes questions — taken from recent Cambridge IGCSE 0620 examiner reports and mark schemes — and how to avoid them.

✕Saying alkenes undergo substitution
0620/42 — recurring
Why it happens
Confusing with alkanes.
How to avoid it
Alkenes: ADDITION (across the C=C). Alkanes: SUBSTITUTION (with UV).
✕Saying alkene + bromine goes from colourless → orange
Why it happens
Inverting.
How to avoid it
ORANGE → colourless (decolourised). The bromine is consumed.
✕Forgetting catalysts/conditions in hydration or hydrogenation
Why it happens
Speed.
How to avoid it
Hydration: $\text{H}_{3}\text{PO}_{4}$ , 300°C, 60 atm. Hydrogenation: Ni, ~150°C.

Practice questions

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

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Alkenes — frequently asked questions

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Alkenes

Short Study Notes in the form of Slides

Short Study Notes — Alkenes

Detailed Notes

What you’ll learn

How it’s examined

1Test for an alkene

2Addition of bromine to ethene

3Hydration of ethene to ethanol

4Hydrogenation of ethene

Alkene general formula

Alkene

Addition reaction

Bromine-water test

✕Saying alkenes undergo substitution

✕Saying alkene + bromine goes from colourless → orange

✕Forgetting catalysts/conditions in hydration or hydrogenation

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Alkenes — frequently asked questions