Introduction

Hydrocarbon = a compound containing ONLY carbon and hydrogen atoms. No oxygen, nitrogen, halogens, or other elements.

Examples: methane (CH₄), ethane (C₂H₆), propene (C₃H₆), hexane (C₆H₁₄), benzene (C₆H₆). NOT a hydrocarbon: methanol (CH₃OH — has O); chloromethane (CH₃Cl — has Cl); ethanoic acid (CH₃COOH — has O).

Hydrocarbons come mainly from CRUDE OIL (a mixture of mainly alkane hydrocarbons) and NATURAL GAS (mostly methane). They are the basis of all major fuels — petrol, diesel, kerosene, LPG, natural gas — and the feedstock for most plastics and many other chemicals.

Homologous series. A FAMILY of organic compounds sharing five features:

1. Same general formula (e.g. alkanes all fit CₙH₂ₙ₊₂).
2. Same functional group — the same reactive 'site' that determines the chemistry.
3. Similar chemical reactions (because of the shared functional group).
4. Smooth trend in physical properties — mp, bp, viscosity vary systematically with chain length.
5. Adjacent members differ by CH₂ in molecular formula.

The 'homo-' in homologous means 'same' — same chemistry, just different chain lengths.

The FOUR homologous series in Pearson Edexcel 4CH1.

(1) Alkanes — saturated hydrocarbons.

• General formula: $C_n H_{2n+2}$.
• Functional group: none (all single bonds).
• Examples: methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀, pentane C₅H₁₂, hexane C₆H₁₄.
• Chemistry: combustion (used as fuels); substitution with halogens in UV light.

(2) Alkenes — unsaturated hydrocarbons with one C=C double bond.

• General formula: $C_n H_{2n}$ (n ≥ 2).
• Functional group: C=C double bond.
• Examples: ethene C₂H₄, propene C₃H₆, butene C₄H₈, pentene C₅H₁₀, hexene C₆H₁₂.
• Chemistry: addition reactions (with H₂, Br₂, H₂O, HBr); decolourise bromine water; polymerise to form plastics.

(3) Alcohols — compounds with the −OH (hydroxyl) functional group.

• General formula: $C_n H_{2n+1}\text{OH}$ (or equivalently C_n H_{2n+2}O).
• Functional group: −OH (hydroxyl).
• Examples: methanol CH₃OH, ethanol C₂H₅OH (CH₃CH₂OH), propan-1-ol C₃H₇OH, butan-1-ol C₄H₉OH.
• Chemistry: combustion (fuels — bioethanol); oxidation to carboxylic acids; esterification with carboxylic acids.

(4) Carboxylic acids — compounds with the −COOH (carboxyl) functional group.

• General formula: $C_n H_{2n+1}\text{COOH}$ (or C_n H_{2n}O_2).
• Functional group: −COOH (carboxyl — a C=O and an O–H on the same carbon).
• Examples: methanoic acid HCOOH (formic acid, in ant stings), ethanoic acid CH₃COOH (in vinegar), propanoic acid C₂H₅COOH, butanoic acid C₃H₇COOH (rancid butter).
• Chemistry: weak acids — partially ionise to release H⁺; react with metals, bases, carbonates; esterification with alcohols.

Why functional groups matter.

The functional group is the 'business end' of an organic molecule. Two compounds with the same functional group react in the same way — even if they have different chain lengths. So once you know that ethene + Br₂ gives an addition product, you also know:

• Propene + Br₂ → 1,2-dibromopropane.
• But-1-ene + Br₂ → 1,2-dibromobutane.
• Hex-1-ene + Br₂ → 1,2-dibromohexane.

All alkenes react the same way with Br₂ — the C=C double bond gets the addition. This is the power of homologous series in organic chemistry: a few principles cover thousands of compounds.

Why properties trend smoothly.

As you add CH₂ to the chain, you add 14 atomic mass units (12 + 2). The molecule gets heavier; its molar mass increases linearly with chain length. Boiling point also increases (almost linearly for the first few members; logarithmically for very long chains). Other physical properties similarly trend smoothly.

The reason: as the molecule gets larger, it has more electrons and a larger surface area → stronger intermolecular forces (van der Waals / London dispersion) → more energy to overcome on melting / boiling. Bigger molecules → higher mp/bp.

IUPAC names follow a systematic pattern: PREFIX (chain length) + SUFFIX (functional group) with position numbers as needed.

Chain-length prefixes (memorise the first six):

Number of C atoms	Prefix
1	meth-
2	eth-
3	prop-
4	but-
5	pent-
6	hex-

Functional-group suffixes:

Series	Functional group	Suffix
Alkane	(no functional group)	-ane
Alkene	C=C	-ene
Alcohol	−OH	-anol
Carboxylic acid	−COOH	-anoic acid
Ester (Paper 2 / MR 6)	−COO−	-anoate

How to build a name.

1. Count the carbon atoms in the longest chain → choose the prefix.
2. Identify the functional group → choose the suffix.
3. If needed, add a POSITION NUMBER to show where the functional group is (e.g. but-1-ene vs but-2-ene).

Examples.

• 1 C + alkane → methane (CH₄).
• 2 C + alkene → ethene (C₂H₄).
• 3 C + alcohol → propan-?-ol. Two possible isomers: propan-1-ol (CH₃CH₂CH₂OH) and propan-2-ol ((CH₃)₂CHOH).
• 4 C + acid → butanoic acid (C₃H₇COOH). The carboxylic acid group is ALWAYS at C1 by definition; no position number needed.

Naming alkenes — position of the double bond.

The position of the C=C must be specified for chains of 4 or more carbons:

• C₂H₄ → ethene (only one position).
• C₃H₆ → propene (only one position: CH₂=CHCH₃; if double bond were 'between C2 and C3' it would just be propene from the other end).
• C₄H₈ → but-1-ene (CH₂=CHCH₂CH₃) or but-2-ene (CH₃CH=CHCH₃) — DIFFERENT compounds.

The number 1, 2, etc. shows the LOWER-numbered C of the C=C bond. Always choose the numbering that gives the lower locant.

Naming alcohols — position of the −OH.

Similar logic. For propan-1-ol vs propan-2-ol: in 1-ol the OH is on C1; in 2-ol it's on C2. Different compounds with different properties.

Naming branched alkanes.

For molecules with a branch (alkyl group like methyl, ethyl), use the format: locant + branch name + main chain name.

Example: (CH₃)₃CH — this is propane with a methyl branch on the central (C2) carbon → 2-methylpropane. Use 'methyl' for CH₃, 'ethyl' for C₂H₅, etc.

Example: CH₃CH(CH₃)CH₂CH₃ — this is a butane chain with a methyl branch on C2 → 2-methylbutane.

Numbering rule. Always number the carbons in the main chain to give the LOWEST possible locants to substituents. Number from whichever end of the chain gives the lower number.

Naming carboxylic acids.

The −COOH is ALWAYS at C1 of the chain (by definition). The chain includes the C of the COOH group. So:

• 1 C → methanoic acid HCOOH (just H–C(=O)–OH).
• 2 C → ethanoic acid CH₃COOH.
• 3 C → propanoic acid CH₃CH₂COOH.
• 4 C → butanoic acid CH₃CH₂CH₂COOH.

Older / common names (sometimes used outside IUPAC).

• Methanoic acid = formic acid.
• Ethanoic acid = acetic acid (the acid in vinegar).
• Methanol = wood alcohol (highly toxic — causes blindness).
• 2-methylpropane = isobutane.
• 2,2-dimethylpropane = neopentane.

IUPAC names are PREFERRED in 4CH1 mark schemes. Common names usually accepted as alternatives.

Tip: building a name systematically.

For an unknown organic compound:

1. Find the longest carbon chain containing the main functional group.
2. Count the carbons in that chain → choose prefix (meth-, eth-, …).
3. Identify the functional group → choose suffix (-ane, -ene, -anol, -anoic acid).
4. Identify any branches (alkyl groups like methyl, ethyl) → add as 'locant + branch name + parent name'.
5. Number the chain to give lowest locants to the functional group + branches.
6. Put it all together: locant of group + prefix + suffix.

Examples:

• CH₃CH₂CH₂CH₃ → 4 C chain, alkane → butane.
• CH₂=CHCH₂CH₃ → 4 C chain, alkene, C=C between C1 and C2 → but-1-ene.
• CH₃CH(CH₃)CH₂CH₂OH → 4 C chain (longest), alcohol on C1, methyl branch on C3 → 3-methylbutan-1-ol.

Saturated. A compound containing ONLY single covalent bonds between carbon atoms (and to H atoms). Each carbon has 4 single bonds — bonded to the maximum number of atoms possible.

• Alkanes are saturated.
• Cycloalkanes (rings of CH₂ groups) are also saturated.
• Their carbons are sp³ tetrahedral, bond angles ~ 109.5°.

Unsaturated. A compound containing AT LEAST ONE C=C double bond (or C≡C triple bond). Carbons in the double bond are sp² hybridised (planar, 120° angles).

• Alkenes have ONE C=C → 'monounsaturated' (one double bond per molecule).
• 'Polyunsaturated' compounds have multiple C=C (relevant for vegetable oils — many double bonds per molecule).

Why 'unsaturated' is called unsaturated. An unsaturated compound can ACCEPT MORE ATOMS via addition reactions. It's not 'full' of atoms — it could gain more.

For ethene (CH₂=CH₂), adding 2 H atoms across the C=C gives ethane (CH₃CH₃) — a saturated compound. Ethene COULD HOLD 2 more H atoms; ethane already holds the maximum.

Key chemistry: alkenes undergo ADDITION reactions.

The C=C double bond is the alkenes' functional group. It is:

1. Electron-rich — the π bond has high electron density above and below the plane of the molecule.
2. Weaker — the π bond (~ 264 kJ/mol) is weaker than the σ bond (~ 348 kJ/mol). Easier to break.
3. Exposed — sticks out from the molecule where reagents can attack.

Small molecules (Br₂, H₂, H₂O, HBr) ADD ACROSS the C=C. Each atom (or fragment) attaches to one of the two former double-bonded carbons. The C=C becomes a C–C single bond.

Example: $\text{CH}_2{=}\text{CH}_2 + \text{Br}_2 \rightarrow \text{CH}_2\text{Br}{-}\text{CH}_2\text{Br}$ (1,2-dibromoethane).

The bromine-water test for unsaturation.

This is THE standard 4CH1 test to distinguish alkanes (saturated) from alkenes (unsaturated):

Procedure. Add a few cm³ of BROMINE WATER (orange-brown, Br₂ dissolved 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 addition across the C=C.

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 under UV conditions — not in this test.)

Mark-scheme word: 'decolourised' — not 'clear'.

'Clear' just means transparent (both orange bromine water and pure water are CLEAR). Use decolourised or 'turns from orange to colourless'.

Why alkanes don't react.

Alkanes have only single C–C and C–H bonds. There's no C=C for Br₂ to add to. The single bonds are strong (~ 348-412 kJ/mol) and there's no electron-rich site for Br₂ to attack at room T. Without UV light (to break Br–Br and start substitution), the alkane is unreactive.

Why alkenes react fast.

The C=C is electron-rich; Br₂ is attracted to it. Br₂ partially polarises as it approaches: the closer Br becomes slightly δ+, attracted to the C=C electrons. Then Br₂ adds across the double bond instantly. No catalyst, no UV, no heat needed.

Test for OIL purity.

Vegetable oils contain many C=C bonds (multiple per molecule — polyunsaturated). They DECOLOURISE bromine water quickly. After hydrogenation (to make margarine), most of the C=C bonds are saturated → the product is less reactive towards bromine water. This is a way of testing how 'hydrogenated' a fat is.

Quick comparison table.

Feature	Alkane	Alkene
Functional group	(none)	C=C
Saturation	Saturated	Unsaturated
Bromine water	NO change	DECOLOURISED
Addition reactions	NO	YES (with Br₂, H₂, H₂O, HBr)
Reaction with halogens	Yes — substitution, UV needed	Yes — addition, no UV
Polymerisation	NO	YES
Example	hexane	hex-1-ene

Structural isomers are compounds with the SAME molecular formula but DIFFERENT structural formulae — the atoms are arranged differently (different connectivity / branching).

They have:

• Same number of each type of atom (same molecular formula, e.g. C₄H₁₀).
• Same molar mass (M_r).
• Same general chemistry (if in the same homologous series).
• DIFFERENT structural arrangement.
• Slightly DIFFERENT physical properties (bp, mp).

Why isomers exist.

For organic molecules with 4+ carbons, there's more than one way to join the carbon atoms together (especially with branches). As chain length increases, the number of possible isomers grows rapidly:

Molecular formula	Number of structural isomers
CH₄ (methane)	1
C₂H₆ (ethane)	1
C₃H₈ (propane)	1
C₄H₁₀ (butane)	2
C₅H₁₂ (pentane)	3
C₆H₁₄ (hexane)	5
C₇H₁₆	9
C₈H₁₈ (octane)	18
C₉H₂₀	35
C₁₀H₂₂	75
C₂₀H₄₂	366,319

The two isomers of butane (C₄H₁₀).

Isomer 1 — n-butane (straight chain).

  H   H   H   H
  |   |   |   |
H-C - C - C - C-H
  |   |   |   |
  H   H   H   H

Condensed: CH₃CH₂CH₂CH₃. IUPAC name: butane (or n-butane). Boiling point: −1 °C.

Isomer 2 — 2-methylpropane (branched).

        CH₃
        |
H₃C - CH - CH₃

Or fully displayed: the central C has 1 H and 3 methyl groups. Condensed: (CH₃)₃CH or CH(CH₃)₃. IUPAC name: 2-methylpropane (older: isobutane). Boiling point: −12 °C.

Both have C₄H₁₀ molecular formula but different structures.

The three isomers of pentane (C₅H₁₂).

1. n-pentane — straight 5-C chain. CH₃CH₂CH₂CH₂CH₃. bp 36 °C.
2. 2-methylbutane — 4-C chain with methyl branch on C2. (CH₃)₂CHCH₂CH₃. bp 28 °C.
3. 2,2-dimethylpropane — 3-C propane chain with TWO methyl branches on C2. C(CH₃)₄. bp 10 °C.

Effect of branching on boiling point.

The straight-chain isomer has the HIGHEST bp; the most-branched isomer has the LOWEST bp.

Why? Bp depends on intermolecular forces (van der Waals). Two factors:

1. Number of electrons — same for all isomers (same molecular formula, same M_r).
2. Surface area in contact between molecules:
   • STRAIGHT chains have a LARGE surface area along their length — close contact between adjacent molecules → strong intermolecular forces.
   • BRANCHED chains are more COMPACT / SPHERICAL — less surface contact between molecules → weaker intermolecular forces.

So branched isomers have weaker intermolecular forces → lower bp.

This is why petrol manufacturers prefer BRANCHED isomers (like 2,2,4-trimethylpentane, 'iso-octane'): they have higher octane ratings (resist 'knock' in engines) but similar volatility to straight-chain octanes.

Chemistry of isomers.

Within the same homologous series (e.g. all alkanes), isomers have the SAME chemistry — they all undergo combustion, substitution with halogens in UV, etc. The differences are only in physical properties (bp, mp, density slightly).

ACROSS different homologous series, structural isomers have VERY DIFFERENT chemistry. For example, C₂H₆O can be:

• Ethanol (CH₃CH₂OH) — an alcohol; reacts with sodium; oxidises to ethanoic acid; bp 78 °C.
• Dimethyl ether (CH₃OCH₃) — an ether; unreactive with sodium; doesn't oxidise; bp −24 °C.

Same molecular formula (C₂H₆O), but totally different functional groups → very different chemistry. (Ethers are not in 4CH1 core, but the concept is useful.)

Drawing isomers — common mistakes.

A frequent error is drawing two structures that are actually the SAME compound rotated or flipped. To check whether two structures are genuinely different isomers:

• Number the carbons in the longest chain in each.
• Check whether the connectivity (which C is joined to which) is the same.

For example, CH₃CH₂CH(CH₃)CH₃ and (CH₃)₂CHCH₂CH₃ might LOOK different, but both are 2-methylbutane (same compound — just written from different ends).

As you move along a homologous series (e.g. alkanes from methane to pentadecane), several physical properties change smoothly:

(1) Boiling point INCREASES.

Alkane	n	Bp (°C)	State at rt
Methane CH₄	1	−161	Gas
Ethane C₂H₆	2	−89	Gas
Propane C₃H₈	3	−42	Gas
Butane C₄H₁₀	4	−1	Gas
Pentane C₅H₁₂	5	36	Liquid
Hexane C₆H₁₄	6	69	Liquid
Heptane C₇H₁₆	7	98	Liquid
Octane C₈H₁₈	8	126	Liquid
Decane C₁₀H₂₂	10	174	Liquid
Hexadecane C₁₆H₃₄	16	287	Liquid
Octadecane C₁₈H₃₈	18	317	Solid (wax)

Pattern: bp rises by ~ 20-30 °C per CH₂ added. After ~ C18, alkanes are SOLID at room T (waxes).

(2) Melting point INCREASES. Same reasoning as bp.

(3) Viscosity INCREASES. Long molecules tangle more → liquid is 'thicker' and flows slower. Petrol (C5-C10) is runny like water; engine oil (C20+) is thick and slow-flowing.

(4) Flammability DECREASES. Combustion requires VAPOUR. Short alkanes vaporise easily (high volatility) → ignite easily with a small spark. Long alkanes have low volatility → harder to ignite. Petrol ignites with a spark; diesel needs compression; bitumen barely burns.

(5) Volatility DECREASES. Long alkanes are less likely to evaporate at room T.

(6) Density INCREASES slightly. Heavier molecules pack down a bit more efficiently.

WHY do these trends occur — intermolecular forces.

The bonds within the molecule (covalent C–C, C–H) are NOT broken on melting or boiling. What IS overcome is the INTERMOLECULAR FORCES between molecules — specifically van der Waals forces (also called London dispersion forces).

Van der Waals arise from TEMPORARY DIPOLES: at any instant, the electron cloud of a molecule is slightly more on one side, making that side temporarily δ⁻ and the other side δ⁺. The δ⁻ side attracts the δ⁺ side of a neighbouring molecule. These forces are WEAK (much weaker than covalent bonds) but they ARE the forces holding the liquid together.

Two factors increase van der Waals forces:

1. More electrons → larger temporary dipoles → stronger forces. Longer alkanes have more electrons (e.g. methane has 10 electrons; decane has 82).

2. Larger surface area in contact between adjacent molecules → more positions for van der Waals interactions → stronger overall attraction. Longer molecules have more surface contact.

Both factors increase as chain length increases → stronger intermolecular forces → more energy needed to separate molecules → higher mp and bp.

Why this is NOT about 'stronger covalent bonds'.

A common error: 'longer alkanes have stronger covalent bonds, so they're harder to boil'. WRONG. Every alkane has the SAME TYPES of covalent bonds (C–H, C–C, all single, similar strength). The number of bonds is more, but boiling doesn't break ANY of them — it only overcomes the (much weaker) INTERMOLECULAR forces.

Same trends across other homologous series.

The same physical-property trends apply to alkenes, alcohols, carboxylic acids, etc. As chain length increases within ANY homologous series, bp/mp/viscosity rise; flammability/volatility fall.

(Alcohols have an additional intermolecular force called HYDROGEN BONDING from the −OH group, which makes them have much HIGHER bp than the corresponding alkane. e.g. methanol bp +65 °C vs methane bp −161 °C — same molecular size but H-bonding adds a lot. Hydrogen bonding is A-level content, not 4CH1 core — but worth knowing.)

Industrial use of the trends.

Crude oil is separated by FRACTIONAL DISTILLATION using exactly these bp trends. Each fraction has a similar bp range and contains alkanes of similar chain lengths:

Fraction	Carbon range	Bp range	Use
Refinery gases	C1-C4	< 25 °C	Cooking gas
Petrol	C5-C10	40-180 °C	Cars
Kerosene	C10-C16	175-300 °C	Jet fuel
Diesel	C14-C20	250-340 °C	Trucks
Lubricating oil	C20-C50	350-400 °C	Machine oil
Bitumen	C70+	> 400 °C	Roads