Organic compounds and homologous series (Chemistry HL)

The big idea: Organic chemistry is the chemistry of carbon compounds. There are millions of them, so chemists organise them into families called homologous series.

A homologous series is a family of organic compounds that share the same general formula and the same functional group, where each member differs from the next by a CH₂ unit.

- Functional group — the reactive atom or group of atoms that gives a series its characteristic chemistry (e.g. the C=C of alkenes, the –OH of alcohols). - General formula — a single formula, in terms of the number of carbons n, that fits every member (e.g. alkanes are C_nH_2n+2).

The four features of a series: Every homologous series has these four features:

- Members fit one general formula. - Successive members differ by CH₂. - They show a gradual change in physical properties (e.g. boiling point rises down the series). - They have similar chemical properties (because they share the same functional group).

Because every member differs by CH₂, each series can be summed up by a single general formula written in terms of the number of carbon atoms, n. Substitute n = 1, 2, 3, … to generate the members.

Building the alkanes: The alkane general formula is C_nH_2n+2:

- n = 1 → CH₄ (methane) - n = 2 → C₂H₆ (ethane) - n = 3 → C₃H₈ (propane) - n = 4 → C₄H₁₀ (butane)

Notice each step adds one C and two H — exactly one CH₂ unit.

Why properties change gradually: As the chain gets longer, the molecules get bigger and heavier, so the intermolecular forces between them get stronger. That is why boiling point rises steadily down a homologous series — a smooth, predictable trend, not random jumps.

A key way to classify organic molecules is by whether they contain a carbon–carbon double bond. This is the difference between saturated and unsaturated compounds.

Same carbons, fewer hydrogens: Compare butane (C₄H₁₀, saturated) with but-1-ene (C₄H₈, unsaturated): both have 4 carbons, but the double bond means the alkene holds 2 fewer hydrogens. The C=C is the functional group that makes alkenes reactive (e.g. they decolourise bromine water).

How this is tested: S3.2 reliably opens with a quick recall mark. The most common ask is to state the general formula of a named homologous series (alkanes, alkenes or alcohols) on Paper 2, or a Paper 1A MCQ that tests whether you can spot the CH₂ difference or classify a molecule as saturated/unsaturated.

The general formula must be written in terms of n — a specific formula like C₂H₄ will not earn the mark.

Memorise the three formulas: Learn these cold: alkanes C_nH_2n+2, alkenes C_nH_2n, alcohols C_nH_2n+1OH. The alkene is the alkane minus 2 H (the double bond), which is the detail students forget.