IB Chemistry SL — Structure 3.2: Functional Groups

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What defines a homologous series?

This is the Standard Level lesson, covering S3.2.1–S3.2.6. Taking Chemistry at Higher Level? The HL version adds stereoisomers — cis–trans (E/Z) and optical isomers (enantiomers).

👆 Explore each functional group · flip through the isomers

1. Functional groups & classes S3.2.2

A functional group is the reactive part of a molecule that gives a whole family its characteristic properties. Organic compounds are classified by the functional group they contain. Pick one:

Functional group explorer

Functional group

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Class · name ending

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Example

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Note

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2. Homologous series S3.2.3–4

A homologous series is a family of compounds with the same functional group and general formula, where each member differs from the next by CH₂. The alkanes are the simplest — general formula C_nH_2n+2:

The first alkanes

Name	Formula	Boiling pt / °C
Methane	CH₄	−162
Ethane	C₂H₆	−89
Propane	C₃H₈	−42
Butane	C₄H₁₀	−1
Pentane	C₅H₁₂	36

Boiling point rises down the series: longer chains have more electrons and a bigger surface, so stronger London (dispersion) forces between molecules.

3. Structural isomers S3.2.6

Structural isomers have the same molecular formula but different connectivities (the atoms are joined up differently). Flip through the C₄H₁₀ and C₃H₈O examples:

Alcohols, halogenoalkanes and amines are also classed as primary (1°), secondary (2°) or tertiary (3°) by how many carbon atoms are attached to the carbon carrying the functional group — 1, 2 or 3 respectively.

Going to HL? The HL lesson adds stereoisomers: cis–trans (E/Z) isomers around a C=C double bond or ring, and optical isomers (enantiomers) around a chiral carbon — non-superimposable mirror images. → Open the HL version

Learn Functional Groups step by step

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What defines a homologous series?✗ Compounds with the same number of carbons. ✓ Same functional group and general formula, with each member differing by CH₂ — and showing a gradual trend in properties.

Are butane and 2-methylpropane the same compound?✗ Yes, both are C₄H₁₀. ✓ No — they are structural (chain) isomers: same molecular formula, different connectivity, so different properties (e.g. boiling point).

What is the name ending for an aldehyde vs a ketone?✗ Both end in -one. ✓ Aldehydes end in -al (C=O at the end of the chain); ketones end in -one (C=O in the middle).

Why do boiling points rise down the alkane series?✗ The covalent bonds get stronger. ✓ Larger molecules have stronger London (dispersion) forces between them; the covalent bonds within molecules are unchanged.

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Functional Groups