IB Chemistry HL — Structure 3.2: Functional Groups

This is the Higher Level lesson. The core S3.2.1–S3.2.6 (functional groups, naming, structural isomers) is shared with SL; the HL-only content — stereoisomers (S3.2.7) — carries a purple HL badge.

Core — SL & HL

HL AHL only

👆 functional groups · isomers · stereoisomers

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.

4. Stereoisomers HL

Stereoisomers have the same structural formula (atoms joined in the same order) but a different arrangement in space. There are two kinds.

Cis–trans (E/Z) isomers S3.2.7

Around a C=C double bond there is no rotation, so groups are fixed on one side or the other. In cis (Z) the two like groups are on the same side; in trans (E) they are on opposite sides. Toggle but-2-ene:

Optical isomers (enantiomers) S3.2.7

A chiral carbon has four different groups attached. Its molecule and its mirror image cannot be superimposed — like your left and right hands. These non-superimposable mirror images are enantiomers. They are identical in most properties, but they rotate plane-polarised light in opposite directions; a 50:50 mixture (a racemic mixture) has no net rotation.

Four different groups (W, X, Y, Z) on the central carbon → two mirror-image forms that can't be superimposed. Real examples: lactic acid and most amino acids.

Common mistakes examiners see

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.

What makes a carbon atom chiral?✗ It has a double bond. ✓ It has four different groups attached, giving non-superimposable mirror images (enantiomers). HL

How do two enantiomers differ physically?✗ Different melting and boiling points. ✓ They are identical in most properties but rotate plane-polarised light in opposite directions. HL

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Functional Groups