The organic toolkit: the reactive groups that define each family of carbon compounds, how to name them, and how isomers share a formula but differ in structure.
Theme · StructureStructure 3.2
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
—
Class · name ending
—
Example
—
Note
—
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 CnH2n+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
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.