Functional Group – IGCSE Chemistry Definition
IGCSE Chemistry definition of functional group: the atom or group of atoms responsible for a compound's characteristic reactions. Covers key groups for 0620.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
Functional groups are the organising concept behind organic chemistry at IGCSE. Each homologous series is defined by its functional group, and the functional group determines the compound’s reactions. Knowing which group does what is the key to answering organic chemistry questions efficiently.
The 0620 definition
A functional group is an atom or group of atoms in an organic molecule that is responsible for the characteristic chemical reactions of that compound.
The functional groups you must know
| Homologous series | Functional group | Structure | Example |
|---|---|---|---|
| Alkenes | C=C double bond | C=C | Ethene, CH₂=CH₂ |
| Alcohols | Hydroxyl group | −OH | Ethanol, CH₃CH₂OH |
| Carboxylic acids | Carboxyl group | −COOH | Ethanoic acid, CH₃COOH |
| Esters | Ester link | −COO− | Ethyl ethanoate, CH₃COOC₂H₅ |
Note: alkanes do not have a functional group — they contain only C−C and C−H single bonds, which is why they are relatively unreactive.
How functional groups determine reactions
| Functional group | Characteristic reactions |
|---|---|
| C=C | Addition reactions (with Br₂, H₂, H₂O, HBr); addition polymerisation |
| −OH | Combustion; oxidation to carboxylic acid; ester formation with carboxylic acids |
| −COOH | Reactions with metals, bases, carbonates (like an acid); ester formation with alcohols |
| −COO− | Hydrolysis (breaking down with water/acid/alkali) |
Identifying functional groups
When given a structural formula, look for the characteristic group:
- A C=C anywhere in the chain → alkene
- An −OH at the end of a carbon chain → alcohol (not water, not a carboxylic acid)
- A −COOH at the end → carboxylic acid
- A −COO− linking two carbon chains → ester
Worked exam question
Compound A has the formula CH₃CH₂OH. Compound B has the formula CH₃COOH. (a) Identify the functional group in each compound. [2] (b) State the homologous series each compound belongs to. [2]
Mark scheme
(a) A: −OH / hydroxyl group [1]; B: −COOH / carboxyl group [1]
(b) A: alcohols [1]; B: carboxylic acids [1]
Common exam mistakes
- Confusing the −OH in alcohols with the −OH in carboxylic acids. In a carboxylic acid, the −OH is part of the −COOH group, which behaves completely differently from an alcohol’s −OH.
- Saying alkanes have a “C−H functional group” — alkanes have no functional group, which is why they are the least reactive organic series.
- Writing “OH” for the alcohol group instead of “−OH” — include the bond dash to show it is attached to a carbon chain.
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