A functional group is an atom or group of atoms that gives an organic compound its characteristic physical and chemical properties. In IB Chemistry, functional groups are used to classify compounds and predict how they will react.
How Functional Groups Work
An organic molecule contains a carbon-based structure, but a particular part of that structure usually determines its characteristic reactions. For example, ethanol contains the hydroxyl group, –OH, which makes it an alcohol and allows it to undergo reactions such as oxidation.
Functional groups also influence physical properties by changing the intermolecular forces between molecules. The hydroxyl group forms hydrogen bonds, so alcohols generally have higher boiling points than alkanes of comparable molar mass.
| Functional group | Organic class and example |
|---|---|
Carbon-carbon double bond, C=C | Alkene, such as ethene |
Hydroxyl, –OH | Alcohol, such as ethanol |
Carbonyl within a chain, –CO– | Ketone, such as propanone |
Terminal carbonyl, –CHO | Aldehyde, such as ethanal |
Carboxyl, –COOH | Carboxylic acid, such as ethanoic acid |
Halogen, –X | Halogenoalkane, such as chloroethane |
Compounds with the same functional group often show similar chemical properties and belong to the same homologous series. Successive members usually differ by CH₂ and show gradual trends in physical properties as molecular size increases.
A common misconception is that the functional group is the entire molecule. It is only the specific reactive region; a molecule may also contain a hydrocarbon chain and can contain more than one functional group.
IB Exam Technique
This content is assessed at both SL and HL in Structure 3.2. When asked to identify or classify an organic compound, locate the functional group in its full, condensed, stereochemical, or skeletal formula, then give the correct organic class. Do not classify a compound using only the length of its carbon chain.
