Functional Groups
Applied Science: BTEC Level 3 National Extended Diploma, topic 4.3.2
Homologous Series
A homologous series is a set of compounds with the same functional group. Members of a homologous series have very similar properties because they have the same functional group.
General formulae
- Every homologous series has its own general formula:
- E.g. Alkanes: CnH2n+2
- E.g. Alkenes: CnH2n
- E.g. Alcohols: CnH2n+2O
- The general formula gives the molecular formula for each compound in the homologous series.
- Different molecules in the series differ mainly in the length of their aliphatic chains.
- Aliphatic chains are carbon chains that do not have a benzene ring.
Functional groups
- A functional group is a group of atoms that characterise the chemistry of a molecule. Functional groups cause specific reactions to happen with certain reagents.
- Examples of functional groups are:
- Hydroxyl group (-OH).
- Carboxylic acid group (-COOH) (pictured).
- Carbon-carbon double bond (C=C).
Alkyl group
- An alkyl group is a saturated hydrocarbon chain attached to a molecule.
- The letter R can be used to represent alkyl groups.
Aliphatic
- Aliphatic compounds contain carbon chains that do not involve a benzene ring.
- Aliphatic chains can be saturated, unsaturated, straight, branched or circular.
Alicyclic
- Alicyclic compounds are aliphatic compounds that have non-aromatic rings but no side chains.
- E.g. Cyclohexane.
Aromatic
- Aromatic compounds contain at least one benzene ring.
- Benzene rings tend to make these compounds smell.
- This is why these compounds are described as aromatic.
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Saturated/unsaturated
- Saturated compounds contain single bonds only between carbon atoms (C-C).
- Unsaturated compounds contain at least one different carbon-carbon bond such as C=C or an aromatic ring.
Test yourself on Functional Groups
Which of these general formulae are correct?
Why do members of a homologous series have very similar properties?