The classification of organic compounds by functional group, homologous series and general formulae, IUPAC nomenclature, the different ways of representing organic molecules, and the meaning of structural isomerism.

What this dot point is asking

CCEA wants you to classify organic compounds by their functional group, understand homologous series and general formulae, name organic compounds using IUPAC rules, represent organic molecules in different ways (molecular, structural, displayed and skeletal), and understand what structural isomerism means. This is the foundation of the whole organic chemistry unit: every reaction and analysis builds on naming and recognising functional groups.

Functional groups and homologous series

Key homologous series include alkanes (general formula CnH2n+2, only single C to C bonds, fairly unreactive), alkenes (CnH2n, containing a C to C double bond, more reactive), alcohols (CnH2n+1OH, containing the hydroxyl group), and carboxylic acids (containing the carboxyl group). Members of a series share similar chemical properties because they have the same functional group, but show a gradual trend in physical properties such as boiling point, which rises as the chain lengthens and intermolecular forces increase. This is why you can learn the chemistry of a whole series from one or two examples.

IUPAC nomenclature

To name a compound: find the longest carbon chain (the stem), identify the functional group (the suffix), then number the chain from the end that gives the functional group or substituents the lowest numbers. So a three-carbon chain with a double bond starting at carbon 1 is prop-1-ene, and a four-carbon alcohol with the hydroxyl on carbon 2 is butan-2-ol. Correct naming is essential because it specifies the exact structure unambiguously, which matters for predicting reactions.

Representing organic molecules

The same molecule can be shown in several ways, each useful for a different purpose. The molecular formula gives the actual numbers of each atom (for example C3H8O for propanol). The empirical formula gives the simplest whole-number ratio of atoms. The structural formula shows how the atoms are arranged in a condensed way (CH3CH2CH2OH). The displayed (full structural) formula shows every atom and every bond as lines, useful for seeing the functional group. The skeletal formula shows the carbon chain as a zig-zag of lines with hydrogens on carbon omitted, useful for larger molecules. Being able to convert between these representations is a routine exam skill. Where two compounds share a molecular formula but differ in structure, they are structural isomers, which is explored in its own dot point.

Examples in context

Example 1. Ethanol in everyday life. Ethanol (CH3CH2OH) is the alcohol in alcoholic drinks, a fuel and a solvent. Its name shows a two-carbon chain (eth-) with single bonds (-an-) and a hydroxyl group (-ol). As a member of the alcohol homologous series it shares the hydroxyl chemistry of methanol and propanol, illustrating how one functional group defines a family.

Example 2. Skeletal formulae for complex molecules. Drug and biological molecules are large, so chemists draw them as skeletal formulae, showing only the carbon framework and functional groups. This makes the reactive parts easy to see without drawing every hydrogen, which is why pharmacology and biochemistry rely on this representation. It connects organic naming to the health-science context of the qualification.

Try this

Q1. Define a functional group. [1 mark]

• Cue. The atom or group of atoms that gives a molecule its characteristic chemical reactions.

Q2. State the general formula of the alkenes and explain why they are more reactive than the alkanes. [2 marks]

• Cue. CnH2n; the carbon-to-carbon double bond is a reactive site, unlike the unreactive single bonds in alkanes.

Q3. Give the IUPAC name of CH3CH2OH. [1 mark]

• Cue. Ethanol.