Homologous series, formulae and nomenclature

A homologous series is a family of compounds with the same functional group and the same general formula, in which each member differs from the next by CH₂. Because of that, their chemical properties are similar and their physical properties change gradually.

• Alkanes: C_nH_2n+2 · Alkenes: C_nH_2n · Alcohols: C_nH_2n+1OH

Types of formula: empirical (simplest ratio, CH₂ for ethene) · molecular (actual atoms, C₂H₄) · structural (condensed, CH₃CH₂OH) · displayed (every bond drawn) · skeletal (bonds only, carbons implied).

Naming, in order:

• find the longest carbon chain containing the principal functional group → stem (meth, eth, prop, but, pent, hex);
• add the suffix for the functional group (-ane, -ene, -ol, -al, -one, -oic acid);
• number from the end that gives the lowest locant to the functional group;
• add side-chain prefixes in alphabetical order (di-, tri- do not count for alphabetising).

Curly arrows. A curly arrow always shows the movement of a pair of electrons. It starts at a lone pair or the centre of a bond (the electron source) and points to where that pair ends up. A double-headed arrow = a pair; a single-headed (fish-hook) arrow = a single electron, used only in radical mechanisms.

Your turn

Structural isomerism

Structural isomers have the same molecular formula but a different structural formula — the atoms are joined together in a different order. There are three types:

• Chain isomerism — the carbon skeleton is arranged differently: butane (straight) and 2-methylpropane (branched), both C₄H₁₀.
• Position isomerism — the same functional group in a different place: propan-1-ol and propan-2-ol, both C₃H₈O.
• Functional group isomerism — a different functional group entirely: propanal (an aldehyde) and propanone (a ketone), both C₃H₆O.

Branching changes physical properties. A branched isomer has a smaller surface area of contact between molecules, so the van der Waals forces are weaker and its boiling point is lower than the straight-chain isomer. 2-methylpropane boils at −12 °C; butane at −1 °C.

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Quick check

E/Z (cis–trans) isomerism

Stereoisomers have the same structural formula but a different arrangement in space. The first kind arises at a C=C double bond, because the π bond prevents rotation.

Two conditions are needed: a C=C double bond, and two different groups attached to each of the two carbon atoms.

Assigning E or Z (the Cahn–Ingold–Prelog rules): on each carbon, compare the two attached atoms and give priority to the higher atomic number. Then:

• the two priority groups on the same side → Z (from the German zusammen, together);
• on opposite sides → E (entgegen, opposite).

If the first atoms tie, look at the next atoms out along each branch.

Why cis/trans is not enough: the old cis/trans labels only work when each carbon carries an identical pair of groups (one of them usually H). E/Z works in every case, which is why AQA prefers it. Note also that 1,1-dichloroethene shows no E/Z isomerism: one carbon carries two identical chlorines.

Quick check

Optical isomerism and chirality

The second kind of stereoisomerism needs a chiral centre: a carbon atom attached to four different groups (an asymmetric carbon, often marked with an asterisk).

Such a molecule has two forms that are non-superimposable mirror images of one another — like your left and right hands. They are called enantiomers (optical isomers).

• Enantiomers have identical physical and chemical properties in an ordinary (achiral) environment — same melting point, same reactions.
• They differ in one physical respect: they rotate the plane of plane-polarised light by equal amounts in opposite directions.
• They also behave differently in a chiral environment — which is why one enantiomer of a drug can be therapeutic while the other is inactive or harmful.

A racemic mixture (racemate) is a 50:50 mixture of the two enantiomers. The rotations exactly cancel, so it is optically inactive.

Where racemates come from — and why it matters. The nucleophilic addition of HCN to a carbonyl forms a racemate: the planar C=O group can be attacked from either face with equal probability, so the two enantiomers are produced in equal amounts. This is a real problem in the pharmaceutical industry, because separating enantiomers is difficult and expensive.

Which kind of isomerism?

Tap the compound or pair, then tap the kind of isomerism it shows.

Quick check

Match the isomerism term to its definition

Tap a term on the left, then its definition on the right.

Your turn

Quick check

Naming, E/Z and chirality — the classic slips

• E/Z needs TWO DIFFERENT groups on EACH carbon of the C=C. But-1-ene (CH₂=) and 1,1-dichloroethene both fail that test, so neither shows E/Z isomerism.
• Priority is by ATOMIC NUMBER, not by group size. Br beats Cl; Cl beats C; C beats H. If the first atoms tie, move one atom further out.
• A chiral centre needs FOUR DIFFERENT groups. Check all four — two identical methyls kills it instantly.
• Number the chain to give the LOWEST locant to the principal functional group, and put the prefixes in alphabetical order.

Why racemates matter commercially: the two enantiomers of a drug can have completely different biological effects, because receptors are themselves chiral. Synthesising a single enantiomer (rather than separating a racemate afterwards) is difficult, but it halves the amount of drug needed and removes the risk of an unwanted isomer.

Quick check

Your turn

The big ideas to know

That is AQA 3.3.1 and 3.3.7. Press Finish to see your score.