Nomenclature, functional groups and homologous series. Structural, displayed, skeletal and molecular formulae. Structural isomers, E-Z stereoisomers and the use of CIP priority. Reaction mechanisms, free radicals, nucleophiles and electrophiles, and curly arrows.

What this dot point is asking

AQA wants you to name organic compounds using IUPAC rules, classify them by functional group and homologous series, draw and interpret the four types of formula, identify structural and E-Z stereoisomers, and use the language of mechanisms (free radicals, nucleophiles, electrophiles, curly arrows).

Nomenclature and homologous series

IUPAC names are built from a stem (longest carbon chain: meth, eth, prop, but, pent), a suffix for the principal functional group (-ane, -ene, -ol, -al, -one, -oic acid) and prefixes for substituents, numbered to give the lowest locants.

The four formulae

• Molecular formula: actual number of each atom, e.g. $C_4H_{10}$.
• Empirical formula: simplest whole-number ratio, e.g. $C_2H_5$ for $C_4H_{10}$.
• Structural formula: shows the arrangement of atoms without drawing every bond, e.g. $CH_3CH_2CH_2CH_3$.
• Displayed formula: every atom and every bond drawn out in full.
• Skeletal formula: carbon chain as a zig-zag, with carbon atoms at each vertex and end, hydrogens on carbon omitted, and functional groups shown explicitly.

Skeletal formulae are the working language of organic chemistry beyond the simplest molecules because they are quick to draw and make the functional groups and ring systems obvious. Knowing how to convert between the formula types, and how to read a skeletal structure to count carbons and hydrogens, is assumed in almost every organic question.

Isomerism

Structural isomers have the same molecular formula but a different structural arrangement (chain, position or functional-group isomerism).

E-Z (cis-trans) stereoisomerism occurs when there is restricted rotation about a $C=C$ double bond and each carbon of the double bond carries two different groups. CIP priority rules assign higher priority to the atom of higher atomic number; if the two higher-priority groups are on the same side it is Z (from the German zusammen, together), on opposite sides it is E (entgegen, opposite). The older cis (same side) and trans (opposite side) labels are used when the two carbons each carry one H and one identical group, but CIP is the general system AQA expects.

The language of mechanisms

Try this

Q1. Give the empirical formula of $C_6H_{12}$. [1 mark]

• Cue. $CH_2$.

Q2. Explain why but-2-ene shows E-Z isomerism but but-1-ene does not. [2 marks]

• Cue. But-2-ene has two different groups on each double-bond carbon and restricted rotation; but-1-ene has two H atoms on one carbon.