Isomerism: structural and stereoisomerism

Isomers have the same molecular formula but a different arrangement of atoms. Structural isomers differ in how the atoms are bonded together. Three types:

• Chain isomerism — different carbon-skeleton arrangements. e.g. butane vs 2-methylpropane ($\text{C}_4\text{H}_{10}$).
• Positional isomerism — the functional group is in a different position on the same chain. e.g. butan-1-ol vs butan-2-ol ($\text{C}_4\text{H}_{10}\text{O}$).
• Functional group isomerism — the same molecular formula but a different functional group. e.g. ethanol ($\text{CH}_3\text{CH}_2\text{OH}$) vs methoxymethane ($\text{CH}_3\text{OCH}_3$), both $\text{C}_2\text{H}_6\text{O}$; or an aldehyde vs a ketone ($\text{C}_3\text{H}_6\text{O}$: propanal vs propanone).

Geometrical (cis–trans) isomerism is a type of stereoisomerism that arises because rotation about a C=C double bond is restricted (the π bond would have to break).

Two conditions are needed:

1. a C=C double bond (restricted rotation), and
2. two different groups on each of the double-bond carbons.

• cis isomer — the two identical/priority groups are on the same side.
• trans isomer — they are on opposite sides.

(Use of E/Z nomenclature is acceptable but not required at 9701.)

Optical isomerism arises from a chiral centre: a carbon atom bonded to four different atoms or groups.

• Such a molecule exists as two optical isomers (enantiomers) — non-superimposable mirror images of each other (like left and right hands).
• Enantiomers have identical physical and chemical properties, except they rotate the plane of plane-polarised light in opposite directions (one clockwise, one anticlockwise).

Worked. Butan-2-ol, $\text{CH}_3\text{CH(OH)CH}_2\text{CH}_3$: carbon 2 is bonded to –H, –OH, –CH₃ and –CH₂CH₃ (four different groups) → it is a chiral centre → two enantiomers exist.

(A molecule can have more than one chiral centre, but knowledge of meso compounds and diastereoisomers is not required.)

To identify a chiral centre: look for any carbon bonded to four different groups (mark it with an asterisk). In cyclic compounds, the two 'directions' around the ring count as the differing groups.

To identify cis–trans: check the molecule has a C=C with two different groups on each double-bond carbon.

To deduce all isomers of a molecular formula: systematically vary

1. the carbon skeleton (chain isomers),
2. the position of the functional group (positional isomers),
3. the functional group itself (functional group isomers), then check each for stereoisomerism.

Worked. $\text{C}_4\text{H}_{10}\text{O}$ alcohols: butan-1-ol, butan-2-ol, 2-methylpropan-1-ol, 2-methylpropan-2-ol — and butan-2-ol additionally shows optical isomerism.