Isomerism: structural and E/ZEdexcel A-Level Chemistry: Revision notes

Structural isomers have the same molecular formula but different structural formulae, because their atoms are bonded together in a different order. They often differ in physical properties and sometimes in chemical properties.

There are three types you must know: chain, position and functional group isomerism. To find all the isomers of a formula, work systematically: first the longest chain, then shorter main chains with one branch placed in every different position, then two branches.

• Chain isomers have different carbon skeletons (straight or branched), e.g. butane and 2-methylpropane; but-1-ene and 2-methylpropene.
• Position isomers have the same carbon skeleton and the same functional group, but the group is in a different position, e.g. propan-1-ol and propan-2-ol; 1-bromobutane and 2-bromobutane.
• Functional group isomers have different functional groups, e.g. but-1-ene (alkene) and cyclobutane (cycloalkane); propan-1-ol (alcohol) and methoxyethane (ether).

Two molecules that look different when drawn may be the same compound: rotating about single bonds changes nothing, so check by naming each.

C₅H₁₂ (3 isomers): pentane; 2-methylbutane; 2,2-dimethylpropane. (3-methylbutane is 2-methylbutane numbered the wrong way.)

C₄H₉Br (4 isomers): 1-bromobutane; 2-bromobutane; 1-bromo-2-methylpropane; 2-bromo-2-methylpropane.

C₅H₁₀ alkenes (6 including E/Z): pent-1-ene; (E)-pent-2-ene; (Z)-pent-2-ene; 2-methylbut-1-ene; 3-methylbut-1-ene; 2-methylbut-2-ene.

Stereoisomers have the same structural formula but a different arrangement of atoms in space. E/Z isomerism is a type of stereoisomerism that occurs in alkenes.

The C=C double bond contains a π bond, which prevents rotation, so groups attached to the carbon atoms are fixed on one side or the other of the bond.

A molecule shows E/Z isomerism only if each carbon atom of the C=C is bonded to two different atoms or groups.

• But-2-ene, CH₃CH=CHCH₃: yes (H and CH₃ on each carbon).
• But-1-ene, CH₂=CHCH₂CH₃: no (two H on one carbon).
• 2-methylpropene, (CH₃)₂C=CH₂: no.

On each carbon of the C=C, decide which attached group has the higher priority: the group whose first atom has the higher atomic number (Cahn–Ingold–Prelog rules). If the first atoms are the same, compare the next atoms along.

• Z (from German zusammen, together): the two higher-priority groups are on the same side.
• E (from entgegen, opposite): the two higher-priority groups are on opposite sides.

For example, in CH₃CH=CHCH₂CH₃ the methyl group and the ethyl group are the higher-priority groups (CH₃ beats H; CH₂CH₃ beats H); when they are on the same side the isomer is (Z)-pent-2-ene.

Where two substituents on the double bond are the same, as in but-2-ene (a CH₃ and H on each carbon), the older terms cis (same groups on the same side) and trans (opposite sides) are also used: cis is Z and trans is E.