I85PREP
⌘ K
Cambridge IAL Chemistry study materials
Cambridge IAL Chemistry study resources

Cambridge IAL Chemistry: Organic Chemistry

Cambridge IAL Chemistry · Work through your exam, one topic at a time.

All guide sections

Organic Chemistry

Organic chemistry in Cambridge IAL Chemistry tests structure, naming, isomerism, reaction mechanisms and functional-group interconversions. Marks are lost less often from not knowing a reaction than from an incomplete mechanism or an imprecise condition.

Core rules

  • Name by the longest chain containing the principal functional group; number to give the lowest locants; cite substituents alphabetically. Structural isomers differ in connectivity; stereoisomers differ only in spatial arrangement. A chiral centre has four different groups and gives a pair of enantiomers.
  • E/Z isomerism requires restricted rotation and two different groups on each C=C carbon; assign priorities by atomic number (CIP).
  • Electrophilic addition to alkenes proceeds via a carbocation; unsymmetrical alkenes give the more stable carbocation. Free-radical substitution has initiation, propagation and termination steps.
  • Nucleophilic substitution: tertiary halogenoalkanes favour SN1 via a carbocation; primary favour SN2 with backside attack. Aqueous conditions give substitution; alcoholic conditions with heat favour elimination.
  • Carbonyls undergo nucleophilic addition; aldehydes are oxidised by Tollens’ or Fehling’s, ketones are not. Alcohols are oxidised by acidified dichromate(VI); primary alcohols give aldehydes on distillation or carboxylic acids under reflux.

Worked example

Hydrolysis of 2-bromo-2-methylpropane with aqueous sodium hydroxide. The carbon bearing Br is tertiary, so the C–Br bond breaks first to form a tertiary carbocation and Br−. A lone pair on water or OH− attacks the carbocation, and deprotonation gives 2-methylpropan-2-ol. The mechanism is SN1; the rate depends only on the halogenoalkane concentration. Curly arrows must start at the C–Br bond and at the oxygen lone pair, with the correct dipoles and charges shown.

Traps

Drawing curly arrows from atoms rather than bonds or lone pairs; omitting the positive charge on a carbocation; using Tollens’ for a ketone; forgetting that reflux with acidified dichromate gives a carboxylic acid from a primary alcohol; assigning E/Z without CIP priorities; and giving SN2 for a tertiary halogenoalkane. Always match reagent and condition to product.