Organic chemistry rewards recognising which of a small number of reaction types is happening. If you can name the reaction, the mechanism follows.
Substitution: one atom or group replaces another (e.g., halogenation of alkanes with light: CH₄ + Cl₂ → CH₃Cl + HCl). Free-radical mechanism.
Addition: unsaturated bond opens up and two groups attach (e.g., alkene + H₂ → alkane; alkene + HBr → haloalkane). Markovnikov's rule applies to asymmetric addition.
Elimination: opposite of addition — two groups leave, forming an unsaturated bond (e.g., alcohol + concentrated H₂SO₄ heat → alkene + water).
Esterification: carboxylic acid + alcohol ⇌ ester + water, catalysed by concentrated H₂SO₄, reversible. The classic exam favourite.
Oxidation: alcohol → aldehyde → carboxylic acid, with CuO or acidified KMnO₄ as oxidant.
For each type, memorise one canonical example. When a question appears, ask: which functional group is present, and which of these five reactions can it undergo?