Organic Chemistry: Reactions, Mechanisms and Synthesis
Core concepts
Build your answers around mechanism, not memorised equations. For substitution, distinguish SN1 (carbocation intermediate, racemisation, favoured by tertiary substrates and polar protic solvents) from SN2 (concerted backside attack, inversion, favoured by methyl/primary substrates and polar aprotic solvents). For elimination, E1 accompanies SN1; E2 requires an anti-periplanar arrangement and a strong base. Electrophilic aromatic substitution (nitration, halogenation, sulfonation, Friedel–Crafts) needs the stability of the arenium ion; activating groups (OH, NH₂, alkyl) direct ortho/para, while deactivating groups (NO₂, COOH, SO₃H) direct meta, and halogens are deactivating but ortho/para directing.
Revise carbonyl chemistry (nucleophilic addition, aldol, Cannizzaro, esterification), oxidation–reduction reagents (KMnO₄, LiAlH₄, NaBH₄, Tollens' and Fehling's tests), and functional-group interconversion. Know Markovnikov's rule and its peroxide exception.
Traps to watch
- Assuming SN1 and SN2 differ only in rate; stereochemistry is the discriminating feature.
- Forgetting that a halogen is deactivating yet ortho/para directing.
- Applying Markovnikov's rule to HBr in the presence of peroxides.
- Confusing Tollens' (aldehydes) with Fehling's (aliphatic aldehydes).
Worked example
Predict the major product when propene reacts with HBr in the presence of benzoyl peroxide. Peroxide generates a bromine radical, which adds to give the more stable secondary radical; hydrogen abstraction then gives 1-bromopropane, the anti-Markovnikov product. Rule of thumb: peroxide present means radical mechanism means anti-Markovnikov.