Organic Reaction Mechanisms and Synthesis
What this domain tests
Curly-arrow mechanisms, reagents and conditions, and planning short syntheses.
Core rules
- Free-radical substitution: initiation (UV splits the halogen), propagation, termination.
- Electrophilic addition to alkenes follows Markovnikov's rule because the more substituted carbocation is more stable; peroxides can reverse the regiochemistry by a radical pathway.
- Halogenoalkanes: tertiary and secondary favour SN1 via a carbocation; primary favour SN2 with backside attack and inversion. Hot ethanolic conditions favour elimination; aqueous conditions favour substitution.
- Carbonyls undergo nucleophilic addition; aldehydes are oxidised by Tollens' or Fehling's reagents, ketones are not.
- Benzene undergoes electrophilic substitution rather than addition because the aromatic ring is stabilised.
Worked example
Propene + HBr gives mainly 2-bromopropane. The first step is protonation of the double bond. Attack at the terminal carbon gives the secondary carbocation CH3CH+CH3; attack at the middle carbon would give a less stable primary carbocation. Bromide then attacks the secondary carbocation.
Traps
Drawing arrows from atoms rather than from bonds or lone pairs; forgetting that a carbocation can rearrange; using SN2 conditions for a tertiary halide; oxidising a primary alcohol with excess oxidant and expecting the aldehyde; writing addition to benzene where substitution occurs.