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Also known as: Unimolecular Nucleophilic Substitution
A two-step mechanism: the leaving group departs first to form a planar carbocation intermediate, then the nucleophile attacks. Since the carbocation is sp²-hybridized (planar), the nucleophile can attack from either face, leading to racemization.
Ionization: The C-LG bond breaks heterolytically. The leaving group departs with the bonding electrons, forming a carbocation. This is the rate-determining step.
Slow step — determines rate
Nucleophilic capture: The nucleophile (or solvent) attacks the planar carbocation from either face, giving a mixture of retention and inversion products.
Fast step — leads to racemization
(CH₃)₃CBr
H₂O
(CH₃)₃COH
(R)-3-Bromo-3-methylhexane
MeOH
(R/S)-3-Methoxy-3-methylhexane
Useful for solvolysis reactions where the solvent is the nucleophile. Common in biological systems (glycosylation).