Typical reaction: the reaction of hydroxide ions with bromomethane in aqueous ethanol solvent. The mixture is heated under reflux.

The mechanism:

Points:

• The approach of the nucleophile is along the C-Br axis, which is the approach of lowest energy.
• The use of a chiral halogenoalkane gives inversion (the Walden inversion) of the molecule, so the product alcohol is optically active.
• There is no intermediate; the negative ion is a transition state, and the reaction is a one-step reaction.
• The mechanism is favoured by halogenoalkanes which would give primary carbocations if S_N1 were the pathway, and by a decreased solvent polarity.

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