Typical reaction: the reaction of 2-bromo-2-methylpropane with hydroxide ions, using a solvent of aqueous ethanol. The mixture is heated under reflux.

The mechanism:

Points:

• This mechanism has a true intermediate, the carbocation; the formation of this is the rate limiting step.
• The carbocation is planar. In principle this suggests that attack from the nucleophile is equally likely from either side, so that if the starting halogenoalkane was chiral, the product alcohol would be a racemic mixture of the two mirror images and would not show optical activity.
• In practice there is some steric hindrance from the departing bromide ion, and the product mixture is roughly 60% inverted configuration, 40% retained.
• This mechanism is favoured by a polar solvent, and by branching of the carbon chain so that the carbocation formed is a tertiary carbocation.

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