10.12 : 通过取代反应制备醇

Recall nucleophilic substitution reactions, where a functional group is substituted with another.

These reactions require an sp³-hybridized electrophile with a good leaving group and can be used to synthesize alcohols via an S_N1 or an S_N2 mechanism.

Primary alkyl halides, preferentially, undergo substitution reactions via the S_N2 mechanism with strong nucleophiles like sodium hydroxide to yield primary alcohols. The competing E2 elimination process gives an alkene as the minor product.

The synthesis of secondary alcohols from secondary alkyl halides via substitution is less favorable because competing elimination reactions lead to a mixture of alcohols and alkenes as the final products.

In the synthesis of tertiary alcohols, tertiary alkyl halides undergo substitution reactions via the S_N1 mechanism with weak nucleophiles like water.

However, if water is replaced with a strong nucleophile like sodium hydroxide, the tertiary substrate favors the E2 reaction, producing an alkene.

If the tertiary halide is chiral, then an S_N1 reaction gives a racemic mixture of tertiary alcohols.

The competing elimination reaction can be minimized by applying a relatively low temperature for the synthesis of alcohols in a weak base or neutral medium.