10.5: Preparation of Alcohols via Substitution Reactions
Alcohols can be synthesized from alkyl halides via nucleophilic substitution reactions. The highly polar carbon-halogen bond in the substrate makes halide a good leaving group. The hydroxide ion or water can act as a nucleophile to take the place of halide and form an alcohol. The substitution reactions occur via two different reaction pathways, SN1 or SN2, depending on the nature of carbon attached to the halide.
Primary alcohols are synthesized from primary alkyl halides, and the reaction proceeds via the SN2 mechanism. The nucleophile attacks the halogen-bearing carbon from the side opposite to the carbon-halogen bond. However, in the presence of a strong nucleophile, a competing elimination reaction occurs as well.
Figure_1: Parallel reactions of 1-bromobutane into substitution products and elimination products (proton abstraction).
The synthesis of secondary alcohols from secondary alkyl halides via substitution reaction is not favored since a mixture of products is formed from the competing SN2 and E2 reaction routes.
Figure_2: Parallel reactions of 2-bromo-3-methylbutane into substitution products and elimination products (proton abstraction).
Tertiary alkyl halides undergo SN1 reaction with a weak base such as water to produce tertiary alcohols along with alkene as a minor product due to a competing E2 elimination reaction.
Figure_3: Parallel reactions of tertiary alkyl halides to elimination and substitution products.
If a strong nucleophile like sodium hydroxide is used, the E1 reaction dominates over SN1.
The nature of the reactant determines the stereochemistry of the product formed. If the halogen in the alkyl halide is connected to a chiral carbon, the resulting alcohol is a mixture of two enantiomers.
Figure_4: Substitution reaction over an asymmetric carbon to yield a racemic mixture of optically active alcohols as the product