Preparing alcohols

Acid-catalysed hydration: an alkene reacts with water in the presence of acid to give an alcohol. With an unsymmetrical alkene the addition follows Markovnikov's rule.

Hydration runs in three steps: the alkene takes a proton from H₃O⁺ to form a carbocation; water attacks the carbocation; the protonated alcohol then loses H⁺ to give the alcohol.

Hydroboration–oxidation: diborane, (BH₃)₂, adds across the C=C to give a trialkylborane, which aqueous sodium hydroxide and hydrogen peroxide oxidise to the alcohol. The yield is excellent.

In hydroboration, boron bonds to the double-bond carbon carrying more hydrogens, so the net result resembles anti-Markovnikov addition of water: propene gives propan-1-ol. H.C. Brown reported the method in 1959; he and G. Wittig were jointly awarded the 1979 Nobel Prize in Chemistry.

Aldehydes and ketones are reduced to alcohols by hydrogen over finely divided Pt, Pd or Ni, or by sodium borohydride (NaBH₄) or lithium aluminium hydride (LiAlH₄). Aldehydes give primary alcohols; ketones give secondary alcohols.

Carboxylic acids are reduced to primary alcohols by LiAlH₄, a strong reducing agent. LiAlH₄ is costly, so industry first turns the acid into an ester and then reduces the ester with hydrogen over a catalyst.

Grignard reagents add to the C=O of aldehydes and ketones to give an adduct, which is hydrolysed to the alcohol.

With a Grignard reagent, methanal gives a primary alcohol, any other aldehyde gives a secondary alcohol and a ketone gives a tertiary alcohol.