Where do H and OH land?

Propene or 2-methylpropene hydrated two ways: acid hydration via the more stable carbocation (Markovnikov) vs hydroboration–oxidation with boron going to the CH₂ end.

Make propan-1-ol from propene.

Hydroboration–oxidation gives propan-2-ol.

• 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.

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