Preparation of alcohols and phenols

The essentials

Alcohols are prepared mainly from alkenes, from carbonyl compounds by reduction, and from carbonyl compounds by reaction with Grignard reagents. Phenol is prepared from haloarenes, benzenesulphonic acid, diazonium salts or cumene.

Key points

• Acid-catalysed hydration of alkenes: follows Markovnikov's rule; propene gives propan-2-ol.
• Hydroboration–oxidation: B₂H₆ then H₂O₂/OH⁻; gives the anti-Markovnikov product; propene gives propan-1-ol.
• Reduction: aldehydes give primary alcohols and ketones give secondary alcohols with H₂/Pd, NaBH₄ or LiAlH₄. Carboxylic acids are reduced to primary alcohols by LiAlH₄.
• Grignard reagents: with methanal (HCHO) they give primary alcohols, with other aldehydes secondary alcohols, and with ketones tertiary alcohols, after hydrolysis.
• Phenol from chlorobenzene: fused with NaOH at 623 K and 300 atm, then acidified.
• From benzenesulphonic acid: fused with NaOH, then acidified.
• From diazonium salts: warming the aqueous diazonium salt solution gives phenol.
• Cumene process: cumene (isopropylbenzene) is oxidised in air to cumene hydroperoxide, which with dilute acid gives phenol and acetone. Most phenol is made this way, and acetone is a useful by-product.

Exam tips

• Ask: how many R groups end up on the carbinol carbon? This tells you which carbonyl compound to choose with a Grignard reagent.

Common mistakes

• Using a ketone with a Grignard reagent to make a secondary alcohol; ketones give tertiary alcohols.