SolutionsH₂O Addition

1. Dilute acid or oxymercuration

Predict the result of the following reactions:

1-Methylcyclobutene, 1-methylcyclopentene and 1-methylcyclohexene with dilute H₂SO₄ and with the mercury procedure: 1) (AcO)₂Hg; 2) NaBH₄; 3) H₂O, HO⁻

Both procedures give the same tertiary alcohol: 1-methylcyclobutanol, 1-methylcyclopentanol and 1-methylcyclohexanol

Both methods follow the Markovnikov rule and here they give the same product, because the tertiary carbocation formed in acid has no rearrangement that would make it more stable. Oxymercuration is preferable when the carbocation can rearrange.

2. Which alkene?

Which alkenes would give 2-methylbutan-2-ol by acid-catalysed hydration?

2-Methylbut-1-ene and 2-methylbut-2-ene. In both cases protonation gives the same tertiary carbocation, (CH₃)₂C⁺–CH₂CH₃, which takes up water. Also 3-methylbut-1-ene, through hydride shift of the secondary carbocation.

3. Equilibrium

Hydration of an alkene and dehydration of an alcohol are the same reaction in opposite directions. How is the equilibrium shifted one way or the other?

Dilute acid (plenty of water) and moderate temperature favour hydration. Concentrated acid and heat, removing the alkene by distillation as it forms, favour dehydration (Le Châtelier's principle).

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