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Preparation of Alcohols (i)
from Alkenes

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On this page, we will study how alcohols are prepared by hydration of alkenes. The term hydration means addition of a molecule of water.

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By acid catalysed hydration of alkenes

Alkenes react with water in the presence of acid as catalyst to form alcohols.

Preparation of alcohols by hydration of alkenes example: ethene to ethanol

In case of unsymmetrical alkenes, OH is added to the carbon having less number of hydrogen atoms (Markovnikov's rule).

Preparation of alcohols by hydration of alkenes example: Markovnikov's rule is applied if hydrocarbon is unsymmetrical

What should be the product when 2-methylbut-2-ene undergoes hydration?

Preparation of alcohols by hydration of alkenes example : markovnikov's rule

Mechanism of Hydration of Alkenes

The following steps are involved in the reaction:

Step 1: Alkene undergoes protonation to form carbocation by electrophilic attack of H3O+.

Hydration of alkenes mechanism: conversion of H2O to H3O+
Hydration of alkenes mechanism step 1

Step 2: Water, being a nucleophile, attacks on the carbocation.

Hydration of alkenes mechanism step 2

Step 3: Deprotonation occurs to form an alcohol.

Hydration of alkenes mechanism step 3

Oxymercuration/ Demercuration

The reaction of alkenes with mercuric acetate [(CH3COO)2Hg] followed by reduction gives alcohol according to markovnikov's rule.

Oxymercuration/Demercuration

Oxymercuration/ Demercuration of propene gives :

Preparation of alcohols by oxymercuration-demercuration

Hydroboration-oxidation

Alkenes in the presence of diborane (B2H6) and hydrogen peroxide give alcohols according to anti-Markovnikov's rule.

Hydroboration-oxidation reaction

Hydroboration-oxidation of 2-methylpropene gives :

Preparation of alcohols by hydroboration-oxidation

Question : How will you synthesise 1-phenylethanol from a suitable alkene?

Answer : Hydration of ethenylbenzene gives 1-phenylethanol.

hydration of ethenylbenzene

Question : How will you synthesise pentan-2-ol from an appropriate alkene?

Answer : pentan-2-ol (pentan-2-ol) can be synthesised from pent-1-ene.

hydrogen bond in propanol

It should be noted that hydration of pent-2-ene also gives pentan-2-ol; however, in that case we get a mixture of two alcohols — pentan-2-ol and pentan-3-ol.

hydrogen bond in propanol

Hence, pent-1-ene should be used and not pent-2-ene.

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