Preparation of Alcohols (i)
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.
In case of unsymmetrical alkenes, OH is added to the carbon having less number of hydrogen atoms (Markovnikov's rule).
What should be the product when 2-methylbut-2-ene undergoes hydration?
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+.
Step 2: Water, being a nucleophile, attacks on the carbocation.
Step 3: Deprotonation occurs to form an alcohol.
The reaction of alkenes with mercuric acetate [(CH3COO)2Hg] followed by reduction gives alcohol according to markovnikov's rule.
Oxymercuration/ Demercuration of propene gives :
Alkenes in the presence of diborane (B2H6) and hydrogen peroxide give alcohols according to anti-Markovnikov's rule.
Hydroboration-oxidation of 2-methylpropene gives :
Question : How will you synthesise 1-phenylethanol from a suitable alkene?
Answer : Hydration of ethenylbenzene gives 1-phenylethanol.
Question : How will you synthesise from an appropriate alkene?
Answer : (pentan-2-ol) can be synthesised from pent-1-ene.
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.
Hence, pent-1-ene should be used and not pent-2-ene.