6.2 Dehydration

  Alcohols, Phenols and Ethers

a). Dehydration to alkene: Heating of most of the alcohols with a strong acid causes a loss of water molecule to form alkenes

(i). The reaction is an elimination and is favored at a high temperature most commonly used acids in the laboratory are ${H_2}S{O_4}$ and ${H_3}P{O_4}$ while alumina is often used in industries.

(ii). The order of dehydration among three types of alcohol is $3^\circ > 2^\circ > 1^\circ .$Thus dehydrating conditions becomes milder as we proceed from $1^\circ $to $3^\circ $alcohols.

This behavior is related to relative stabilities of carbocations $\left( {3^\circ > 2^\circ > 1^\circ } \right)$.

(iii). Since carbocations are formed as intermediate rearranged olefins, where possible are formed.

(iv). Whenever dehydrations produces two different alkenes major product is formed according to Staff's rule.

Such reaction which can produce two or more structural isomers but one of them is in a greater amount than the other are called regioselective. In case if only one product is formed, a reaction is termed as regiospecific.

(b). Intra-molecular Dehydration to ether: Primary alcohols when heated in presence of an acid catalyst (usually H2SO4) undergo intermolecular dehydration to form ether.

Dehydration to ether takes place at a lower temperature than dehydration to an alkene.

This method is not used for the preparation of unsymmetrical ethers because reaction leads to mix. of products

\[R - O - H{\text{ }} + {\text{ }}HO - R'\xrightarrow{{{H^ + }}}R - O - R + {\text{ }}R' - O - R{\text{ }} + R' - O - R'\]