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Question

(i) Name the starting material used in the industrial preparation of phenol.

(ii) Write complete reaction for the bromination of phenol in aqueous and non-aqueous medium.

(iii) Explain why Lewis acid is not required in bromination of phenol?


Solution

(i) The starting material used in the industrial preparation of phenol is cumene.


(ii) Phenols, when treated with bromine water, gives polyhalogen derivatives in which all the hydrogen atoms present at ortho and para positions with respect to — OH group are replaced by bromine atoms.

However, in a non-aqueous medium such as CS2, CCI4, CHCI3, monobromophenols are obtained.

In aqueous solution, phenol ionizes to form phenoxide ion. This ion activates the benzene ring to a very large extent and hence the substitution of halogen takes place at all three positions.
On the other hand, in non-aqueous solution ionization of phenol is greatly suppressed. Therefore, the ring is activated slightly and hence monosubstitution occur.

(iii) Lewis acid is an electron deficient molecule. In bromination of benzene, Lewis acid is used to polarise Br2 to form Br+ electrophile.
In the case of phenol, an oxygen atom of phenol itself polarises the bromine molecule to form Br+ ion (electrophile).  It means that the polarisation of bromine occurs even in the absence of Lewis acid. This is because of the highly activating effect of -OH group on the benzene ring.

Mechanism of bromination of phenol.

Mechanism of bromination of benzene
AlBr3+BrBr[AlBr4]+Br+

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