Q. 725.0( 1 Vote )

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

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

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

Answer :

(a) The starting material is Cumene (isopropylbenzene) for the industrial preparation of phenol.

(b) The reaction of bromine with phenol is known as bromination of phenol.

Bromination can occur in two ways.

i) In an aqueous medium – In aqueous solution, bromine water( Br2 + H2O)is formed which reacts with phenol to form trisubstituted phenol called 2,4,6-tribromo phenol.

ii) In a non-aqueous medium – In absence of water, phenol reacts with low polar solvents like CS2 or CHCl3 to form two monosubstituted (ortho and para) bromophenols.

(c) In an Electrophilic substitution reaction, lewis acid is used to generate the required electrophile (E+)by means of polarization.During bromination of phenol, the oxygen atom of phenol can polarize Br-Br bond to generate Br+ ion. So, the presence of lewis acid is not required.

This process continues which leads to the formation of trisubstituted product 2,4,6-tribromophenol.In aqueous solution, phenoxide ion donates electrons to the benzene ring to a large extent, thus the ring gets activated. But in a non-polar solvent, only -OH group donates electrons to ring to a small extent. Thus monosubstituted product o-bromophenol and p- bromophenol are formed. p- bromophenol is formed as a major product.

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