Q.Prepare carbolic acid from benzene sulphonic acid. Write a chemical equation for the action of neutral ferric chloride on phenol.

Alkali fusion of sodium benzenesulphonate followed by acidification gives phenol; phenol's characteristic test is a violet colour with neutral ferric chloride.

Preparation of carbolic acid (phenol) from benzenesulphonic acid — alkali fusion method:

Sodium benzenesulphonate is fused with solid NaOH at about 623 K, converting the sulphonate group to a phenoxide:

$C_6H_5SO_3Na + NaOH \xrightarrow{\text{fuse, }623K} C_6H_5ONa + Na_2SO_3$

The fused mass is dissolved in water and acidified with dilute HCl, liberating phenol:

$C_6H_5ONa + HCl \rightarrow C_6H_5OH + NaCl$

Action of neutral ferric chloride on phenol: phenols give a characteristic violet/purple colouration with neutral $FeCl_3$ solution, due to the formation of a coloured iron–phenolate complex:

$6C_6H_5OH + FeCl_3 \rightarrow H_3[Fe(OC_6H_5)_6] + 3HCl$

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