Question

What does phenol form by reaction with $HCN$ and $HCl$.

Answer 1

Hint: Phenol is an aromatic compound in which electrons are delocalized. When phenol is reacted with the mixture of $HCN$and $HCl$ it forms aldimine as the intermediate. The catalyst we use for this reaction is $ZnC{l}_2$. Thus we do hydrolysis of the intermediate product to get the final product.

Complete answer:
Phenol is an aromatic compound which contains hydroxyl groups. The mixture of $HCN$and $HCl$is reacted with phenol in the presence of a catalyst known as zinc chloride. Further an intermediate product is formed which on hydrolysis yields us $p-$hydroxybenzaldehyde. The whole reaction will take place in the following steps:
$1.$In the first stage of reaction, the mixture of hydrochloric acid and hydrogen cyanide is reacted to form a complex in the presence of the zinc chloride.
$HCl\text{ + HCN }\stackrel{ZnC{l}_2}{\to }\text{ Cl - CH = NH}$
The formed complex will further react with the phenol and form an intermediate product.
$2.$ Now the given complex will react with phenol and produce an intermediate product known as aldimine. Then the aldimine so formed will undergo hydrolysis.

$3.$Now the formed intermediate, Aldimine, is further undergoing hydrolysis with $H_3{O}^{+}$. On hydrolysis of the above intermediate we get $p-$hydroxybenzaldehyde as our main product.

The product so formed is $p-$hydroxybenzaldehyde. The reaction is also known as the Gattermann Reaction used for synthesis of aromatic aldehydes.

Note:
 The intermediate formed during the reaction is a result of zinc chloride. Here zinc chloride acts as a catalyst. We can use others like Aluminium Chloride in place of Zinc Chloride. On hydrolysis there is addition of oxygen and an amine group if formed as a by-product.