Question

How is Benzene converted to Salicylic acid?

Answer 1

Hint: Salicylic acid is $2{\text{ - Hydroxybenzoic acid}}$. Therefore it contains a $OH$group and a $COOH$group. We have to substitute these two groups on the given benzene. For this we will convert benzene into phenol and will add carboxylic acid .

Complete answer:
Benzene can be converted into Salicylic acid using the following steps :
Step $1.$We will first convert Benzene to phenol. There are many through which it can be converted into phenol. One of them is mentioned below:

$\xrightarrow{{{H_2}S{O_4}}}$

$\xrightarrow{{NaOH}}$

Benzene Benzene Sulfonic acid Phenol
 Here Benzene reacts with sulfuric acid to form Benzene Sulfonic acid. Then this acid further reacts with $NaOH$at ${200^ \circ }C$ . This will give us phenol.
Step $2.$ Now the phenol will react with $C{O_2}{\text{ / NaOH}}$and then we have to do protonation with help of ${H_2}S{O_4}$. The phenol reacts with $C{O_2}{\text{ / NaOH}}$ under pressure which will give us sodium salt or salicylic acid. Then the protonation of this salt yields us salicylic acid.

$\xrightarrow{{C{O_2}{\text{ / NaOH}}}}$

       

$\xrightarrow{{{H_2}S{O_4}}}$

                                                                                                                                  Salicylic acid
Thus we get Salicylic acid from Benzene by using these steps. This reaction is also known as the Kolbe Schmitt Reaction. It is also known as beta hydroxy acid.

Note:
The reaction takes place at high pressure because of the stability of benzene. For protonation we use ${H_2}S{O_4}$ which gives ${H^ + }$. It is preferable to get the hydroxyl group first on the ring because if the carboxyl group is attached first then the ring becomes meta directing in nature. Hence salicylic acid cannot be formed in this way. Thus the hydroxyl group is added first since it is ${\text{o - p}}$activating group. Thus we get salicylic acid as our product.