Question

Which of the following product is formed when salicylic acid is treated with $\mathrm{(CH_3CO)_2O}$ in the presence of acid ?

• A.

  

• B.

  

• C.

  

• D.

  

Hint:

The anhydride acetylates the phenol −OH, not the −COOH, giving aspirin.

Answer 1

The Correct Option is D

Concept: This is a 'which group reacts' question. Salicylic acid is a benzene ring carrying two groups, a phenol group $\mathrm{-OH}$ and a carboxylic acid group $\mathrm{-COOH}$. Acetic anhydride is a reagent that adds an acetyl group, so we just need to spot which of these two groups it attacks.

Step 1: Find the group that reacts
An anhydride like $\mathrm{(CH_3CO)_2O}$ likes to put its acetyl group onto an $\mathrm{-OH}$ group. So here it goes for the phenolic $\mathrm{-OH}$. The $\mathrm{-COOH}$ group is left untouched.

Step 2: Build the product
The H of the phenol $\mathrm{-OH}$ is swapped for an acetyl group, so $\mathrm{-OH}$ becomes $\mathrm{-OCOCH_3}$. The $\mathrm{-COOH}$ group stays exactly as before, sitting right next to it on the ring.

Step 3: Name the product
A benzene ring carrying $\mathrm{-OCOCH_3}$ next to $\mathrm{-COOH}$ is 2-acetoxybenzoic acid, the common medicine we all know as aspirin. That is the structure drawn in option (D).

Answer: Option (D), aspirin (acetylsalicylic acid).