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

Step 1: Recognise the molecule.
Salicylic acid is a benzene ring carrying two groups that sit next to each other, a phenol $\mathrm{-OH}$ and a carboxylic acid $\mathrm{-COOH}$. The reagent acetic anhydride, $\mathrm{(CH_3CO)_2O}$, is well known for adding an acetyl group, so the question is really asking which of the two groups gets acetylated.

Step 2: Pick the group that reacts.
Acetic anhydride attaches its acetyl unit onto an $\mathrm{-OH}$ type group. The phenolic $\mathrm{-OH}$ is the one that takes part. The $\mathrm{-COOH}$ group is left untouched in this reaction.

Step 3: Build the product in words.
The hydrogen of the phenol $\mathrm{-OH}$ is swapped for an acetyl group, so that part becomes an ester, $\mathrm{-O\,CO\,CH_3}$. The neighbouring $\mathrm{-COOH}$ stays exactly as before. A ring carrying $\mathrm{-OCOCH_3}$ right beside a $\mathrm{-COOH}$ is the familiar painkiller aspirin, also called acetylsalicylic acid.

Step 4: Match the option.
The structure that shows an acetylated phenol next to an intact carboxylic acid is option (D).
\[ \boxed{\text{Option (D): aspirin (acetylsalicylic acid)}} \]