Q5P - Organic Chemistry (Mcmurry)

Question

p-Nitrobenzyl alcohol is more acidic than benzyl alcohol, but p-methoxy-benzyl alcohol is less acidic. Explain.

Step-by-Step Solution

Answer

Due to the electron-withdrawing group in p-nitrobenzyl alcohol, it is more acidic than benzyl alcohol. 

Due to the electron-donating group in p-methoxy-benzyl alcohol, it is less acidic than benzyl alcohol.

Step 1: Relative acidity of a substituted phenol

The relative acidity of a substituted phenol is checked by identifying the substituent present on the aromatic ring. If the substituent is an electron-withdrawing group, it is more acidic, and if it is an electron-donating group, the phenol is less acidic.

Step 2: The acidic nature of p-nitrobenzyl alcohol and p-methoxy-benzyl alcohol

We know a nitro group is an electron-withdrawing group, and in p-nitrobenzyl alcohol, the presence of the nitro group (${\text{-NO}}_{\text{2}}$) stabilizes the phenoxide anion, and thus, it is more acidic than benzyl alcohol.

On the other hand, p-methoxy-benzyl alcohol has a methoxy group (${\text{-OCH}}_{\text{3}}$), an electron donating group. It destabilizes the alkoxide ion, making the compound less acidic than benzyl alcohol.