Question

Phenol is less acidic than

A) p-nitrophenol

B) Ethanol

C) Cresol

D) Benzyl alcohol

Answer 1

Hint: You need to know that the compound will be more acidic when the conjugate base after releasing the hydrogen ion is more stable. If the conjugate acid is less stable, then the compound will be less acidic.

Complete step by step answer:

We know that phenol contains only OH groups. If it releases the  $ {{\text{H}}^{\text{ + }}} $ ion, the conjugate base is phenoxide ion. In p-nitrophenol, after the releasing of  $ {{\text{H}}^{\text{ + }}} $ ions, the negative charge on the phenoxide ion will be stabilized by strong –M effect of nitro group in the para position. So, the conjugate base is more stable than phenol. So, p-nitrophenol is more acidic than phenol.

Therefore, it is clear that phenol is less acidic than p-nitrophenol.

So, the correct option is (A).

Additional Information:

Anything which stabilizes the conjugate base will increase the acidity. Therefore $p_{Ka}$ is also a measure of how stable the conjugate base is. Put another way, strong acids have weak conjugate bases, and vice versa.

Note: In ethanol, the +I effect decreases the stability of the conjugate base. So, it is less acidic than phenol. In cresol, since there is no mesomeric effect in the meta position, it shows +I effect and decreases the stability of the conjugate base. Hence it is less acidic than phenol. In benzyl alcohol, the conjugate base is destabilized by the +I effect. Also benzyl alcohol is less acidic than phenol.