Question

Predict the increasing order of acidity in the following compounds.

• A. Phenol<paracresol<paranitrophenol<metanitrophenol
• B. Paracresol<phenol<metanitrophenol<paranitrophenol
• C. Metanitrophenol<paranitrophenol<paracresol<phenol
• D. Paranitrophenol<metanitrophenol<paracresol<phenol

Answer 1

Answer: (B)

Paracresol<phenol<metanitrophenol<paranitrophenol

In para cresol, there is +I effect of methyl group thereby is less acidic than phenol. In meta nitrophenol, -I effect of nitro group makes the compound more acidic than phenol and para cresol.
On comparing m-nitrophenol and p-nitrophenol, the nitro group attached to a conjugated system shows strong -M effect, and this decreases the electron density more at the ortho and para positions than at the meta position. This factor helps greater stabilisation of the conjugate base (nitro phenoxide ion) of the ortho and para isomers than that of the meta isomer.