What is the key factor that makes ortho-nitrophenol more acidic than phenol?
The -NO₂ group in ortho-nitrophenol exerts a strong electron-withdrawing effect, stabilizing the phenoxide ion and increasing acidity compared to phenol.
Ref: NCERT Class 12 Chemistry > Chapter 7: Alcohols, Phenols and Ethers > Topic: Physical Properties and Acidity of Alcohols and Phenols