Which one is the most acidic compound?
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Answer
(3) [figure]
Solution
Acidity of a phenol depends on the stability of its phenoxide ion.
∵ -NO2 at ortho/para positions withdraws electrons (-I and -R effects) and disperses the negative charge of the phenoxide ion ⇒ acidity increases.
∵ -CH3 is electron releasing ⇒ it destabilises the phenoxide ion ⇒ acidity decreases.
As per NCERT, pKa values: 2,4,6-trinitrophenol = 0.71, p-nitrophenol = 7.1, phenol = 10.0, p-cresol = 10.2.
∴ Order of acidity: 2,4,6-trinitrophenol > p-nitrophenol > phenol > p-cresol.
∴ 2,4,6-Trinitrophenol (picric acid) is the most acidic.
Answer: (3)