JEE Main · 2023 · 29 Jan 2023 · Shift 1 · Alcohols Phenols and Ethers

The increasing order of

pK_{a}

for the following phenols is
(1) 2, 4-Dinitrophenol
(2) 4 - Nitrophenol
(3) 2,4,5-Trimethylphenol
(4) Phenol
(5) 3-Chlorophenol

(A)1 < 2 < 5 < 4 < 3✓ Correct
(B)1 < 2 < 4 < 5 < 3
(C)2 < 1 < 5 < 4 < 3
(D)2 < 1 < 4 < 5 < 3

Answer: (A)

Solution

The acidity of a phenol rises with electron-withdrawing substituents, which stabilise the phenoxide ion, and falls with electron-releasing groups; the

p K_{a}

runs the opposite way to acidity. The two nitro groups of 2,4-dinitrophenol make it the strongest acid and the single para-nitro group of 4-nitrophenol the next, giving them the two lowest

p K_{a}

values. A ring chlorine withdraws electrons only weakly through its inductive effect, leaving 3-chlorophenol a little more acidic than phenol itself, while the three electron-releasing methyl groups of 2,4,5-trimethylphenol make it the weakest acid of all. The

p K_{a}

therefore increases in the order 2,4-dinitrophenol, 4-nitrophenol, 3-chlorophenol, phenol, 2,4,5-trimethylphenol.

JEE Main · 2023 · 29 Jan 2023 · Shift 1 · Single correct option

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JEE Main · 2023 · 29 Jan 2023 · Shift 1 · Alcohols Phenols and Ethers

Solution

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