JEE Main · 2019 · 10 Jan 2019 · Shift 1 · Alcohols Phenols and Ethers

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The increasing order of pKa values of the following compounds is

A

B

C

D

(A)C $<$ B $<$ A $<$ D
(B)B $<$ C $<$ D $<$ A
(C)D $<$ A $<$ C $<$ B
(D)B $<$ C $<$ A $<$ D✓ Correct

Answer: (D)

Solution

A lower

p K_{a}

means a stronger acid, so I rank these phenols by acidity and then invert. The para-nitrophenol is the most acidic because the nitro group withdraws electrons and, from the para position, stabilises the phenoxide by direct resonance; the meta-nitrophenol is a little less acidic since from the meta position the nitro group acts mainly by induction without that resonance help; plain phenol is weaker still; and para-methoxyphenol is the least acidic because the electron-donating methoxy group destabilises the phenoxide. Acidity thus runs nitro(para) over nitro(meta) over phenol over methoxy, so the increasing order of

p K_{a}

is the reverse,

B < C < A < D

JEE Main · 2019 · 10 Jan 2019 · Shift 1 · Single correct option

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JEE Main · 2019 · 10 Jan 2019 · Shift 1 · Alcohols Phenols and Ethers

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