Question

The most acidic compound among the following is:

• A. phenol
• B. ethanol
• C. 3,5-dinitrophenol
• D. 4,4-methoxyphenol

Answer 1

The correct option is C

3,5-dinitrophenol

Explanation for correct option

(C) 3,5-dinitrophenol

• A substance to be a stronger acid, the structure after removal of a proton should be stable enough.
• Here the removal of proton will be from the hydroxyl group.
• The Nitro group is a strong electron-withdrawing group with $-\mathrm{R}$effect.
• This $-\mathrm{R}$effect will stabilize the resulted phenolate ion by further resonance.
• Therefore 3,5-dinitrophenol is a strong acid.

• Hence option C is correct.

Explanation for incorrect options

(A) Phenol.

• The phenolate ion after the removal of a proton is stabilized through resonance.
• But there are no other groups like nitro here to further stabilize this resonance.
• Thus it can't be the stronger acid.

• Therefore option (A) is incorrect.

(B) Ethanol

• There is no factor to stabilize the negative charge created on the oxygen atom after the removal of the proton here.
• An ethyl group is a $+\mathrm{I}$ group with electron releasing nature. Thus it will only destabilize the negative charge. So the ion will not be stable.
• Therefore removal of the proton is not easy, thus ethanol is not a strong acid.
• 
• Hence option B is incorrect.

(D) 4,4-methoxyphenol

• The Methoxy group $\left(-{\mathrm{OCH}}_3\right)$is a strong electron releasing group with $+R$ effect.
• This electron-releasing nature will further destabilize the negative charge created on the oxygen after the removal of the proton.
• Therefore this will not be a strong acid.
• Thus option D is also incorrect.

Hence the correct option is (C) 3,5-dinitrophenol.