The best proton donor is:
A. \(H_2O\)
B. \(CH_3CH_2CH_2OH\)
C. \(CH_3CH_2CH_2CH_2OH\)
D. \(CH_3CH_2\underset{\underset{\large OH}{|}}{C}HCH_3\)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → A only
A proton donor is an acid (Brønsted). Alcohols are weaker acids than water. The alkyl group is electron-releasing (+I effect): it increases the electron density on oxygen, so the O–H bond releases its proton less easily, and the alkoxide ion \( \ce{RO^{-}} \) is less stable than \( \ce{OH^{-}} \). Water reacts with an alkoxide ion:
\( \displaystyle \ce{RO- + H-O-H -> R-O-H + OH-} \)
Water gives its proton to the alkoxide ion, so water is a better proton donor (stronger acid) than an alcohol, and the alkoxide is a stronger base than hydroxide.
(A) \(H_2O\) — Correct. No alkyl group; it is the strongest proton donor of the four.
(B) \(CH_3CH_2CH_2OH\) — Incorrect. Propan-1-ol is a primary alcohol; the propyl group (+I) makes it a weaker acid than water.
(C) \(CH_3CH_2CH_2CH_2OH\) — Incorrect. Butan-1-ol is also a primary alcohol and a weaker acid than water (about the same as propan-1-ol).
(D) \(CH_3CH_2\underset{\underset{\large OH}{|}}{C}HCH_3\) — Incorrect. Butan-2-ol is a secondary alcohol; two alkyl groups on the carbinol carbon make it even less acidic (acid strength 1° > 2° > 3°).
NCERT (2026-27 edition):
Acidity of Alcohols – …
