Which of the following species is the strongest base? (R = alkyl group)
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → RO−
Each species is the conjugate base of an acid: the weaker the acid, the stronger its conjugate base.
\( \ce{RO^-} \) comes from an alcohol, ROH; \( \ce{HO^-} \) from water; \( \ce{C6H5O^-} \) from phenol; p-nitrophenoxide from p-nitrophenol.
Order of acid strength: p-nitrophenol > phenol > water > alcohol.
So the order of base strength is the reverse: \( \ce{RO^-} \) > \( \ce{HO^-} \) > \( \ce{C6H5O^-} \) > p-\( \ce{O2N-C6H4-O^-} \).
An alkoxide ion even takes a proton from water, which shows it is a stronger base than hydroxide:
\( \displaystyle \ce{RO^- + H2O -> ROH + OH^-} \)
Why the other options are incorrect
Option (1) HO− — Incorrect. Water is a stronger acid than an alcohol, so \( \ce{HO^-} \) is a weaker base than \( \ce{RO^-} \).
Option (3) C6H5O− — Incorrect. In the phenoxide ion the negative charge is delocalised over the ring, which stabilises it; phenol is more acidic than alcohols, so phenoxide is a weaker base.
Option (4) p-O2N–C6H4–O− — Incorrect. The electron withdrawing nitro group at the para position spreads the negative charge further, making p-nitrophenol more acidic than phenol; its anion is the weakest base of the four.
NCERT (2026-27 edition):
7.4.4 Chemical Reactions – “Alcohols are, however, weaker acids than water. … Also, in the above reaction, we note that an alkoxide ion is a better proton acceptor than hydroxide ion, which suggests that alkoxides are stronger bases (sodium ethoxide is a stronger base than sodium hydroxide).”