Hydrogen bonding plays a central role in the following phenomena:
- (A)Ice floats in water.✓ Correct
- (B)Higher Lewis basicity of primary amines than tertiary amines in aqueous solutions.✓ Correct
- (C)Formic acid is more acidic than acetic acid.
- (D)Dimerisation of acetic acid in benzene.✓ Correct
Answer: (A), (B), (D)
Solution
Hydrogen bonding is crucial in:
(A) Ice floats in water: Ice has an open hexagonal structure due to extensive hydrogen bonding, making it less dense than liquid water. Correct.
(B) Primary amines more basic than tertiary in aqueous solution: In water, primary amines can form more hydrogen bonds with water molecules (via N-H bonds) than tertiary amines (which have no N-H bonds). This stabilizes the protonated form (RNH₃⁺) of primary amines more effectively, making them stronger bases in aqueous solution. Correct.
(C) Formic acid more acidic than acetic acid: This is due to the electron-donating inductive effect (+I) of the methyl group in acetic acid, which destabilizes the carboxylate anion and makes acetic acid less acidic. Hydrogen bonding is not the primary factor. Incorrect.
(D) Dimerisation of acetic acid in benzene: Acetic acid forms dimers in non-polar solvents like benzene through intermolecular hydrogen bonding between carboxyl groups. Correct.
Correct answers: (A), (B), (D).
(A) Ice floats in water: Ice has an open hexagonal structure due to extensive hydrogen bonding, making it less dense than liquid water. Correct.
(B) Primary amines more basic than tertiary in aqueous solution: In water, primary amines can form more hydrogen bonds with water molecules (via N-H bonds) than tertiary amines (which have no N-H bonds). This stabilizes the protonated form (RNH₃⁺) of primary amines more effectively, making them stronger bases in aqueous solution. Correct.
(C) Formic acid more acidic than acetic acid: This is due to the electron-donating inductive effect (+I) of the methyl group in acetic acid, which destabilizes the carboxylate anion and makes acetic acid less acidic. Hydrogen bonding is not the primary factor. Incorrect.
(D) Dimerisation of acetic acid in benzene: Acetic acid forms dimers in non-polar solvents like benzene through intermolecular hydrogen bonding between carboxyl groups. Correct.
Correct answers: (A), (B), (D).
JEE Advanced · 2014 · 25 May 2014 · Paper 1 · One or more correct options