Question

Consider the following molecules and statements related to them :

(a) (B) is more likely to be crystalline than (A)
(b) (B) has higher boiling point than (A)
(c) (B) dissolves more readily than (A) in water
Identify the correct option from below :

• A. only (a) is true
• B. (a), (b) and (c) are true
• C. (b) and (c) are true
• D. (a) and (b) are true

Hint:

(A) is salicylic acid (ortho, intramolecular H-bonding) and (B) is p-hydroxybenzoic acid (para, intermolecular H-bonding). Intermolecular H-bonding raises melting/boiling point, crystallinity and water solubility.

Answer 1

The Correct Option is B

(A) is 2-hydroxybenzoic acid (salicylic acid); the -OH and -COOH groups are ortho and form an intramolecular hydrogen bond (chelation).
(B) is 4-hydroxybenzoic acid; the groups are far apart, so the molecules form extensive intermolecular hydrogen bonds with each other and with water.
(a) Intermolecular H-bonding gives a stronger, regular lattice, so (B) is more likely to be crystalline: true.
(b) More intermolecular H-bonding means a higher boiling point for (B): true.
(c) (B) can H-bond with water molecules, while in (A) the -OH and -COOH are already engaged internally, so (B) dissolves more readily in water: true.
All three statements are true.
Answer: (2) (a), (b) and (c) are true