Q.47 Give below are two statements : One is labelled as Assertion A and the other is labelled as Reason R:
Assertion A : Treatment of monosaccharides with sodium amalgam reduces their aldehyde and keto groups to
corresponding acid :
Reason R : D-Dulcitol and D-Sorbitol are the conesponding alcohols ofD-galactose and D-glucose,
respectively.
In the light of the above statements. choose the most appropriate answer from the options given below :
1. Both A and R are correct and R is the correct explanation of A
2. Both A and R are correct but R is NOT the correct explanation of A
3. A is correct but R is not correct
4. A is not correct but R is correct

Correct Answer: 4. A is not correct but R is correct

Sodium amalgam reduces monosaccharide carbonyls (aldehyde/ketone) to alcohols, not oxidizes to acids; R correctly identifies dulcitol from D-galactose and sorbitol from D-glucose.

Assertion (A) Analysis

Assertion A is incorrect. Sodium amalgam (Na/Hg) in water generates nascent hydrogen, reducing aldose (-CHO → -CH₂OH) or ketose (>C=O → >CH-OH) groups to polyols (sugar alcohols). Examples: D-glucose → D-sorbitol, D-galactose → D-dulcitol. Oxidation to acids uses nitric acid (→ aldaric acids like glucaric) or Br₂ (→ aldonic like gluconic). No reduction produces carboxylic acids.

Reason (R) Analysis

Reason R is correct. D-Dulcitol (galactitol) is the polyol from complete D-galactose reduction; D-sorbitol (glucitol) from D-glucose. Both aldohexoses lose chirality at C1 upon -CHO → -CH₂OH, yielding straight-chain hexitols.

Option Breakdown

• Option 1: Both correct, R explains A—False. A wrong.

• Option 2: Both correct, R not explanation—False. A incorrect.

• Option 3: A correct, R incorrect—False. R right.

• Option 4: A incorrect, R correct—True. Matches carbohydrate chemistry facts.

Monosaccharides sodium amalgam reduction dulcitol sorbitol converts sugar carbonyls to alcohols (not acids as A wrongly claims), with R correctly naming products—essential for carbohydrate chemistry in competitive exams.

Reduction Reaction Mechanism

Na/Hg + H₂O → nascent [H] (mild reductant):

• Aldoses: R-CHO → R-CH₂OH (primary alcohol).

• Ketoses: R₂C=O → R₂CH-OH (secondary alcohol).
  No effect on -OH groups. Yields optically active polyols unless meso form.

Examples:

Common Exam Confusion

• Reduction (NaBH₄, Na/Hg): Carbonyl → alcohol.

• Mild oxidation (Br₂): Aldehyde → aldonic acid (gluconic).

• Strong oxidation (HNO₃): Both ends → aldaric acid (saccharic).

A traps with reversed oxidation/reduction.

Structures Comparison

GATE/CSIR Application

Tests reaction specificity: R exemplifies correct products, unrelated to A’s error. Memorize reagents/outcomes for assertion-reason formats.