Question

A carbohydrate (A), when treated with dilute HCl in alcoholic solution gives two isomers (B) and (C). B on reaction with bromine water gives a monocarboxylic acid 'Z' and 'C' is a ketohexose. What is A?

• A. Starch
• B. Maltose
• C. Sucrose
• D. Lactose

Hint:

Bromine water test distinguishes Aldoses (Glucose) from Ketoses (Fructose). Aldoses are oxidised (decolourising bromine water); Ketoses are not.

Answer 1

The Correct Option is C

Step 1: Analyze Hydrolysis: Compound A hydrolyzes to give two isomers B and C. This suggests A is a disaccharide made of two hexose units (\(\text{C}_6\text{H}_{12}\text{O}_6\)).
Step 2: Characterize B and C: - C is a ketohexose: The most common natural ketohexose is Fructose. - B reacts with Bromine Water: Bromine water oxidises aldehydes to acids but does not affect ketones. Since B reacts to form a monocarboxylic acid Z (Gluconic acid), B must be an aldose. The isomer of Fructose that is an aldose is Glucose.
Step 3: Identify A: A yields Glucose and Fructose on hydrolysis. The disaccharide composed of Glucose and Fructose is Sucrose (Invert Sugar). Reaction: \(\text{C}_{12}\text{H}_{22}\text{O}_{11} (\text{Sucrose}) + \text{H}_2\text{O} \rightarrow \text{Glucose} + \text{Fructose}\). Analysis of other Options: - Maltose \(\rightarrow\) Glucose + Glucose (Same units, not isomers in this context). - Lactose \(\rightarrow\) Glucose + Galactose (Both are aldoses).
Required Answer: Option (C).