JEE Chemistry 2025 Question with Solution

Given: We need the pair of reactants that forms the dipeptide Gly-Ala with elimination of $\mathrm{HCl}$.

Find: Which option gives the peptide bond in the correct order, glycine followed by alanine.

A dipeptide forms when the carboxyl-derived reactive group of one amino acid reacts with the amino group of another amino acid. If an acid chloride is used, the elimination is $\mathrm{HCl}$ instead of water.

For Gly-Ala, the glycine part must provide the acyl fragment and alanine must provide the amino group.

Thus the suitable reactants are:

$$\mathrm{NH_2-CH_2-COCl} \quad \text{and} \quad \mathrm{NH_2-CH-COOH}$$

The acid chloride of glycine reacts with the amino group of alanine to form the peptide bond, eliminating $\mathrm{HCl}$.

So the required pair is option A.

Other options fail because:

• two acid chlorides do not provide the required free amino acid partner properly,
• free acids without activation do not eliminate $\mathrm{HCl}$ for peptide formation,
• reversing the acid chloride partner would not give Gly-Ala in the required order.

Therefore, the correct option is A.

Given: The product is Gly-Ala and elimination is $\mathrm{HCl}$.

Find: The reactant pair matching this directly.

Shortcut idea: If $\mathrm{HCl}$ is eliminated, one reactant should be an acid chloride. In the name Gly-Ala, the first amino acid contributes the acyl part of the peptide bond.

So glycine must be present as:

$$\mathrm{NH_2-CH_2-COCl}$$

and alanine must be the amino acid with a free amino group:

$$\mathrm{NH_2-CH-COOH}$$

This matches option A immediately.

Therefore, the correct option is A.