JEE Main 2025 · 4 April · morning · Single correct · +4 / −1
Identify the pair of reactants that upon reaction, with elimination of HCl will give rise to the dipeptide Gly-Ala.
- (a) NH₂-CH₂-COOH and NH₂ - CH(CH₃; |) - COOH
- (b) NH₂-CH₂-COCl and N(⊕)H₃-CH(|)(CH₃)-COCl
- (c) NH₂-CH₂-COCl and NH₂-C(|)(CH₃)H-COOH
- (d) NH₂-CH₂-COOH and NH₂-CH(CH₃ |)-COCl
Answer and solution
Answer (c) NH₂-CH₂-COCl and NH₂-C(|)(CH₃)H-COOH
Solution
Chemistry ma’am · 3 steps
Gly-Ala means glycine gives the C=O and alanine gives the NH, so glycine must carry the COCl.
- Read the name. A peptide is written from the N-terminus to the C-terminus. In Gly-Ala, glycine keeps its free NH₂, alanine keeps its free COOH, and the peptide bond joins glycine's carbonyl to alanine's nitrogen.
- Where HCl comes from. Losing HCl means an acid chloride (-COCl) has reacted with an amine (-NH₂). The Cl leaves from the carbonyl side and the H leaves from the nitrogen side. So glycine must be the acid chloride, and alanine must have a free NH₂ and an untouched COOH.
H₂N-CH₂-COCl + H₂N-CH(CH₃)-COOH → H₂N-CH₂-CO-NH-CH(CH₃)-COOH + HCl
- So. The pair is glycyl chloride and alanine, which is option (c).
The idea to keep. In a peptide name, the first residue gives the C=O of the peptide bond and the next one gives the N–H. Whichever residue is written first is the one that must be activated as the acid chloride.
Answer from the NTA final answer key. Solution by PrepWiser.