A dipeptide, " x " on complete hydrolysis gives " y " and " z ". " y " on treatment with aq. HNO₂ produces lactic acid. On the other hand " z " on heating gives the following cyclic molecule.
Based on the information given, the dipeptide X is :

Solution

Chemistry ma’am · 4 steps

Read each clue as one amino acid: lactic acid means alanine, the bare ring means glycine.

1. Clue for y. Aqueous HNO₂ replaces the -NH₂ of an amino acid with -OH and releases N₂. The carbon skeleton does not change.

   CH₃CH(NH₂)COOH →(HNO₂) CH₃CH(OH)COOH + N₂ ↑

2. So y is alanine. Lactic acid is 2-hydroxypropanoic acid, CH₃CH(OH)COOH. Put NH₂ back in place of OH and you get alanine.
3. Clue for z. On heating, two molecules of an amino acid join head to tail and lose two H₂O to form a cyclic diamide (a 2,5-diketopiperazine). In the ring shown, the carbons between NH and C=O are plain CH₂ groups with no side chain. So the side chain is R = H, and z is glycine, H₂NCH₂COOH.
4. So. Hydrolysis of x gives alanine and glycine, so the dipeptide is alanine-glycine, option (d).

The idea to keep. HNO₂ swaps NH₂ for OH and keeps the skeleton, so read the hydroxy acid back to the amino acid. In a diketopiperazine, the ring substituents tell you the side chain R.

Answer from the NTA final answer key. Solution by PrepWiser.