Why does only one molecule of secondary amine react with diethyl oxalate and two molecules of primary amine react with diethyl oxalate?
I feel secondary amine also should have reacted similar to primary amine.
From JEE-Main 2015:
Reaction with diethyl oxalate
1°, 2°, and 3° amines can be distinguished by their reactions with diethyl oxalate. Primary (1°) amines react with diethyl oxalate forming N,N-oxamide, which is a solid.
Secondary (2°) amines react with diethyl oxalate forming oxamic ester, which is a liquid.
Tertiary (3°) amines do not react with diethyl oxalate.