Reaction of amines with diethyl oxalate (Hofmann amine separation method)

Question

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.

Reihani · Answer

When a 1° amine attacks to a carboxyl group, The structure below (left) is generated which lowers the energy level of the intermediate such that the second amine can easily react with the other carboxyl group.
It is clear that 2° amines (right) don't have such proton for hydrogen bonding with the oxygen. Why the second 2° amine doesn't attack to the other carboxyl at all is maybe due to the decreased partial positive charge of the amide (less electronegativity of nitrogen) toward the carboxyl.
Finally, 3° amines don't have even a leaving proton to substitute with the ethoxide.

Guest · Answer

The tetrahedral intermediate in the primary amine attack case would have an intramolecular hydrogen bond to stabilize.  In the secondary amine case, no such H-bond is available.

Reaction of amines with diethyl oxalate (Hofmann amine separation method)

Reaction with diethyl oxalate

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