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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.

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Secondary (2°) amines react with diethyl oxalate forming oxamic ester, which is a liquid.

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Tertiary (3°) amines do not react with diethyl oxalate.

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    $\begingroup$ Probably a steric issue. Seems reasonable. $\endgroup$ – Zhe Jan 10 '18 at 3:47
  • $\begingroup$ You may want to look at the Hinsberg test for distinguishing between amines. $\endgroup$ – user55119 Jul 21 '18 at 21:51
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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.

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