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Would some be able to explain why when I mix a carboxylic acid, like ethanoic acid, and an amine, say ethylamine, I get ethylammonium ethanoate. But when I mix a bi-functional group carboxylic acid (say ethanedioic acid) and a bi-functional group amine (say 1,2-diaminoethane) I get a condensation reaction and an amide bond is formed.

This also leads to why does ethanoic and ammonia yield ammonium ethanoate and not ethanamide. I understand that the ammonia is acting as a base in this reaction, but why doesn't it favour being a base rather than a nucleophile?

To me both of the reaction seen the same. Can someone elaborate this for me, or am I missing a key piece of knowledge.

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    $\begingroup$ Did you actually do this? Yes, acetic acid and ethylamine form salts and I would expect the same result with oxalic acid and ethylene diamine at ambient temperature. At elevated temperature, the first example would yield n-ethylacetamide and the latter a polymeric amide since both components are bifunctional. Mixing 1,6-ethylenediamine and adipic acid at ambient temperature you do not get nylon. By the way, real chemists, at least of my generation, say acetic acid and not ethanoic acid. $\endgroup$ – user55119 Feb 3 at 21:54
  • $\begingroup$ So is it a temperature that stops the anime acting as a base and starts acting as a nucleophole? But why? I only use ethanoic acid as that what I have to use for my exams as it’s IUPAC standards $\endgroup$ – H.Linkhorn Feb 3 at 21:58
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    $\begingroup$ At ambient temperature the amine reacts as a base forming a salt. At elevated temperature the amine will act as a nucleophile. In your first example a sealed tube would be required to avoid sublimation. This is not a normal non-industrial way to make amides. Usually the acyl chloride or anhydride of the acid is reacted with the amine. $\endgroup$ – user55119 Feb 3 at 22:20
  • $\begingroup$ Unrelated to the question, but the preferred IUPAC name is actually “acetic acid”, not the systematic name “ethanoic acid”. $\endgroup$ – orthocresol Feb 4 at 2:30
  • $\begingroup$ I should add that oxalic acid and ethylenediamine can also form the 6-membered monocycle piperazine-2,3-dione. $\endgroup$ – user55119 Feb 4 at 2:40
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You have a false dichotomy.

Diamines and diacids can form salts with doubly charged anions and cations. Sadly, simply searching SigmaAldrich’s catalogue for the phrase diammonium did not turn up any example; however, I assume this is because these salts are unlikely to be used and thus wouldn’t sell. Chemically, proton transfer is one of the fastest reactions possible and thus simply mixing a diacid and a diamine in a suitable solvent would result in the diammonium diate salt as it would for an ammonium monoate salt.

Amide bond formation is a different reaction that typically requires activation or harsher conditions (e.g. heating). It is a condensation that results in the release of water. All that is required is the presence of an amino group and a carboxylic acid. So aminoethane and ethanoic acid can be heated in solution to produce N-ethylacetamide.

The one difference between the mono and the di systems is that the mono system has no of polymerising or cyclising. So the amide bond will always consist of one acid fragment and one amine fragment while in the case of a diamine and a diacid polymerisation or cyclisation can occur.

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  • $\begingroup$ So what is it exactly about the “harsher conditions” that make the amine act as a nucleophile and not a base? $\endgroup$ – H.Linkhorn Feb 4 at 14:30
  • $\begingroup$ @H.Linkhorn In introductory chemistry, you would probably use heat. The salt forms immediately at any temperature at which the suitable solution is liquid. For the condensation reaction to occur, the activation barrier must be overcome which is easily achieved by heating up. Another one is removal of water e.g. using molecular sieves. In preparative chemistry, you would usually not use the actual acid but an activating ester (e.g. phenyl thiocarboxylate) or in situ activator such as DCC. $\endgroup$ – Jan Feb 4 at 16:16
  • $\begingroup$ So, does that mean that when you are forming polyamides, a salt if formed first and then you have to heat the mixture to drive off the water leading to polymerisation? $\endgroup$ – H.Linkhorn Feb 4 at 17:12

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