Amino Acid A + Amino Acid B -> Peptide chain + H20

Amine + Carboxylic Acid -> Salt

Why do they have different outcomes when breaking it down they are both the reaction between the 2 function groups: carboxyl(-COOH) and amino (-NH2)


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These two cases are not all that different.

In mild conditions amines + carboxylic acids end up producing salts. So do amino acids. Mind you, there is no need to use two of them, for each amino acid already has $\ce{-COOH}$ and $\ce{-NH2}$. In a way, each amino acid is a salt in itself.

If you remove water, then amino acids produce compounds with $\ce{-CONH}-$ link. So do amines with carboxylic acids. You must have heard of nylon; indeed, it is a polymer, not unlike peptides.


The issue here is that $\rightarrow$ means two different things in the reactions you've written.

You can't just add two amino acids together and get a peptide. That's the overall transformation, but it takes place via several steps and requires other reagents or catalysts.

If you just mix two amino acids together, e.g., a sample of a single amino acid, you get the second reaction. The case of an amino acid, it doesn't quite make a salt because the acid and base are on the same molecule. Instead, you get a zwitterion. And indeed, amino acids do exists as zwitterions in solution at physiological pH.


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