Question

Give reason for the following :

The melting points of α-amino acids are generally higher than that of the corresponding carboxylic acids.

Hint:

Amino acids are zwitterions held by strong electrostatic forces in the crystal.

Answer 1

Step 1: Why the melting points are higher:
In the solid state an alpha-amino acid does not exist as \(\mathrm{H_2N-CHR-COOH}\). The acidic COOH gives its proton to the basic NH2 inside the same molecule.
This makes a dipolar ion (zwitterion) \(\mathrm{{}^{+}H_3N-CHR-COO^{-}}\). Zwitterions attract each other by strong electrostatic (ionic type) forces, like a salt.
A carboxylic acid has only hydrogen bonds and dipole-dipole forces between molecules. These are weaker than ionic attractions.
More energy is needed to break the crystal of an amino acid, so its melting point is higher.

Final Answer:
Amino acids exist as zwitterions with strong electrostatic attraction between them, so their melting points are higher than those of the corresponding carboxylic acids.