When pure crystalline amino acids are heated, decomposition generally occurs before the solid melts. Account for this observation. (Hint: Crystalline amino acids exist as \(\mathbf{H}_{3} \mathrm{NCRHCOO}^{-}\) called zwitterions.)

A zwitterion is a molecule with both positive and negative charges, but with an overall net charge of zero. In the case of amino acids, the zwitterionic form has a positively charged amino group (\(\mathrm{NH}_{3}^{+}\)) and a negatively charged carboxylate group (\(\mathrm{COO}^{-}\)). This unique structure allows amino acids to stabilize through strong electrostatic interactions between the oppositely charged groups.

In crystalline amino acids, the zwitterionic form allows the molecules to arrange themselves in a stable lattice structure. The oppositely charged groups in each amino acid molecule form strong ionic bonds with each other, leading to the formation of a highly stable crystal lattice.

When the crystalline amino acids are heated, the thermal energy causes the molecules to vibrate and eventually break the ionic bonds in the crystal lattice. However, since the zwitterionic structure of amino acids involves relatively strong ionic bonds, it requires a significant amount of energy to break these bonds. This energy is typically higher than the energy needed to break the covalent bonds within the molecule itself.

As the crystalline amino acids are heated, the covalent bonds within the amino acids begin to break first due to the lower energy required. This leads to the decomposition of the amino acid molecules into simpler compounds before the ionic bonds in the crystal lattice have a chance to break completely. Consequently, decomposition occurs before the solid amino acids have an opportunity to melt into a liquid state. In conclusion, the observation that pure crystalline amino acids decompose before melting when heated can be attributed to their zwitterionic structure, which creates strong ionic bonds in the crystal lattice. The energy required to break these ionic bonds is generally higher than the energy needed to break the covalent bonds within the amino acid molecules, resulting in decomposition prior to melting.