Question

The melting points and solubility in water of amino acids are generally higher than those of the corresponding halo acids. Explain.

Answer 1

Both acidic (carboxyl) and basic (amino) groups are present in the same molecule of amino acids. In aqueous solutions, the carboxyl group can lose a proton and the amino group can accept a proton, thus giving rise to a dipolar ion known as a zwitter ion.

\[\begin{array}{cc}
\phantom{.........}\ce{O}\phantom{........................}\ce{O}\phantom{....}\\
\phantom{......}||\phantom{.........................}||\phantom{.}\\
\ce{R - CH - C - O - H <=> R - CH - C - O-}\\
|\phantom{.........................}|\phantom{........}\\
\ce{:NH2}\phantom{..................}\ce{\underset{(Zwitter ion)}{^+NH3}}\phantom{.....}
\end{array}\]

Due to this dipolar behaviour, they have strong electrostatic interactions within them and with water. But halo-acids do not exhibit such dipolar behaviour.

For this reason, the melting points and the solubility of amino acids in water is higher than those of the corresponding halo-acids.