Question

The melting points of amino-acids are higher than the corresponding halo-acids because?
(a) Amino acids exist as zwitterion resulting in strong dipole-dipole interaction.
(b) Amino acids are optically active.
(c) Due to higher molecular mass of $-{ NH }_{ 2 }$ group molecular mass of amino acids is higher.
(d) They interact with water more than halo-acids and have salt-like structure.

Answer 1

In Amino acids, $-{ NH }_{ 2 }$ (basic) and $-COOH$ (acidic) groups are present as functional groups and they can be classified as $\alpha -,\quad \beta -,\quad \gamma -,\quad \delta -,$ etc. depending upon the relative position of the $-{ NH }_{ 2 }$ group with respect to the $-COOH$ group .

Complete step by step answer:
There are a total of twenty six $\alpha -$amino acids that are found in proteins and they differ based on the nature of their side chain groups that determines the properties of the protein.
$\alpha -$amino acids possess the following properties:

-They are colourless, non-volatile crystalline solids.
-They melt with decomposition at very high temperatures.
-They are soluble in water but are insoluble in non-polar solvents.
-They exist as internal salt or zwitterion: Since amino acids contain both an acidic group (carboxyl group) and a basic group (amino group), they neutralise each other through the transfer of a proton from the carboxyl group to the amino group present in the molecule. The resultant ion is called a dipolar ion.

It is because of this zwitterion structure that the $\alpha -$amino acids have a crystalline structure and possess a high melting point.

Halo-acids are halogen derivatives of the carboxylic acids example 2-Chlorocarboxylic acids. Because they do comprise of halogen and –COOH groups, these molecules will possess dipole-dipole interactions but amino acids consist of stronger dipole-dipole interactions as well as electrostatic forces of attractions because of their dipolar nature.

Hence the correct answer is (a) Amino acids exist as zwitterion resulting in strong dipole-dipole interaction.

Note:
The chemical properties of the members of a homologous series similar though the first member may vary considerably from the rest of the members. The successive members of a homologous series differ by a $C{{H}_{2}}$ group or by 14 mass units.