Question

Which amino acid is achiral ?
A.Alanine
B.Valine
C.Proline
D.Glycine

Answer 1

The amino acid which is achiral is the one which does not have any chiral centre . The mirror images of the achiral amino acid are superimposable on each other .

Complete step by step answer:
A molecule or compound is said to be chiral if it cannot be superimposed on its mirror image by any of the different combinations of translations and rotations.
For a molecule to be chiral it should have at least one chiral centre or stereocenter , that is, the central atom which bears different substituents on it such that if we interchange any two substituents it forms a stereoisomer.
For example, the chiral carbon atom is one around which four different groups are attached.
On the other hand , an achiral molecule is one which is superimposable on its mirror image .This means the central carbon atom contains two similar groups around itself.
Amino acids contain an amino group and an acidic group as substituents on the central carbon which is the alpha carbon atom. Amino acids are substituted methanes . The substituents can be hydrogen, carboxyl group, amino group and a variable group. The variable group could be hydrogen (in glycine), a methyl group (in alanine) , a hydroxy methyl group (in serine).
All the amino acids are chiral except glycine. This means glycine is the only amino acid which contains two same groups and two different groups around the central carbon atom, that is, there are two hydrogen atoms , one amino group and one carboxyl group. So the mirror images can be superimposed on each other.
Hence, option D is correct .

Note: All amino acids on treatment with ninhydrin ( 2,2 - dihydroxybenzene - 1,3 - dione ) give purple colouration . This test is called ninhydrin test and is used for the detection of amino acids .