Question

Write the structure of a D-aldohexose that gives a meso compound on treatment with NaBH4 in water.

Answer 1

Explanation:

Meso compounds are optically inactive stereoisomers. Despite of having chiral carbon, they do not show  optical activity because it has a plane of symmetry in its structure itself. It is superposable on its mirror image.

Aldo sugars on reaction which reducing agents such as , NaBH₄ reduces the carbonyl group in the sugar to alcohol and gives corresponding alditol.

The D- aldohexose which on reduction gives a meso alditol are allose and galactose.

The structure is shown in the image below. Thus, in allitol and galactitol formed have internal plane of symmetry which makes the optically inactive.