Question

Why are carbohydrates generally optically active?

Answer 1

Hint: The carbohydrates which possess one or more such carbon chiral centres are mostly optically active. The molecules which rotate the plane-polarized light to the left are called laevorotatory (–) substances.

Complete answer:
> When light vibrating in all directions is passed through a polarizer filter, the emerging light or the polarized light vibrates only in one direction and some substances in its path can turn this direction of light towards the right direction or left direction. The molecules which rotate the plane of polarized light are known as optically active.
> The molecules in which a carbon atom is connected to all the four different atoms or groups is known as a chiral carbon or centre.
> Carbohydrates which possess one or more such carbon chiral centres are mostly optically active. The molecules which rotate the plane-polarized light to the left are called laevorotatory (–) substances.
Example is (-)-glucose
> The molecules which rotate the plane-polarized light to the right are called dextrorotatory (+) substances.
Examples is (+)-glucose
> Most biologically important molecules have chiral centres, and hence are optically active. Often, living systems contain only one of the two forms, i.e. either it contains (+) or (-). The molecules which have more than one chiral centre, maybe (+) or (-) or optically active (meso) form. Tartaric acid exists in L (+), D (-) and meso tartaric forms. Laevorotatory or dextrorotatory is determined from the experimental determination of the optical activity of the molecule. Carbohydrates are generally optically active because they have one or more chiral carbon atoms. The molecules of carbohydrates are chiral because of the presence of many asymmetric carbon atoms. Thus, carbohydrates are generally optically active.

Note: Not all carbohydrates are optically active. The monosaccharides generally are optically active and it is because they contain chiral carbon atoms, and chirality results in optical activity.