Question

How can carbohydrates be classified based on optical activity?

Answer 1

Classification of carbohydrates based on optical activity:

1. Optical activity of a molecule is defined as its ability to rotate a plane-polarized (light vibrating in one direction) light that passes through it.
2. Optical activity of a molecule depends on the chirality of the molecule.
3. A chiral molecule is a molecule that is a chiral carbon, i.e., a carbon with four different groups attached to it.
4. Some molecules can rotate plane-polarized light towards the left, thus known as laevorotatory (-) and some can rotate it towards the right, hence known as dextrorotatory (+).
5. Glucose molecule having an optically active chiral carbon that can rotate a plane-polarized light passing through it is optically active.
6. Eg, D-glucose (+) and L-Glucose (-), which are the glucose molecules rotating a plane-polarized light towards the right and left, respectively.
7. Though there are two optical isomers of glucose, only the D-Glucose occurs abundantly in nature and biological solutions.
8. A meso compound is a non-optical mixture of two optically active isomers.
9. Hence, a meso compound is optically inactive and does not rotate a plane-polarized light despite having two optically active stereoisomers.
10. Molecules having more than one chiral carbons can exist in meso forms.
11. Example of a meso compound is tartaric acid, etc.