Space Isomery (continued)

Optical isomer

Optical spatial isomerism is one that has a carbon with four different ligands. We call this carbon asymmetric. It is identified with an asterisk (*).

Asymmetric or chiral carbon - It is the carbon atom that is bonded to four different groups.

Example: lactic acid or 2-hydroxypropanoic acid.

Note that there are four different groups in this molecule: H, CH3OH, COOH.

If we could place a mirror in front of the lactic acid molecule, we would visualize the following molecule.

Lactic acid is given the name D and its specular image (mirror image) is named L.

Substances that deflect polarized light to the right are called dextrogira (from Latin dexter, right) and substances that shift the polarized light to the left are called levogira (from Latin laevus, left).

Then the two lactic acid molecules divert polarized flat light, but one way and the other. We can write:

- dextrogyric lactic acid, d-lactic acid or (+) lactic acid
- levogyric lactic acid, l-lactic acid or (-) lactic acid

The physical properties of these isomers are the same. The only difference is the polarization of polarized flat light.

The two isomers of lactic acid can be called pair of enantiomorphs or optical antipodes.

There are some molecules that have several asymmetric or chiral carbons. Take the example of α-hydroxy-β-methyl succinic acid:

In this substance we have two asymmetric carbons that are different from each other.

This compound has four optically active and distinct isomers which can be represented as follows:

The difference between the four compounds is indicated by the positions of the OH and CH groups.3, which are sometimes on the left and sometimes on the right of the formula.

The isomers A and B are optical antipodes, one being dextrogyrous and the other levogyrous.
The isomers C and D are optical antipodes, one being dextrogyrous and the other levogyrous.

Note that A and B; C and D are mirror images of each other. It is as if there is a mirror in front of each of these molecules.

Optical isomers that are not enantiomorphs to each other are called diastereoisomers. In this case they are A and C; A and D; B and C; B and D.

To know how many optical isomers there are, we can calculate according to the number of asymmetric carbons.


n = number of asymmetric carbons

Example: In the α-hydroxy-β-methyl succinic acid molecule there are two asymmetric carbons. Then applying the formula:

2² = 4

There are four isomers for this compound. Two dextrogiros and two levogiros are formed.