Optical Isomerism

Optical Isomerism

Optical Isomerism

Optical isomerism is a phenomenon that occurs when two substances have the same chemical formula and structure, but their mirror images cannot be superimposed to each other. Optical isomers are also known as enantiomers.

Optical Isomerism


The structures exhibiting Optical isomerism are called optical isomers. Optical isomers have the ability to rotate the plane polarized light. This property is often referred as optical activity. Optical activity is the ability of a substance to rotate the plane polarized light (PPL) passes through it. The amount of rotation depends on the concentration of the substance, the wavelength of the light, and other factors.

When a beam of PPL passes through a solution of certain organic molecules, the PPL is rotated through an angle α. Not all organic substances exhibit this property, but those that exhibit are said to be optically active. The angle of rotation can be measured with an instrument called a polarimeter. When a solution of known concentration of an optically active material is placed in the polarimeter, the beam of light is rotated either to the right (clockwise) or to the left (anti-clockwise). So the compounds which rotate the plane polarized light to the right (clockwise) is said to be Dextrorotatory, and those which rotate the plane polarized light to the left is said to be Levorotatory. Dextrorotatory is indicated by + sign, while Levorotatory by a minus sign (─)


Optical Activity

Condition of Optical Activity

For a compound to show optical activity, it must be asymmetrical, lacking a plane of symmetry (POS), center of symmetry (COS), and alternating axis of symmetry (AAOS). These symmetrical features allow the compound to be superimposable on its mirror image, preventing optical activity.

What is Plane Polarized Light?

Ordinary light consists of electromagnetic waves that oscillate in an infinite number of planes at right angles to the direction of light travel. When a beam of ordinary light is passed through a device called a polarizer, or a Nicol prism which is made of calcite or CaCO3, light is found to vibrate in only one plane is said to be plane-polarized light. Light waves in all other planes are blocked out. In other words we can say that plane-polarized light is light that vibrates in a single plane due to the process of polarization.



Chiral and Achiral Molecules

Chiral molecules are non-superimposable on its mirror images. For example, a left hand does not posses a plane of symmetry, and its mirror image is not another left hand but a right hand. The two hands are not identical and cannot be superimposed. If we lay one hand on the other, the fingers and the thumbs would clash.

Chirality with two hands


Achiral molecules are symmetric at all the centres. For example, a butterfly and its mirror image are superimposed on each-other because it has a plane of symmetry. Therefore, butterfly (almost all living beings) are achiral.

Achiral Molecules with Butterfly


Chiral Center or Chiral Carbon

Chiral Center_Chiral Carbon_Asymmetric Carbon A carbon having four different atoms or group of atoms is called chiral carbon or chiral center or stereogenic center. Chiral center is indicated by sterisk mark (*) next to the asymmetric carbon. Chiral carbon lacks the plane of symmetry and therefore, called as dissymmetric or Asymmetric. A chiral object cannot be superimposed on its mirror image as shown above. Knowing the number of asymmetric carbon atoms, you can calculate the maximum possible number of stereoisomers for any given molecule.
If 'n' is the number of asymmetric carbon atoms then the maximum number of isomers = 2n.


Conditions for Optical Isomerism

The necessary conditions for a molecule to exhibit optical isomerism is that it should be dissymmetric , That is the molecule should not be superimposible on its mirror image. In simple molecules, the dissymmetry (also called chirality) resulted from the presence of a chiral carbon atom.

The non-superimposable mirror image forms of a chiral carbon are called Enantiomers. They represent two optical isomers (+), and (-). Their opposite rotatory abilities are due to the opposite arrangements of atoms or groups around a chiral carbon.

Remember
It is true that molecules containing chiral carbons are optically active, but it is not always so.
There are some compounds such as meso-tartaric acid which have chiral carbon but is not optically active. On the other hand, there are some compounds which do not have chiral carbons but are optically active i.e., substituted allenes and biphenyls.

Molecule does not have asymmetric carbon also optical active

Also read Isomerism | Geometrical Isomerism | Coformational Isomerism


Daily
Quiz