Optical Isomerism

Isomerism of Class 11

Optical Isomerism

In optical isomerism we have a much more subtle phenomenon than even the geometrical isomerism. While the geometrical isomers differ in physical properties such as melting point, boiling point, density etc., and also in certain chemical properties, the optical isomers will have the same chemical reactions and will be alike in all physical properties mentioned above. They can only be distinguished by their 'action on plane-polarized light'. This property which is often referred to as the Optical activity requires a brief discussion.

What is Optical Activity?     Light is propagated by a vibratory motion of the 'ether' particles present in the atmosphere. Thus in ordinary light vibrations occur in all planes at right angles to the line of propagation. In plane polarized light the vibrations take place only in one plane, vibrations in other planes being cut off. Plane polarized light can be obtained by passing ordinary light through a Nicol prism.

Certain organic compounds, when their solutions are placed in the path of a plane polarized light, have the remarkable property of rotating its plane through a certain angle which may be either to the left or to the right. This property of a substance of rotating the plane of polarized light is called Optical activity and the substance possessing it is said to be Optically active.

The observed rotation of the plane of polarized light (determined with the help of polarimeter) produced by a solution depends on : (a) the amount of the substance in tube ; (b) on the length of the solution examined ;  (c) the temperature of the experiment;  and (d) the wavelength of the light used.

For the measurement of optical rotations, a term Specific Rotation is introduced. This is a physical constant characteristic of a substance as much as the melting point, boiling point, density or its refractive index. It is defined as the number of degrees of rotation observed when light is passed through 1 decimeter (10 centimeters) of its solution having concentration 1 gram per milliliter. The specific rotation of a given substance can be calculated by using the following expression.

The rotation may be different in different solvents and this needs to be mentioned while reporting the specific rotation. Thus,

Definition and Examples of Optical Isomerism. The simple organic compounds which show optical activity are :

All these substances are known to exist in three forms :

1.         One rotating the plane of polarized light to the left. This form is named Laevorotatory (Latin, laevous = left) or  (–)- form.

2.         One rotating the plane of  polarized light  exactly to the same extent but to the right. This form is named Dextrorotatory (Latin, dexter = right) or (+)- form.

3.         An inactive from which does not rotate the plane of polarized light at all. This is a mixture of equal amounts of (+)– and (–)– forms and hence its optical inactivity. It is named (±)– mixture or Racemic mixture (Latin, racemic = mixture of equal components).

Thus three lactic acids are known. They are : (a) (+)- lactic acid, (b) (–)- lactic acid, and (c) (±)- mixture. Since the (±)- acid is only a mixture of (+)- and (–)- forms, in reality lactic acid exists in two forms, the (+)-lactic acid and the (–)-lactic acid. These two acids are exactly identical in physical and chemical properties but differ in their action on the plane polarized light. They have different sign of specific rotation. Such forms of the same compound which differ only in their optical properties are called Optical isomers and the phenomenon is termed Optical isomerism.

Sometime back d or l method was used to designate the direction of plane polarised light. Thus d is synonymous with (+) and the letter l with (–). The three optical isomers of lactic acid, for example, could be represented as :d-lactic acid instead of (+)– lactic acid ; l-lactic acid instead of (–)– lactic acid; and dl-lactic acid instead of (±)– lactic acid.

The stereoisomers, which are related as mirror image-object are called enantiomers. One isomer and its enantiomer are mirror images of each other and they are not super imposable. Which of the two forms drawn is dextro or laevo, cannot be known by looking at their structures, it can only be determined experimentally using polarimeter. An equimolar mixture of two enantiomers of lactic acid shows no rotation of plane polarized light, thus it is optically inactive. It is called racemic form or racemic mixture and is designated as (±). Physical properties of racemic mixture are different from the properties of pure enantiomer.

Asymmetric Carbon Atom

A carbon atom is described as being asymmetric when four different atoms or groups are bonded to it. Thus it can be represented as

An asymmetric carbon in formula is usually indicated by an asterisk (*) placed near it. All organic compounds containing one asymmetric carbon atom (lactic acid, amyl alcohol, etc.) are optically active.

Asymmetric or Dissymmetric Molecules or Chirality

An asymmetric object cannot be superimposed on its mirror image. Thus the right hand produces a mirror image which is identical with your left hand. The two hands are nonsuperimposable which is clearly evident if you try to put your right hand in the left-handed glove. On the other hand, a symmetric object like a ball can be superimposed on its mirror image which is another similar ball. Example of asymmetric objects:

Thus the molecule of bromochlorofluoromethane is asymmetric because it is non-superimposable on its mirror image.

Chirality: This term has been recently used to describe such molecules that have no elements of symmetry (plane of symmetry or centre of symmetry). Thus asymmetrical molecules are also called chiral molecules. An asymmetrical carbon is a chiral centre.

Criteria to display optical activity

Although the ultimate criterion to know whether a compound will exhibit optical activity or not is the non-superimposability of the mirror image (chirality) but other tests may also be used that are simpler to apply. One such test is the presence of any element of symmetry like plane of symmetry, centre of symmetry.

(i)            Plane of symmetry: A plane of symmetry (also called a mirror plane) is a plane passing through the molecule such that the molecule is divided into 2 equals parts, one part being the mirror image of other (the plane acting as a mirror). For example,

Such an isomer is called meso isomer, which is optically inactive due to the presence of plane of symmetry.

(ii)           Centre of symmetry: A center of symmetry is an imaginary point within the molecule such that a straight line drawn from any part or element of the molecule to the center and extended an equal distance on the other side encounters an equal part or element. For example, 2, 4-dimethyl cyclobutane-1, 3-dicarboxylic acid possess a centre of symmetry, which is the centre of the ring. Centre of symmetry can be at the centre of molecule or over an atom or midway between a bond.

Another important example of the compound having a centre of symmetry is the trans form of dimethyl keto piperazine. The compound exists in two forms-cis and trans. The cis form of the compound exists in two enantiomeric forms but the trans form has a centre of symmetry and therefore, it is optically inactive.

Similarly, a-truxillic acid is optically inactive because of the presence of a centre of symmetry.

It must be noted that only even-membered rings possess a centre of symmetry. It is not found in odd-membered rings. Let us see the optical activity of 2-butanol.

Structures I and II are mirror images of each other and are not super imposable. They are enantiomers of 2-butanol. A pair of enantiomers is always possible for molecules that contain one tetrahedral carbon atom with four different groups attached to it. The carbon atom C2 is called a stereocentre.

2,3-dibromopentane has two asymmetric carbon atoms. The total number of stereoisomers is 2n where n is the number of dissimilar asymmetric C atoms. The stereoisomers are

Structures (1) and (2) are mirror images of each other and so are enantiomers. Structures (3) and (4) are also mirror images of each other and they form another set of enantiomers, all stereoisomers 1 to 4 are optically active. Structures (1) and (3) are stereoisomers but they are not mirror images of each other. They are called diastereomers and they have different physical properties like melting point, boiling point and solubilities.

The compound, although it consist chiral centers but found to be optically inactive due to certain element of symmetry is known as Meso compound.

You will be astonished to see that the compounds even if they do not have the optically active carbon can still show optical activity. eg; allenes, cumulenes, biphenyls, spiran derivatives.

By preliminary examination, it seems that the compound possess plane of symmetry and thus, it should be optically inactive. But on closer observation, it is revealed that it is optically active due to the following reasons.

Therefore, within the molecule two distinct planes arise and any one or all of them may be plane of symmetry or may be none. Very clearly none is the plane of symmetry as R1, R2, R3, R4 are all different. If R1 = R2, R3 ¹ R4 then X-Z plane within the compound will bisect it into two equal halves. So, X-Z plane will be the plane of symmetry. For the compounds with cumulative odd number of double bonds and different terminal groups will not show optical activity because clearly the terminal carbon lie in the same plane. Therefore, the compounds become optically inactive. Draw the orbital picture and see on your own.

For the biphenyl compounds having all sp2 hybridised carbon atoms if the o, o¢-substituents are very bulky then to release steric strain, the rotation around C-C bond axis takes place causing loss of planarity of the compound. For example,

 

But in the above compound, there is no plane or centre of symmetry, so it is optically active.

 

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