Isomerism

Isomerism

In chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. The phenomenon is called isomerism.

In other word, isomer any of two or more substances that are composed of the alike elements in the same proportions but differ in properties because of differences in the arrangement of atoms.

Examples: Pentane, 2-methylbutane, and 2,2-dimethylpropane are structural isomers of each other.

There are two main forms of isomerism:

·          Structural isomerism and

·          Stereoisomerism (spatial isomerism).

Structural isomerism

In structural isomers, sometimes referred to as constitutional isomers, the atoms and functional groups are joined together in different ways. Structural isomers have different IUPAC names and may or may not belong to the same functional group. This group includes chain isomerism whereby hydrocarbon chains have variable amounts of branching; position isomerism which deals with the position of a functional group on a chain; and functional group isomerism in which one functional group is split up into different ones.

For example, there are two chain isomers of butane, C₄H₁₀. In n-butane, CH₃CH₂CH₂CH₃, the carbon atoms are joined in a so-called straight, or unbranched, chain. In isobutane, CH₃CH(CH₃)₂, the carbon atoms are joined in a branched chain; the isobutane molecule can be visualized as a carbon atom bonded to one hydrogen atom and to three methyl (CH₃) groups.

Structural isomerism is of five types

Chain isomerism

Chain isomerisms have the same in molecular formula but differ in order in which the carbon atoms are bond to each other.

Example: In n-butane, CH₃CH₂CH₂CH₃

Positional isomerism

Positional isomerisms have the same in molecular formula but differ in the position of a functional group on the carbon chain.

Example: 1- Bromobutane (CH₃CH₂CH₂CH₂Br), 2- Bromobutane (CH₃CHCH₂CH₃) Br

Functional isomerism

Functional isomerisms have the same in molecular formula but different functional group.

Example: Ethylalcohol (CH₃CH₂OH) & Dimethylether (CH₃ O CH₃)

Tautomerism

Tautomers are structural isomers of the same chemical substance that spontaneously interconvert with each other, even when pure. They have different chemical properties, and consequently, distinct reactions characteristic to each form are observed. If the interconversion reaction is fast enough, tautomers cannot be isolated from each other. The phenomenon is called Tautomerism. Tautomerism is a special case of structural isomerism and can play an important role in non-canonical base pairing in DNA and especially RNA molecules.

An example is when they differ by the position of a proton, such as in keto/enol tautomerism, where the proton is alternately on the carbon or oxygen.

Fig : Tautomerism

Stereoisomerism

Stereoisomerism occurs when two or more molecules have the same basic arrangement of atoms in their molecules but differ in the way the atoms are arranged in space. There are two types of stereoisomerism. The first type, geometric isomerism, may occur when a compound contains a double bond or some other feature that gives the molecule a certain amount of structural rigidity. Geometric isomers differ in physical properties such as melting point and boiling point.

Stereoisomerism is two types

1. Geometric isomerism or cis-trans isomerism

2. Optical isomerism

Cis-trans isomerism or geometric isomerism

In organic chemistry, cis-trans isomerism or geometric isomerism or configuration isomerism or is a form of stereoisomerism describing the orientation of functional groups within a molecule. In general, such isomers contain double bonds, which cannot rotate, but they can also arise from ring structures, wherein the rotation of bonds is greatly restricted. Cis and trans isomers occur both in organic molecules and in inorganic coordination complexes.

The terms cis and trans are from Latin, in which cis means "on the same side" and trans means "on the other side" or "across". The term "geometric isomerism" is considered an obsolete synonym of "cis-trans isomerism" by IUPAC. It is sometimes used as a synonym for general stereoisomerism (e.g., optical isomerism being called geometric isomerism); the correct term for non-optical stereoisomerism is diastereomerism.

Fig: geometric isomerism

Optical isomerism

The second type of stereoisomerism is optical isomerism. When plane-polarized light is passed through an optical isomer it is rotated into a different plane of polarization. Optical isomers, also known as chiral molecules or enantiomers, exhibit this optical activity in varying degrees. Optical isomers of a given compound are often identical in all physical properties except the direction in which they rotate light. The molecules of optical isomers are asymmetrical. The simplest optical isomers have a single "asymmetrical carbon atom" in their molecules. An asymmetrical carbon atom has four different atoms or radicals bonded to it, arranged approximately at the corners of a tetrahedron centered on the carbon atom. 

For example, there are two optical isomers of lactic acid. The atom and radical to either side of the carbon atom are visualized as being above the plane of the paper, Thus it is seen that the two molecules are mirror images of each other and, each being asymmetrical, cannot be superposed on each other. The d- and l- prefixes stand for dextro (right) and levo (left). Two optical isomers, such as these, whose molecules are asymmetrical and are mirror images of each other, are called enantiomorphs. When equal amounts of d- and l-enantiomorphs are mixed, the mixture has no effect on polarized light; such a mixture is called racemic.

Fig: optical isomerism

Racemic Mixture

In chemistry, a racemic mixture, or racemate, is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule. The first known racemic mixture was "racemic acid", which Louis Pasteur found to be a mixture of the two enantiomeric isomers of tartaric acid.

A racemic mixture is denoted by the prefix dl- or (±) - (for carbohydrates the prefix dl- may be used), indicating an equal (1:1) mixture of dextro and levo isomers. If the ratio is not 1:1 (or is not known), the prefix d/l-, (+)/(−) or d/l- (with a slash) is used instead.

Properties

A racemate is optically inactive, meaning that there is no net rotation of plane-polarized light. Although the two enantiomers rotate plane-polarized light in opposite directions, the rotations cancel because they are present in equal amounts. In contrast to the two pure enantiomers, which have identical physical properties except for the direction of rotation of plane-polarized light, a racemate sometimes has different properties from either of the pure enantiomers. Different melting points are most common, but different solubilities and boiling points are also possible. Pharmaceuticals may be available as a racemate or as the pure enantiomer, which might have different potencies.