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Isomerism

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Isomerism occurs when two or more different compounds share the same molecular formula and different structural formula. When the atoms or groups are linked together in different ways the phenomenon is referred to as structural isomerism.

When the atoms are linked together in the same way but the spatial arrangements are different, the type of isomerism is known as stereoisomerism. There are two distinct types of stereoisomerism; geometric isomerism and optical isomerism.

Definition

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"The two molecules are said to be isomers if they are made up of the same number and types of atoms, but differ in the arrangements of these atoms."

Isomer is the name given to molecules that contain the same type and number of atoms but that differ in structure. If the isomers differ in the way in which their atoms are bonded in the molecule they are described as structural isomers. In stereoisomers, the spatial arrangements of substituents differ relative to a bond or the isomerism is due to the presence of a chiral center in the molecule.

Structural Isomerism

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Structural isomers have the same molecular formula but have atoms bonded together in a different order. They have different formulas. There are various ways in which structural isomerism can arise.

Structural Isomerism

1. Chain isomerism


There is only one alkane corresponding to each of the molecular formula. With four or more carbon atoms in a chain different arrangements are possible. For example,

Chain Isomerism

2. Position isomerism


This can occur where there is an atom or group of atoms substituted in a carbon chain or ring. These are called functional groups.
Isomerism occurs when the functional group is situated in different positions in the molecule. For example, isomers of C6H4Cl.

Position Isomerism

3. Functional group isomerism


It is sometimes possible for compounds with the same molecular formula to have different functional groups and because they have different functional groups they will belong to different homologous series. For example the compound with molecular formula C3H8O.

Functional Group Isomerism

Stereoisomerism

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When isomerism is caused by the different arrangements of atoms or groups in space, the phenomenon is called stereoisomerism. The stereoisomers have the same structural formulas but differ in the arrangement of atoms in space. In other words, stereoisomerism is exhibited by such compounds which have the same structural formula but differ in configuration.

Stereoisomerism is of two types :
  1. Geometrical or cis-trans isomerism
  2. Optical isomerism
Stereoisomerism

Geometric Isomerism

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Geometric isomerism also known as cis-trans isomerism is one type of stereoisomerism. Stereoisomers have identical molecular formula and the atoms are bonded together in the same order, but the arrangement of atoms in space is different in each isomer.

Geometric isomerism depends on the arrangement of the groups or atoms around the double bond, for example, but-2-ene can have two structures.

Geometric Isomerism

Optical Isomerism

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Optical isomers are two compounds with same structural formula, but one isomer is the mirror image of the other. Optical isomerism occurs when four different groups of atoms are joined to a carbon atom by four single covalent bonds. Optical isomerism occurs because of the tetrahedral bonding around a carbon atom. If the molecule was flat with 90o angles then two different isomers would not be obtained.

An example is amino acid H2N-CH(CH3)-COOH. This compound has a chiral centre, which is shown below. The structures of its two optical isomers are shown below.

Optical Isomerism

Cis-Trans Isomerism

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Cis-trans isomerism (sometimes called geometric isomerism) is one kind of stereoisomerism and it is most easily understood with a specific case. Consider, for example the possible structures of 1,2-dimethylcyclopentane. 

Cis trans isomers differ from one another only in the way that the atoms or groups are positioned in space. Unlike conformers they are not readily inter converted by rotation around carbon-carbon bonds.

Cis-Trans Isomerism

Cis trans isomers can be separated from each other and kept separate usually without inter conversion at room temperature. Cis trans isomerism can be important in determining the biological properties of molecules.

Linkage Isomerism

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The isomerism in which a ligand can form linkages with metal through different atoms. For example, Nitro group (-NO2) can link to metal either through nitrogen atom or through oxygen atom.

For example, [Co(NH3)5ONO]Cl2 - Pentaaminenitrito-O-cobalt(III) chloride and [Co(NH3)5NO2]Cl2 - Pentaaminenitrito-N-cobalt(III) chloride are linkage isomers.