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Organic compounds are mainly composed of carbon atoms which are arranged in a different manner to give certain geometry to the molecule. On the basis of structure, organic compounds can be classified as open chain compound and cyclic compounds. Open chain compounds have branched or un-branched carbon chain in the molecule such as hydrocarbons. On the contrary, cyclic compounds have a ring structure. Cyclic compounds can again classify as heterocyclic compounds and aromatic compounds. Aromatic compounds have conjugated pi electron system which stabilise the molecule.

The presence of any functional group in the molecule affects the physical and chemical properties of the molecule such as hydroxy group in alcohol, -COOH group in carboxylic acid etc. Chemical formula of organic molecules can be represented in different ways such as structural formula, condense formula, molecular formula etc. The structural formula of molecule represents the bonding of carbon atoms in the molecule such as the structural formula of ethane is $CH_3$–$CH_3$. In the line formula of carbon atoms are represented by dots and bonds by lines. The molecular formula of organic molecules indicates the number of atoms in the molecule. For example, the molecular formula of ethane is $C_{2}H_{6}$. Organic compounds which have same molecular formula but different structural formula also show different chemical and physical properties, known as isomers. Let’s discuss about isomers and isomerism.


Isomerism Definition

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Organic compounds are represented by using their molecular and structural formula which shows the number of bonded atoms and their skeleton. There are many organic compounds which have different properties but the same molecular formula.

For example, ethyl alcohol formulated as C2H5OH and dimethyl ether as CH3OCH3. Both compounds have a different functional group (alcoholic group and ether group), therefore show different physical and chemical properties. But the molecular formula for both compounds is the same, that is, C2H6O. Such type of molecules which have the same molecular formula but different molecular structures are known as isomers of each other and this phenomenon is known as isomerism.


Isomerism can be broadly classified as; structural isomerism and stereoisomerism which can be further classified in various categories.
Structural Isomerism and Stereoisomerism

Structural Isomerism

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In this type of isomerism, isomers have different molecular structure because of different arrangement of atoms. They are also known as Constitutional isomers. Structural isomers show different order of connectivity between atoms. Therefore they show different branching and functional groups.

For example, propanal and acetone have the same molecular formula; C3H6O but they have different functional groups. Propanal (CH3CH2CHO) has aldehyde group –CHO while acetone (CH3COCH3) has ketonic >C=O group, therefore they show different chemical and physical properties.
Structural Isomerism
Structural isomerism can be further classified into six sub-groups.

1. Chain isomerism

This type of isomerism shows different skeletons of carbon atom due to branching while keeping the same molecular formula. Because of branching, the linkage of carbon atoms in the parent chain is different which results in different molecules. Such type of isomerism is also known as nuclear isomerism. Generally chain isomerism is exhibited by hydrocarbons like alkanes. Here are some examples of chain isomerism with different molecular formula.
  • With molecular formula C4H10: n-Butane and 2-Methylpropane

Chain Isomerism C4H10

  • With molecular formula C5H12: Pentane, 2-Methylbutane and 2,2-dimethylpropane

Chain Isomerism C5H12

  • With molecular formula C7H16: Heptane, 2-Methylhexane, 3-Methylhexane, 2,3-Dimethylpentane, 2,4-Dimethylpentane, 2,2-Dimethylpentane, 3,3-Dimethylpentane, 3-Ethylpentane, 2,2,3-Trimethylbutane

Chain Isomerism C7H16

  • With molecular formula C6H14: Hexane, 2-Methylpentane, 3-Methylpentane, 2,3-Dimethylbutane, 2,2-Dimethylbutane

2. Positional isomers

In such type of isomerism, isomers have the same functional groups or multiples of them at different positions. For example; pent-1-ene(CH2=CH-CH2-CH2-CH3) and pent-2-ene(CH3-CH=CH-CH2-CH3) have different positions of the double bond in molecules but show the same molecular formula. Therefore they are positional isomers of each other. Similarly propan-1-ol and propan-2-ol differ in the position of the hydroxyl group at C-1 and C-2 respectively, hence they show position isomerism.
Positional Isomers

3. Functional isomerism

Such type of isomers have different functional group, and therefore belong to different families. For example; alcohols and ethers are functional isomers of each other. Some other pairs of functional isomers are as follows.
  • Diens, allenes and alkynes
  • Nitroalkane and alkylnitrile
  • Primary, secondary and tertiary amines
  • Cyanide and isocyanides
  • Aromatic alcohols, phenols and ether
4. Metamerism

In this type of isomerism, isomers differ in structure because of differences in the distribution of carbon atoms about the functional group. Ether and secondary amines are the best examples of metamerism. For example; ethoxypropane and 1-ethoxypropane are metamers of each other.

CH3CH2CH2-O-CH2CH3 and CH3-CH2-O-CH2-CH2-CH3
Ethoxypropane 1-Ethoxypropane

5. Tautomerism

It is also called as keto-enol isomers as in this type of isomerism,
isomers are in dynamic equilibrium and converts in keto and enol form due to
1,3-migration of hydrogen atom. For example vinyl alcohol and acetaldehyde are tautomers of each other. Vinyl alcohol shows enol form while acetaldehyde
consists of carbonyl group and represents keto form.

6. Ring-chain isomerism

In this type of isomerism one isomer possesses an open chain structure while the other has a cyclic. For example; propene and cyclopropane are ring-chain isomers of each other. Other examples of ring-chain isomerism are propyne and cyclopropene, But-1-ene, cyclobutane and methylcyclopropane.

Ring Chain Isomerism

Geometrical Isomerism

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  1. Stereoisomerism is a type of isomerism in which isomers have different arrangement of atoms in a three dimensional space. It can be classified in to two types geometrical and optical isomerism. Geometrical isomerism exhibited by unsaturated hydrocarbons which show restricted rotation of carbon-carbon bond due to the presence of double bond in molecule.
  2. Since pi bond between carbon atoms is formed by side way overlapping of p-orbitals and oriented perpendicular to sigma bond, it restricts the free rotation of carbon-carbon sigma bond and creates different configurations of molecule.
  3. These configurations differ in the spatial arrangement of groups bonded to double bonded carbon atoms and called as geometrical isomers of each other.
  4. Hence the stereoisomerism exhibited by alkenes due to difference in the spatial arrangement of groups about the double bonded carbon atoms is known as geometrical isomerism and isomers are known as geometrical isomers.
  5. These isomers cannot inter convert into each other by free rotation of the carbon-carbon bond, therefore they can exist in two possible configurations one in which similar groups placed on same side cis isomers and another has similar groups on opposite sides known as trans isomer.

Geometrical Isomerism

Hence for geometrical isomerism, there must be a double bond in the molecule and the two atoms or groups bonded on double bonded atoms should be different. Alkenes with general formula; abC=Cab, abC=Ccd, abC=Cax exhibit geometrical isomerism.

While compounds like aaC=Cab or aaC=Cbb cannot show this isomerism. Both geometrical isomers, cis and trans are not inter convertible as the conversion requires about 284 kj/mol of energy which is not available at room temperature.

Cis Trans Isomerism

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Geometrical isomers which are also known as cis-trans isomers show different spatial arrangements of atoms about the double bonded carbon atoms. In cis-isomers, similar groups are seated on the same side and in trans-isomers they are placed on opposite sides.

For example, 2-butene exhibits cis-trans isomerism. In cis-isomer both methyl groups are placed on the same side while in trans-isomer methyl group are placed on opposite sides.
2 Butene Exhibits Cis Transisomerism
Cis-trans isomers show different physical properties like melting point, boiling point, dipole moment, solubility etc. Cis-isomers are more polar compared to trans-isomers because individual dipole does not cancel each other. For example, the dipole moment of cis-2-butene is 0.33 D whereas that of trans-isomer is almost zero. Due to polarity, cis-isomers have a higher boiling point than trans-isomers, but the melting point of trans-isomers is more than cis-isomers due to their symmetrical nature.

Some common examples of cis-trans isomers are as follows:

  1. Maleic acid(cis-isomer) and fumaric acid (trans-isomer)
  2. Cis-1,2-Dichlorocyclohexane and trans-1,2-Dichlorocyclohexane
  3. Cis-1,2-Dichlorocyclopentane and trans-1,2-Dichlorocyclopentane
  4. Cis-1,2-Dicarboxycyclopropane and trans-1,2-Dicarboxycyclopropane
  5. Cis-2-Pentene and trans-2-Pentene
  6. Cis-1,2-Dichloroethene and trans-1,2-Dichloroethene
  7. Oleic acid (cis-isomer) and Elaidic acid (trans-isomer)
  8. Cis-2,3-Dichlorobut-2-ene and trans-2,3-Dichlorobut-2-ene

Coordination compounds can also show cis-trans isomerism due to different possible geometrical arrangements of ligands in heteroleptic complexes. Square planer and octahedral complexes show this type of isomerism. Some examples of cis-trans isomers are platinum amine complex and tetraamminedichlorocobalt(III) ion.
Cis Trans Isomerism

Optical Isomerism

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  1. When plane polarized light is passed through the liquid or dissolved forms of certain substances, the light emerging out does not oscillate in the same plane.
  2. The plane of oscillation gets rotated through some angle towards the left or right of the original plane of oscillation.
  3. The substance which can rotate the plane of polarized light is known as an optically active substance.
  4. The substance which rotates the plane in clockwise direction, towards the right is known as dextro rotatory substance.
  5. This is indicated by putting letter d or (+) sign before the name of the substance. The substances which rotate the plane towards the left, i.e. in anticlockwise direction are known as levo rotatory substance and are indicated by putting l or (-) before the substance name.
  6. For a molecule to be optically active there must be chiral carbon atom in the molecule and no plane of symmetry.

A carbon atom bonded with four different atoms or groups in a molecule is called as chiral carbon atom or chirality center or asymmetrical carbon or stereocenter. For example, center carbon atom of lactic acid is chiral in nature and indicated with an asterisk (*). Similarly amino acid molecules are also chiral due to chiral carbon atom.

Optical Isomerism
Chiral molecules are non-superimposable on their mirror image. The stereoisomerism
related to each other as non-superimposable mirror are known as enantiomers. The enantiomer rotates the plane polarized light through same angle but in opposite direction. Thus if one enantiomer is dextrorotatory, other one will be levorotatory. Enantiomers show same physical and chemical properties but different optical properties.


Linkage Isomerism

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This type of isomerism is exhibited by coordination compounds with ambidentate ligands. A unidentate ligand which can bind to the central metal atom through two possible donor atoms is known as ambidentate ligand. For example, nitro (-NO2) and nitrito (-ONO) are examples of an ambidentate ligand. It can bind with the central metal atom through either nitrogen or oxygen. Similarly other examples of ambidentate ligands are as follow.
  • Cyanon (-CN) and isocyano (NC)
  • Thiocyanato (SCN) and isothiocyanato (NCS)
For example, pentaamminenitrocobalt (III) ion can show linkage isomerism each containing –NO2 group in complex ion. One of the isomer pentaamminenitritocobalt (III) ion is red and another one pentaamminenitrocobalt(III) ion is yellow compound. Both isomers show different chemical properties due to the different linkage of –NO2 group.

Other example of linkage isomers are as follows:
  • [Co(H2O)6SCN] and [Co(H2O)6NCS]
  • [(NH3)2(py)2Co(NO2)2] and [(NH3)2(py)2Co(ONO)2]
  • [Mn(CO)5SCN] and [Mn(CO)5NCS]

Coordination Isomerism

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In coordination compounds, if both the cation and anion are complex, they can show coordination isomerism. Coordination isomers differ in the distribution of ligands in cation and anion. Hence this type of isomerism is because of interchange of ligands between two complex ions. For example, tetraamminezinc(II) tetrachlorocuprate(II) and tetraamminecopper(II) tetrachlorozincate(II) are coordination isomers, as both are inter-convertible into each other by interchange of ligands in cation and anion.

Coordination Isomerism

Other example of coordination isomers are as follow.
  1. [Co(NH3)6][Cr(CN)6] and [Cr(NH3)6][Co(CN)6]
  2. [Co(NH3)6][Cr(C2O4)3] and [Cr(C2O4)3][Co(NH3)6]
  3. [Co(en)3][Cr(CN)6] and [Cr(en)3][Co(CN)6]
  4. [Cu(NH3)4][PtCl4] and [Pt(NH3)4][CuCl4]
  5. [Pt(NH3)4][PtCl4] and [PtCl(NH3)3][PtCl3(NH3)]
More topics in Isomerism
Geometrical Isomerism Linkage Isomerism
Isomerization Structural Isomerism
Cis Trans Isomerism Types of Isomerism
Optical Isomerism Chain Isomerism
EZ Isomerism Conformational Isomerism
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