TYPES OF ISOMERISM

Organic compounds represent 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 same molecular formula.

For example, ethyl alcohol formulated as C₂H₅OH and dimethyl ether as CH₃OCH₃. Both compounds have different functional group (alcoholic group and ether group), therefore show different physical and chemical properties. But the molecular formula for both compounds is same that is C₂H₆O. Such type of molecules; have 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.

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 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 can be further classified in six sub-groups.

1. Chain isomerism

This type of isomers shows different skeletons of carbon atom due to branching while has same molecular formula. Because of branching, the linkage of carbon atom in parent chain is different which results different molecules. Such type of isomerism is also known as nuclear isomerism. Generally chain isomerism exhibits by hydrocarbons like alkanes. Here are some examples of chain isomerism with different molecular formula.

• With molecular formula C₄H₁₀: n-Butane and 2-Methylpropane

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

• 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

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

3. Functional isomerism

Such type of isomers have different functional group, therefore belongs to different families. For example; alcohols and ethers are functional isomers of each other. Some other pairs of functional isomers are as follow.

• 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 difference in distribution of carbon atoms about the functional group. Ether and secondary amines are best example of metamerism. For example; ethoxypropane and 1-ethoxypropane are metamers of each other.

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.

1. Stereoisomerism is a type of isomerism in which isomers have different arrangement of atoms in three dimensional space. It can be classified in 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 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.

Hence for geometrical isomerism, there must be a double bond in molecule and 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.

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 same side while in trans-isomer methyl group are placed on opposite sides.

Cis-trans isomers show different physical properties like melting point, boiling point, dipole moment, solubility etc. Cis-isomers are more polar compare 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 high 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 follow.

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

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

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 optically properties.

This type of isomerism exhibits by coordination compounds with ambidentate ligands. A unidentate ligand which can bind to central metal atom through tow possible donor atoms is known as ambidentate ligand. For example, nitro (-NO₂) and nitrito (-ONO) are example of ambidentate ligand. It can bind with 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 follow.

• [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]

In coordination compounds; if both cation and anion is complex, they can show coordination isomerism. Coordination isomers differ in 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 interconvertible into each other by interchange ligands in cation and anion.

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)]

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