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Types of Isomerism

All the chemical molecules are made of different atoms which are bonded with each other through chemical bonds. The chemical bonds can be either covalent type or ionic type. The covalent bond is formed by equal sharing of valence electrons from the bonded atoms. The number of constituent atoms in a chemical molecule can be represented with the help of structural formula. For example; one molecule of ethanol is made up of 2 C, 6 H and one O atom. Hence the molecular formula of ethanol would be C2H6O. But the molecular formula of a molecule does not provide information about the bonding between the constituent atoms therefore we are using structural formula which represents atoms and chemical bonds between them. For example the structural formula of ethanol would be as given below. 

Structural Formula of Ethanol

So the structural formula of ethanol molecule indicates that there is a methyl group, one methylene group and one hydroxyl group in one ethanol molecule. There are some molecules which have same molecular formula but different structural formula. They are called as isomers of each other and this phenomenon is called as isomerism. In this article; we will discuss about the types of isomerism and also isomerism I coordination compounds.


Different Types of Isomerism

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Isomerism is mainly classified in two types; structural and stereoisomerism. Structural isomers have same molecular formulae but different structural formulae. There are six types of structural isomerism. Let’s discuss different types of structural isomers. Chain isomers have same molecular formula but they have different number of carbon atoms in the parent chain of the molecule. For example; n-pentane and 2-methylbutane have same number of carbon atoms in their molecular formulae but there are 5 carbon atoms in the parent chain of n-pentane and 4 carbon atoms in the parent chain of 2-methylbutane.

Another type of structural isomerism is position isomerism in which the position of functional group varies but the molecular formula formulae remain same for isomers. For example; 2-butene and 1-butene are positional isomers in which the position of double bond varies which effects the physical properties and some of the chemical properties of molecule. Similarly position of functional group varies in different molecules also make positional isomers such as 1-propanol and 2-propanol.

Functional isomerism is another type of structural isomerism in which the functional groups of isomers are different but the molecular formula remains same. For example; alcohol and ether, ester and carboxylic acids are functional isomers of each other. Since they have different functional groups therefore they exhibit different physical and chemical properties. Metamerism is exhibited by the molecules which have different alkyl group at both sides of the functional group such as ether, secondary amines.

They have different alkyl groups but their molecular mass remains same therefore their physical and chemical properties are almost same. Tautomers are also called as Keto-enol isomers as they can interchange in to each other by simple shifting of H-atom and a bond. Ring-chain isomers have same molecular formulae but different arrangement of atoms. One isomer is open chain compounds whereas another must have ring structure of carbon atoms. Stereoisomerism can be classified in two types; cis-trans or geometrical isomerism and optical isomerism.

Types of Isomerism With Examples

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It is easy to understand the isomerism with appropriate examples of molecules. Chain isomerism is mainly found in alkanes such as n-butane and 2-methylpropane which have same molecular formulae but different structural formulae with different number of carbon atoms in the parent chain. 
Chain Isomerism

Similarly ethanol and methoxymethane are good example of functional isomers which have same molecular formula but have different functional groups. Because of the presence of different functional groups, they show different physical and chemical properties.
  • $CH_{3}CH_{2}OH$  (Ethanol)
  • $CH_{3}OCH_{3}$  (Methoxymethane)
Alkenes with different position of double bond in the parent chain such as 1-butene and 2-butene are good examples of positional isomers.
  • $CH_{3}CH$ = $CHCH_{3}$    ( 2-butene)
  • $CH_{3}CH_{2}CH$ = $CH_{2}$ ( 1-butene)
Metamers are commonly found in ethers and secondary amines. For example; diethyl ether and methyl propyl ether are metamers as they have same molecular formula but different structural formulae as given below;
  • $CH_{3}CH_{2}OCH_{2}CH_{3}$ ( Diethyl ether)
  • $CH_{3}OCH_{2}CH_{2}CH_{3}$  (Methyl propyl ether)
Tautomers are also keto-enol forms which are inter-convertible into each other with the shifting of H atom. 


3 Types of Isomers

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On broad scale we can classify isomers as structural, geometrical and optical isomers. Structural isomers have different structural formulae but same molecular formulae. On the contrary, geometrical and optical isomers have same molecular and structural formulae but they have different arrangement of atoms in three dimensional space which make them different in their few properties.

Cis-trans or geometrical isomerism is commonly found in alkenes with abC=Cab type of formula. Here if same groups are placed at the same sides of the double bonded carbon atoms, the isomer would be called as cis-isomer. If the same groups are placed at the opposite sides of the double bonded carbon atoms, they are said to be trans-isomers. Cis and trans isomers cannot convert into each other at room temperature because the double bonded carbon atoms cannot rotate freely. The restricted rotation between carbon atoms forms cis and trans-isomers. Optical isomers are non-super imposable mirror images of each other. They show almost same physical and chemical properties but different optical prpeprties.

Types of Isomerism in Coordination Compounds

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Coordination compounds are also called as complex compounds which are formed by coordination linkage between positively charged metal ion and ligand which is an electron donor species like atom or molecule or ion with extra pair of electrons.

Metal ions have empty or incomplete orbitals which can accept electrons from some electron rich species. Since the bond is formed from ligand to metal ion therefore it is called as dative bond. Coordination compounds also show different kinds of isomerism such as structural isomerism, geometrical isomerism and optical isomerism.

There are mainly four types of structural isomers; linkage, hydrate, ionization and coordination isomers in coordination compounds. Ionization isomers have same composition of ions but they give different ions in solution such as pentaaminebromidocobalt(III) sulphate $[Co(NH_{3})_{5}Br]SO_{4}$ gives sulphate ions which form precipitate with Barium chloride whereas pentaaminesulphatecobalt(III) bromide $[Co(NH_{3})_{5}SO_{4}]Br$ gives negative test for sulphate ion in solution and form precipitate with silver nitrate which confirms the presence of of bromide ions. Solvate or hydrate isomers have the same composition with different number of solvent ligand molecules such as $[Cr(H_{2}O)_{6}]Cl_{3}$, $[Cr(H_{2}O)_{5}Cl]Cl_{2}] \cdot H_{2}O$ and $[Cr(H_{2}O)_{4}Cl_{2}]Cl \cdot 2H_{2}O$. Linkage isomers have ambidentate ligands like $NO_{2}$. Coordination isomers have different distribution of ligands between the cation and the anion like $[Co(NH_{3})_{6}][Cr(CN)_{6}]$ and $[Cr(NH_{3})_{6}][Co(CN)_{6}]$.
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