OH OH O H H H H HO O H H H H H OHHO OH HO OH H OH OH H H H O OH H O H H H HO OH HHO H H OH OH OH H HO OH H O OH HH H H O H OH HO OHHO H H OH OH OH H H HO OH
Isomerism is the phenomenonwhereby certain compounds, withthe same molecular formula, exist indifferent forms owing to theirdifferent organizations of atoms. The concept of isomerism illustratesthe fundamental importance ofmolecular structure and shape inorganic chemistry.
e.g. The formula C4H10 represents two possible structural formulae, butane and methylpropane: STRUCTURAL ISOMERS The structural isomerism or constitutionalisomerism arises when atoms within a moleculeare arranged in different orders. The structuralisomers have the same molecular formula butdifferent structural formulae. The structural isomers usually show differentphysical and chemical properties. The structural isomerism is further divided into: •Chain isomerism •Positional isomerism •Functional isomerism
CHAIN ISOMERISMarises due to different arrangements of carbon atoms leading to linear and branched chains.The chain isomers have same molecular formula but different types of chains i.e., linear and branched.The chain isomers have almost similar chemical properties but different physical properties.For example, the branched chain isomers have lower boiling points than that of their linear counterparts. It is because, the linear ones have more surface area of contact and hence the intermolecular forces of attraction are maximum.
E.g. Two chain isomers possible with the molecular formula, C4H12.i.e., n-butane: a linear chain isomer. isobutane (or 2-methylpropane): a branched isomer
The positional isomerism arises due todifferent positions of side chains, substituents,functional groups, double bonds, triple bondsetc., on the parent chain.E.g. 1) Propyl chloride and isopropyl chloride arethe positional isomers with the molecularformula, C3H7Cl. These isomers arise due todifference in the position of the chloro groupon the main chain.
Ex 2) In the following positional isomers i.e., but-1-ene and but-2-ene the position of double is different but they have same molecular formula, C4H8.Ex 3) The functional group, -OH is at different positions on the main chain of the following alcohols. They have the same molecular formula, C3H7OH.
Ex. 4) In the following positional isomers i.e., ortho xylene, meta xylene and para xylene, the relative positions of methyl groups on the benzene ring are different
The functional isomers have the same molecular formula but possess different functional groups.The functional isomers show different physical as well as chemical properties.*different Functional groupse.g. the molecular formula C3H60 could either apropanal(aldehyde) or propanone(ketone)
For C3H6O2 can form either be a carbolylic acid oran ester
STEREOISOMERSStereoisomers have the same structure and bond order but their atoms and groups of atoms are arranged differently in space. They have different spatial arrangements and their molecules are not superimposable. Types of Stereisomers: 1] Geometric Isomerism (GI) Involves a double bond, usually C=C, that does not allow free rotation about the double bond (unlike a C-C single bond).
Geometric IsomersThey are not superimposable. e.g. cis-but-2-ene and trans-but-2-ene Each the 2 doubly bonded C atom should have 2 different substituents
GI orientations: cis (z) = bulky groups are in the same side trans(E) = bulky groups are in the opposite side GI calculation: GI = 2n where n=no. of double bonds
Ex. How many GI in 1,4-dichlorobuta-1,3-diene Cl-CH=CH-CH=CH-Cl GI= 22 = 4 (cis-cis;cis-trans;trans-cis;trans-trans)
2. ENANTIOMERSEnantiomers are stereoisomers which are mirror image of each other and non-superimposable. There should be at least one chiral carbon(sp3C where it contains 4 different substituents) A chiral compound is an optically active, which can rotate a plane of polarized light. Achiral compound is superimposable on its mirror image Ex: glyceraldehyde & lactic acid
3] DIASTEREOMERS isomers that are non-mirror compounds but contains at least 2 chiral carbons within the molecule. No.of Stereoisomers = 2n where n is the no, of chiral carbon Ex: 2,3-dichloropentane CH3–CH–CH –CH2 – CH3 │ │ Cl Cl No. of stereoisomers= 22 = 4 isomers