ISOMERISM
ISOMERISM
Isomer is the existence of different compounds with the same molecular formula but different structural formulas.
Two types of isomerism ;
1) Constitutional (Structural) isomers
isomers with the same molecular formula but differ in the order of attachment of atoms.
2) Stereoisomers
isomers with the same molecular formula but different arrangement of atoms in space.
CONSTITUTIONAL (STRUCTURAL) ISOMERISM
• Isomerism resulting from different ways of attachment of atoms.
• Three types
a) Chain isomerism
b)Positional isomerism
c) Functional group isomerism
CHAIN ISOMERISM
The isomers differ in the structure of the carbon chain / differ in the length of straight chain or branches
They possess the same functional group and belong to the same homologous series.
POSITIONAL ISOMERISM
• These isomers have a functional group/subtituents group in different positions.
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FUNCTIONAL ISOMERISM
Isomers have different functional groups and belong to
different homologous series with the same general formula.
DEFINITION
Stereoisomers: Isomers that have the same constitution (order in which their atoms are connected) but differ in the arrangement of their atoms in space.
Enantiomers: Stereoisomers that nonsuperimposable mirror images of one another.
Diastereomers: Stereoisomers whose molecules are not mirror images of each .
Cis-trans isomers: Stereoisomers due to restricted rotation about C=C bond in alkenes and C-C bond in cyclic compounds
CIS-TRANS ISOMERS
• Cis-trans (geometric) isomers have same molecular formulas and same order of attachment of atoms but differ in how the atoms/groups of atoms are orientated in space.
• Occurs in compounds in which free rotation is prevented by the presence of C=C or a cyclic structure.
• Requirements for cis-trans isomers;
i. restricted rotation about a C=C in alkenes or a C-C single bond in cyclic compounds.
ii. each carbon atom of a site of restricted rotation has two different groups attached to it.
• Cis isomer has two same atom/group of atoms on the same side of the double bond.
• Trans isomer has two same atom/group of atoms on the opposite sides of the double bond.
• If one of the doubly bonded carbons has two identical groups, cis-trans isomerism is not possible.
• cis-trans isomers have similar chemical properties.
• but different physical properties; because different spatial arrangement of the groups & polarity of the molecules.
Stability: trans-isomer > cis-isomer
Melting point: cis-isomer < trans- isomer
Boiling point: cis-isomer > trans- isomer
CHIRAL
•Left hand and right-hand are mirror images of each other and non-superimposable. Thus, chiral.
•Chirality arises from a lack of symmetry.
• Lack of symmetry is called asymmetry.
•Optically active compounds are chiral compounds
OPTICAL ACTIVITY
• Some of the stereoisomers are optically active. The compound differ in the effect on plane-polarised light.
• Light after passes through a polarizing filter, vibrates in one direction, the light is called plane-polarised light.
• These stereoisomers rotate the polarized light & the angle is measured using a polarimeter.
• Optically active compounds exist in two isomeric forms.
• Isomer rotates clockwise is dextrorotatory isomer; (+) isomer. Isomer rotates anticlockwise is laevorotatory isomer; (-) isomer
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The requirements for optical active compounds :-
i. molecule contains a chirality centre /stereogenic centre / asymmetric carbon(*).
Chirality centre :
a sp3-hybridised carbon atom with 4 different groups attached to it.
ii) molecule is not superimposable with its mirror image.
ENANTIOMERS
• Enantiomers : a pair of stereoisomers with structures that are mirror-images of each other but nonsuperimposable.
• The pair has identical chemical and physical properties.
• The pair differ in the direction of rotation of plane-polarised light but the magnitude of rotation is the same.
RACEMIC MIXTURE
A racemic mixture or racemate is an equimolar (50:50) mixture of enantiomers.
• The mixture shows zero optical rotation (no net rotation of plane-polarized light).
• The rotation of (+)isomer cancels the (-) isomers
• The mixture is optically inactive
Applications of chiral compounds in daily life.
• Many drugs are optically active, with one enantiomer only having the beneficial effect, the other can even be harmful, e.g. Thalidomide.
• In the 1960’s thalidomide was given to pregnant women to reduce the effects of morning sickness.
• This led to many disabilities in babies and early deaths in many cases.
Other examples:
(-) Dopa is used for treatment of Parkinson’s disease but (+) dopa is toxic to human.
(-)-Nicotine is more toxic than (+)-nicotine.
(+)-adrenaline is more active than (-)adrenaline in constricting blood vessel.
