Stereoisomers

 Isomers are different compounds that have

the same molecular formula.Molecules that have the

same number of the same

kind of atoms (and hence

the same molecular

formula) but differ in

chemical and physical

properties.

 Structural isomers are compounds with the

same molecular formula but different

structural formulae.

◦ Carbon-chain

◦ Positional

◦ Functional-group

 Stereoisomers are compounds with the same

structural formula but which have the atoms

arranged differently in space.

 Carbon-chain isomerism – the difference

between the isomers is the length of the

carbon chain.

 Positional isomerism – the isomers have the

same functional group in different locations

on the carbon skeleton.

 Functional-group isomerism – the isomers

are members of different homologous series

and, therefore, have different functional

groups.

 The conversion of one configurational isomer into

another is happening by bond breaking between

the atoms in the molecules.

 The conversion of one conformational isomer into

another is happening by rotation of sigma bond

between two carbon atoms.

biological significance

 The most of the biochemical reactions are stereospecific

and take place with only one of the two enantiomers.

 Biological differences: One enantiomer of Thalidomide is

a cure for morning sickness and the other cause birth

defects, like vestigial limbs.

Optical isomerism

 It occurs in organic compounds when four different

groups are attached to the same carbon atom. (chiral

centre or asymmetric carbon atom *)

 A chiral centre results in two optical isomers

(enantiomers) that are non - superimposable mirror

images of each other.

 Enantiomers rotate the plane of polarisation of plane -

polarised light in opposite directions.

 An equimolar mixture of enantiomers is called a

racemic mixture. It has no effect on plane – polarised

light.

Unpolarized and polarized light

 A solution of one enantiomer rotates the

plane of polarisation of plane-polarised light

in a cloackwise direction (+); the other

enantiomer rotates it in an anticlockwise

direction (-).

Pi disastereoisomers

 They are also called cis-trans isomers (Geometric

isomers).

 It is caused by the presence of a functional group that

restricts rotation.

 Alkenes exhibit geometric isomerism if there are

different groups on each carbon atom of the C=C

bond.

 Cis - - are on the same side of the double bond

 Trans – are on opposite side of the double bond

Sigma disastereoisomers

Partial mirror images.

 Molecules with more than one asymmetric carbon

atom.

 If a molecule has two identical chiral centers, the

isomer with a plane of symmetry will not be optically

active because one chiral centre will rotate the plane

of polarisation of plane-polarised light clockwise and

the other chiral centre will rotate it equally

anticlockwise. (meso form)

 Example: Tartaric acid

Conformational isomerism

Conformation is the different spatial orientation of the

atoms (or groups) in the molecule, created by rotation

of sigma bond between two carbon atoms.

 Newman projection

 Example: Ethane