4.1 STEREOISOMERISM

Steroisomerism

Compounds with the same molecular formula and connectivity but different 3d arrangements of atoms within a space

Structural isomerism

Compounds with the same molecular formula but different arrangement of atoms

Types of steroisomerisms

E-Z isomerisms

Optical isomerism

E-Z isomerism

Occur restricted rotation around c=c double bond

2 different groups on each carbon

• E high priority opposite sides

• Z high priority same side

Optical isomerism

Occur chiral centre Two forms enantiomers

• Rotate plane of polarised light in opposite directions

Chiral centre

Atom bonded to four different groups

Enantiomers

Non-superimposable mirror images

• light clockwise (+)

• Light anticlockwise(-)

Optical activity

Molecules possessing a chiral centre

Rotate plane of polarised light oppssite direction

Racemic mixture

Equimolar mixture of both enantiomers

No net light rotations of plane of polarised light

Rotation of plane of polarised light depends on

Particular enantiomer

Concentration of enantiomer

Length of tube containing solution through which light pass through

Light frequency

Ibuprofen

Chiral centre in racemic mixture

Unwanted enantiomer danger of side effects

Resolution

Separation of 2 enantiomers

Molecules with more than one chiral centre

Diastereoisomers

Physical properties may differ between EZ isomers

Boiling melting point

Solubility

• different polarity caused by spatial arranagement

Why is a molecule non-superimposiable mirror image

Mirror iamges

Cant be placed on top of each other to give same molecule

Chiral centre creates asymmetry in 4d

Explain 50:50 racemic mixture

Rotations of 2 enantiomers cancel each other out

No net rotation of plane polarised light

Optically inactive

Enantiomers different biological effects

Interact different to biological systems

Enzymes, receptors, proteins chiral

Only one enantiomer fits properly

• One may be therapeutically activee other not

Importance of stereoisomerism in pharmaceuticals

Enantiomers of drug can have different biological effects

Chiral so only one enantiomer fit properly and be active

• Thalidomide

One enantiomer R effective and S serious birth defects

• need carefully designed and tested ensure only therapeutically active enantiomer administered

Explain why equimolar has no apparent effect on plane of polarised light

• one isomer rotates the plane to the left and the other to the right

• isomers n equimolar amounts therefore the effect of rotation is cancelled out