Optical Isomerism
All molecules have a mirror image, which is often the same molecule, but sometimes can be a
different molecule
Molecules have optical isomers if their mirror images are non-superimposable
Molecules containing a C with four different group attached to it exhibit optical isomerism –
known as a chiral (asymmetric) carbon atom
Two molecules that are optical isomers of each other are enantiomers
A mixture containing a 50/50 mixture of the two enantiomers is described as a racemate or
racemic mixture
Optical isomers have the same chemical and physical properties as each other, apart from their
effect on plane polarised light (and reactions with other chiral compounds)
Light consists of waves that vibrate in many directions perpendicular to the direction of travel,
but some substances have the ability to remove all the light waves except those vibrating in a
single plane, producing plane polarised light
Optical isomers can rotate the plane of polarised light and so are said to be optically active
o If a racemic mixture is produced in a reaction, and the carbonyl group is planar, the
product if optically inactive as attack by nucleophile is equally likely from either side
One enantiomer rotates it in one direction and the other enantiomer rotates it by the same
amount in the opposite direction
o (+)-enantiomer – clockwise rotation
o (-)-enantiomer – anticlockwise rotation
o (±)-racemate – no overall effect
Many natural occurring compounds contain chiral C atoms and occur as one pure enantiomer
only
Many drugs contain chiral C atoms and occur as enantiomers, but often only one of the
enantiomers has the desired effect
o With some drugs, the opposite enantiomer can have an unwanted effect and so the
drug must be produced as one enantiomer only – wasteful
Nucleophilic Addition
The C=O is polar with the C being δ+ as the O is more electronegative than the C, so the C is
susceptible to attack by nucleophiles
The atoms attached to the C of the C=O are arranged in a trigonal planar shape, so the C, O and
other two atoms are in the same plane
Attack by nucleophiles is equally likely from either side of the carbonyl group, so a racemic
mixture of two enantiomers is produced
All molecules have a mirror image, which is often the same molecule, but sometimes can be a
different molecule
Molecules have optical isomers if their mirror images are non-superimposable
Molecules containing a C with four different group attached to it exhibit optical isomerism –
known as a chiral (asymmetric) carbon atom
Two molecules that are optical isomers of each other are enantiomers
A mixture containing a 50/50 mixture of the two enantiomers is described as a racemate or
racemic mixture
Optical isomers have the same chemical and physical properties as each other, apart from their
effect on plane polarised light (and reactions with other chiral compounds)
Light consists of waves that vibrate in many directions perpendicular to the direction of travel,
but some substances have the ability to remove all the light waves except those vibrating in a
single plane, producing plane polarised light
Optical isomers can rotate the plane of polarised light and so are said to be optically active
o If a racemic mixture is produced in a reaction, and the carbonyl group is planar, the
product if optically inactive as attack by nucleophile is equally likely from either side
One enantiomer rotates it in one direction and the other enantiomer rotates it by the same
amount in the opposite direction
o (+)-enantiomer – clockwise rotation
o (-)-enantiomer – anticlockwise rotation
o (±)-racemate – no overall effect
Many natural occurring compounds contain chiral C atoms and occur as one pure enantiomer
only
Many drugs contain chiral C atoms and occur as enantiomers, but often only one of the
enantiomers has the desired effect
o With some drugs, the opposite enantiomer can have an unwanted effect and so the
drug must be produced as one enantiomer only – wasteful
Nucleophilic Addition
The C=O is polar with the C being δ+ as the O is more electronegative than the C, so the C is
susceptible to attack by nucleophiles
The atoms attached to the C of the C=O are arranged in a trigonal planar shape, so the C, O and
other two atoms are in the same plane
Attack by nucleophiles is equally likely from either side of the carbonyl group, so a racemic
mixture of two enantiomers is produced
