OPTICAL ISOMERISM

Occurrence another form of stereoisomerism
occurs when compounds have non-superimposable mirror
images

Isomers the two different forms are known as optical isomers or
enantiomers they occur when molecules have a chiral centre
a chiral centre contains an asymmetric carbon atom
an asymmetric carbon has four different atoms (or groups)
arranged tetrahedrally around it.

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OPTICAL ISOMERISM

CHIRAL CENTRES

There are four different 2-chlorobutane exhibits optical
colours arranged isomerism because the second

tetrahedrally about the carbon atom has four different
carbon atom atoms/groups attached

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OPTICAL ISOMERISM

SPOTTING CHIRAL CENTRES

Look at each carbon atom in the chain and see what is attached to it. For a chiral centre
you need an asymmetric carbon with four different atoms/groups) arranged tetrahedrally around it.

IF A CARBON HAS MORE THAN ONE OF ANY ATOM/GROUP ATTACHED, IT CAN’T BE CHIRAL

C 3 H’s around it NOT chiral
CH3CH2CH2CH2Cl C 2 H’s around it NOT chiral

1-chlorobutane C 2 H’s around it NOT chiral
C 2 H’s around it NOT chiral

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OPTICAL ISOMERISM

SPOTTING CHIRAL CENTRES

Look at each carbon atom in the chain and see what is attached to it. For a chiral centre
you need an asymmetric carbon with four different atoms/groups) arranged tetrahedrally around it.

IF A CARBON HAS MORE THAN ONE OF ANY ATOM/GROUP ATTACHED, IT CAN’T BE CHIRAL

C 3 H’s around it NOT chiral
CH3CH2CH2CH2Cl C 2 H’s around it NOT chiral

1-chlorobutane C 2 H’s around it NOT chiral
C 2 H’s around it NOT chiral

CH C 3 H’s around it C 2 H’s NOT chiral
3CH2CHClCH3

around it NOT chiral
2-chlorobutane C H, CH3, Cl,C2H5 around it CHIRAL

C 3 H’s around it NOT chiral

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OPTICAL ISOMERISM
SPOTTING CHIRAL CENTRES

Look at each carbon atom in the chain and see what is attached to it. For a chiral centre
you need an asymmetric carbon with four different atoms/groups) arranged tetrahedrally around it.

IF A CARBON HAS MORE THAN ONE OF ANY ATOM/GROUP ATTACHED, IT CAN’T BE CHIRAL

C 3 H’s around it NOT chiral
CH3CH2CH2CH2Cl C 2 H’s around it NOT chiral

1-chlorobutane C 2 H’s around it NOT chiral
C 2 H’s around it NOT chiral

CH C 3 H’s around it C 2 H’s NOT chiral
3CH2CHClCH3

around it NOT chiral
2-chlorobutane C H, CH3, Cl,C2H5 around it CHIRAL

C 3 H’s around it NOT chiral

(CH ound it C NOT chiral
3)2CHCH2Cl C 3 H’s ar

2 CH3’s around it NOT chiral
1-chloro-2-methylpropanane C 2 H’s around it NOT chiral

(CH C 3 H’s around it C NOT chiral
3)3CCl

2-chloro-2-methylpropanane 3 CH3’s around it NOT chiral

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OPTICAL ISOMERISM
Spatial differences between isomers

• two forms exist which are NON-SUPERIMPOSABLE MIRROR IMAGES of each other
• non-superimposable means you you can’t stack one form exactly on top of the other

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OPTICAL ISOMERISM
Spatial differences between isomers

• two forms exist which are NON-SUPERIMPOSABLE MIRROR IMAGES of each other
• non-superimposable means you you can’t stack one form exactly on top of the other

Some common objects are mirror images and superimposable spoons
superimposable but not mirror images books
non-superimposable mirror images hands

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OPTICAL ISOMERISM

Spatial differences between isomers

• two forms exist which are NON-SUPERIMPOSABLE MIRROR IMAGES of each other
• non-superimposable means you you can’t stack one form exactly on top of the other

Some common objects are mirror images and superimposable spoons
superimposable but not mirror images books
non-superimposable mirror images hands

NB For optical isomerism in molecules, both conditions must apply…
they must be mirror images AND be non-superimposable

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OPTICAL ISOMERISM

What is a non-superimposable mirror image?

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OPTICAL ISOMERS – DIFFERENCE
• isomers differ in their reaction to plane-polarised light

• plane polarised light vibrates in one direction only

• one isomer rotates light to the right, the other to the left

• rotation of light is measured using a polarimeter

• rotation is measured by observing the polarised light coming out towards the observer

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OPTICAL ISOMERS – DIFFERENCE
• isomers differ in their reaction to plane-polarised light

• plane polarised light vibrates in one direction only

• one isomer rotates light to the right, the other to the left

• rotation of light is measured using a polarimeter

• rotation is measured by observing the polarised light coming out towards the observer

• If the light appears to have turned to the right turned to the left

DEXTROROTATORY LAEVOROTATORY

d or + form l or – form

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OPTICAL ISOMERS – DIFFERENCE
• isomers differ in their reaction to plane-polarised light

• plane polarised light vibrates in one direction only

• one isomer rotates light to the right, the other to the left

• rotation of light is measured using a polarimeter

• rotation is measured by observing the polarised light coming out towards the observer

• If the light appears to have turned to the right turned to the left

DEXTROROTATORY LAEVOROTATORY

d or + form l or – form

Racemate a 50-50 mixture of the two enantiomers (dl) or (±) is a racemic mixture.
The opposite optical effects of each isomer cancel each other out

Examples Optical activity is common in biochemistry and pharmaceuticals
• Most amino acids exhibit optical activity
• many drugs must be made of one optical isomer to be effective
– need smaller doses (safer and cost effective)
– get reduced side effects
– improved pharmacological activity

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OPTICAL ISOMERISM

The polarimeter

A B

C D
E

F

A.Light source produces light vibrating in all directions
B.Polarising filter only allows through light vibrating in one direction
CPlane polarised light passes through sample
D If substance is optically active it rotates the plane polarised light
E Analysing filter is turned so that light reaches a maximum
F Direction of rotation is measured coming towards the observer

If the light appears to have turned to the right turned to the left
DEXTROROTATORY LAEVOROTATORY

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OPTICAL ISOMERISM – THALIDOMIDE
The one obvious difference between optical isomers is their response to plane
polarised light. However, some naturally occurring molecules or specifically
synthesised pharmaceuticals show different chemical reactivity.

The drug, THALIDOMIDE is a chiral molecule and can exist as two enantiomers. In the
1960’s it was used to treat anxiety and morning sickness in pregnant women.
Tragically, many gave birth to children with deformities and missing limbs.

It turned out that only one of the enantiomers (the structure on the right) was effective
and safe; its optically active counterpart was not. The major problem was that during
manufacture a mixture of the isomers was produced. The drug was banned world-
wide, but not after tens of thousands of babies had been affected.

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OPTICAL ISOMERISM – Other points
The following points are useful when discussing reactions producing optical isomers.

The formation of racemic mixtures is more likely in a laboratory reaction than in a
chemical process occurring naturally in the body.

If a compound can exist in more than one form, only one of the optical isomers is
usually effective.

The separation of isomers will make manufacture more expensive.

A drug made up of both isomers will require a larger dose and may cause problems if
the other isomer is ‘poisonous’ like thalidomide.

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Disclaimer

All data and content provided in this presentation are
taken from the reference books, internet – websites
and links, for informational purposes only.

Thank You
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