Topic 3.7 - Optical Isomerism

What property must a

carbon atom have for the

molecule to display optical

isomerism about that carbon

atom?

4 different substituents attached to one carbon

atom

What are the similarities and

differences between two

optical isomers?

Same atoms and bonds, but they are non-superimposable

mirror images of one another. NOT IDENTICAL in chemical

properties necessarily.

Differ in the way they rotate plane polarised light - rotate

plane of polarisation by the same angle but in different

directions.

What word is used to

describe optically active

molecules?

chiral

What are the pair of isomers

called?

Enantiomers

What is the chiral centre?

The carbon that has four different substituents

attached to it

How is the chiral centre

denoted?

C* (star on C)

Give two examples of chiral

molecules. Draw one of

them (both enantiomers).

All alpha amino acids, except glycine.

Lactic acid / 2-hydroxypropanoic acid

How is light polarised?

By passing it through a polaroid filter, so

oscillations are only in one plane.

What effect does the

racemic mixture have on

plane polarised light?

None, as the rotation by each enantiomer

cancels out to nothing

What effect does the +

isomer have on plane

polarised light?

Rotates plane of polarisation by x 0 clockwise

What effect does the -

isomer have on plane

polarised light?

Rotates plane of polarisation by x o anti clockwise

(same angle, opposite direction)

What is the structure of a

polarimeter?

Light source (unpolarised light) → polarising filter (polarised

light) → polarised light passes through compartment

containing sample → detector determines the angle of

rotation of the plane polarised light

What are polarimeters used

for?

To identify which enantiomer is present, the

purity of the sample, the concentration of the

sample etc.

What is the first stage of the

synthesis of lactic acid

(2-hydroxypropanoic acid)

from ethanal? (Equation).

(Reagents are KCN and HCl but it is acceptable

to write HCN in the balanced equation as this is

the H + from the HCl and -

CN from KCN)

CH 3

CHO + HCN → CH 3

CH(OH)CN

Why is the CH 3 CH(OH)CN

molecule formed chiral?

H, CH 3

, OH and CN groups attached to the

central chiral carbon atom - 4 substituents

What is the second stage of

the synthesis of lactic acid

(2-hydroxypropanoic acid)

from ethanal?

Hydrolysis: CH 3

CH(OH)CN + HCl +2H 2

O →

CH 3

CH(OH)COOH + NH 4

Cl

How does this second stage

affect the chirality?

Does not affect it - still racemic as chirality not

affected by this stage.

Are racemic mixtures

formed in nature? Why?

Not often, as enzyme mechanisms are 3D so

only form one enantiomer

Why is optical isomerism a

problem for the drug

industry?

Sometimes, only one enantiomer is effective due

to enzyme’s active site/cell receptors being 3D.

What are the options to

resolve the issue of only

one enantiomer being

effective?

1. Separate enantiomers - difficult and expensive as have

very similar properties

2. Sell racemate - wasteful as half is inactive

3. Design alternative synthesis to only produce one

enantiomer.

Examples of optically active

drugs?

Ibuprofen, Thalidomide

Why is ibuprofen able to be sold as

a racemate, even though the +

isomer is needed to treat

inflammation?

Sold as 50% racemate.

But body converts 60% of R- isomer to S+ isomer → end up

with 80% S+ isomer