Optical Isomerism

Question 1

What property must a carbon atom have for the

molecule to display optical isomerism about that carbon atom?

Answer 1

4 different substituents attached to one carbon atom

Question 2

What are the similarities and differences between 2 optical isomers?

Answer 2

Same atoms and bonds, but they are non-superimposable mirror images of one another.
Differ in the way they rotate plane polarised light - rotate plane of polarisation by the same angle but in different directions.

Question 3

What word is used to describe optically active

molecules?

Answer 3

chiral

Question 4

What are the pair of isomers called?

Answer 4

Enantiomers

Question 5

What is the chiral centre?

Answer 5

The carbon that has four different substituents attached to it

Question 6

How is the chiral centre denoted?

Answer 6

C* (star on C)

Question 7

Give two examples of chiral molecules.

Answer 7

All alpha amino acids, except glycine.

Lactic acid / 2-hydroxypropanoic acid

Question 8

How is light polarised?

Answer 8

By passing it through a polaroid filter, so oscillations are only in one plane.

Question 9

What effect does the racemic mixture have on plane polarised light?

Answer 9

None, as the rotation by each enantiomer cancels out to nothing

Question 10

What effect does the +
isomer have on plane
polarised light?

Answer 10

Rotates plane of polarisation by x0 clockwise

Question 11

What effect does the -
isomer have on plane
polarised light?

Answer 11

Rotates plane of polarisation by xo anti clockwise

same angle, opposite direction

Question 12

What is the structure of a polarimeter?

Answer 12

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

Question 13

What are polarimeters used for?

Answer 13

To identify which enantiomer is present, the purity of the sample, the concentration of the sample etc.

Question 14

What is the first stage of the synthesis of lactic acid

2-hydroxypropanoic acid) from ethanal? (Equation

Answer 14

(Reagents are KCN and HCl but it is acceptable to write HCN in the balanced eqn as this is the H+ from the HCl and -CN from KCN)
CH3CHO + HCN → CH3CH(OH)CN

Question 15

Why is the CH3CH(OH)CN molecule formed chiral?

Answer 15

H, CH3, OH and CN groups attached to the central chiral carbon atom - 4 substituents

Question 16

What is the second stage of the synthesis of lactic acid (2-hydroxypropanoic acid) from ethanal?

Answer 16

Hydrolysis: CH3CH(OH)CN + HCl +2H2O → CH3CH(OH)COOH + NH4Cl

Question 17

How does this second stage affect the chirality?

Answer 17

Does not affect it - still racemic as chirality not

affected by this stage.

Question 18

Are racemic mixtures formed in nature? Why?

Answer 18

Not often, as enzyme mechanisms are 3D so only form one enantiomer

Question 19

Why is optical isomerism a problem for the drug industry?

Answer 19

Sometimes, only one enantiomer is effective due

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

Question 20

What are the options to resolve the issue of only one

enantiomer being effective?

Answer 20

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

Question 21

Examples of optically active drugs?

Answer 21

Ibuprofen, Thalidomide

Question 22

Why is ibuprofen able to be sold as a racemate, even though the + isomer is needed to treat inflammation?

Answer 22

Sold as 50% racemate.

But body converts 60% of R- isomer to S+ isomer → end up with 80% S+ isomer