3.7 - Optical Isomers

Question 1

What property must a carbon atom have for the molecule to display optical isomerism about that carbon aom?

Answer 1

4 different substituents attached to one carbon atom

Question 2

What are the similarities and differences between two optical isomers?

Answer 2

Same atoms and bonds but they are non-superimposable mirror images of one another. Not identical in chemical properties necessarily.
Differe in the way they rotate plane polarised light - rotate the 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 4 different substituents attached to it

Question 6

How is light polarised?

Answer 6

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

Question 7

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

Answer 7

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

Question 8

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

Answer 8

Rotates plane of polarisation by x^0 clockwise

Question 9

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

Answer 9

Rotates plane of polarisation by x^0 anti-clockwise (same angle, opposite direction).

Question 10

What is the structure of polarimeter?

Answer 10

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 11

What are polarimeters used for?

Answer 11

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

Question 12

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

Answer 12

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

Question 13

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

Answer 13

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

Question 14

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

Answer 14

Hydrolysis: CH3CH(OH)CN + HCl +3H2O -> CH3CH(OH)COOH +NH4Cl

Question 15

How does this second stage affect the chirality?

Answer 15

Does not affect it - still racemic as chirality not affected by this stage.

Question 16

Are racemic mixtures formed in nature? Why?

Answer 16

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

Question 17

Why is optical isomerism a problem for the drug industry?

Answer 17

Sometimes, only one enantiomer is effective due to enzyme’s active site/ cell receptors being 3D.