3.7 Optical isomerism

Question 1

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

Answer 1

Chiral carbon
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, 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

Question 3

What word is used to describe optically active isomers

Answer 3

Chiral

Question 4

What are pairs 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 the chiral centre denoted

Answer 6

C* (star on C)

Question 7

Give 2 examples of chiral molecules

Answer 7

All alpha amino acids, except glycine
Lactic 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 x0 anticlockwise (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 conc of the sample etc

Question 14

What is the first stage 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 equation as this is the H+ from 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

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

Doesnt 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