Organic 2 - Optical Isomers

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 word is used to describe optically active molecules?

Answer 2

chiral

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.
Differ in the way they rotate plane polarised light - rotate plane of polarisation by the same angle but in different directions.

Question 3

What are the pair of isomers called?

Answer 3

Enantiomers

Question 4

What is the chiral centre?

Answer 4

The carbon that has four different substituents attached to it

Question 5

How is the chiral centre denoted?

Answer 5

C* (star on C)

Question 6

Give two examples of chiral molecules. Draw one of them (both enantiomers).

Answer 6

All alpha amino acids, except glycine. Lactic acid / 2-hydroxypropanoic acid

check photo!

Question 7

How is light polarised?

Answer 7

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

Question 8

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

Answer 8

Rotates plane of polarisation by xo anti clockwise (same angle, opposite direction)

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 is the structure of a polarimeter?

Answer 11

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 12

What are polarimeters used for?

Answer 12

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

Question 13

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

Answer 13

(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) CH3CHO + HCN → CH3CH(OH)CN

Question 14

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

Answer 14

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

Question 15

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

Answer 15

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

Question 16

How does this second stage affect the chirality?

Answer 16

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

Question 17

Are racemic mixtures formed in nature? Why?

Answer 17

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

Question 18

Why is optical isomerism a problem for the drug industry?

Answer 18

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

Question 19

What are the options to resolve the issue of only one enantiomer being effective?

Answer 19

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 20

Examples of optically active drugs?

Answer 20

Ibuprofen, Thalidomide

Question 21

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

Answer 21

Sold as 50% racemate.
But body converts 60% of R- isomer to S+ isomer → end up with 80% S+ isomer