Topic 17 Organics II

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
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 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.

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 x^0 clockwise

Question 11

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

Answer 11

Rotates plane of polarisation by x^0 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

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

Are racemic mixtures formed in nature? Why?

Answer 15

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

Question 16

Why is optical isomerism a problem for the drug industry?

Answer 16

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

Question 17

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

Answer 17

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 18

Examples of optically active drugs?

Answer 18

Ibuprofen, Thalidomide