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

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 a chiral centre ?

Answer 5

The carbon that has four different substituents attaches to it

Question 6

How is the chiral centre denoted

Answer 6

C*

Question 7

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

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 plans of polarisation by X0 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 the polariimeters used for?

Answer 13

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

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 equation as this is the H+ from the HCl and -CN from KCN)

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 enantiomer- 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