Organic 7: Optical Isomerism

Question 1

What is optical isomerism?

Answer 1

A form of stereoisomerism
Where there are four different substituents attached to one carbon atom
Results in two isomers that are non-superimposable mirror images of each other
But are not identical
Each of the two enantiomers rotate a plane of polarised light in opposite directions

Question 2

What is chirality?

Answer 2

A molecule can have a chiral centre, or an asymmetric carbon atom, if it has four different substituents attached
Usually indicated by an *
All amino acids except glycine have a chiral centre

Question 3

How is optical rotation measured?

Answer 3

Using a polarimeter:
1) Polarised light (has been passed through a polaroid) passes through two solutions of the same concentration each containing a different optical isomer of the same substance
2) The (+) -isomer rotates the plane of polarisation clockwise
3) The (-) -isomer rotates the plane of polarisation anticlockwise

Question 4

What is a racemic mixture?

Answer 4

A 50:50 mixture of two optical isomers
The rotational effects of each of the two enantiomers cancel out
So plane-polarised light is not rotated

Question 5

Why are racemic mixtures formed?

Answer 5

E.g. in nucleophilic substitution, the cyanide ions have a 50/50 probability of adding above or below the plane
So an equal amount of each enantiomer is formed

Question 6

What properties do optical isomers share?

Answer 6

Solubility
Melting points
Boiling points
(Only differ by rotation of plane-polarised light)

Question 7

Why are racemic mixtures not optically active?

Answer 7

Because racemic mixtures are a 50/50 mixture of two enantiomers
So the rotational effects in opposite directions cancel out
Plane-polarised light is not rotated

Question 8

Define the term “optically active.”

Answer 8

Capable of rotating plane-polarised light