3.3.7 Optical isomerism

What type of isomerism is optical isomerism?

Stereoisomerism

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

4 different atoms/functional groups attached to one carbon atom (chiral carbon)

What are the similarities & differences between two optical isomers?

• Same atoms & bonds, but they are non-superimposable mirror images of one another

• They are not necessarily identical in chemical properties

• They differ in the way they rotate plane polarised light (rotate plane of polarisation by the same angle but in opposite directions)

What word is used to describe optically active molecules?

Chiral

What are the pair of isomers called?

Enantiomers

What is the chiral carbon centre?

The carbon atom that has 4 different atoms/groups of atoms attached to it

How is the chiral carbon/centre denoted?

C* (star on the C)

How is light polarised?

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

What is the definition of a racemic mixture?

Equal mixtures of enantiomers

Why are racemic mixtures formed?

The carbonyl group is planar, so a nucleophile can attack from either above & below the bond (equal probability)

What effect does the racemic mixture have on plane polarised light?

None, as the rotation by each enantiomer cancels out to nothing

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

Rotates plane of polarisation by x° clockwise

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

Rotates plane of polarisation by x° anti-clockwise (same angle, but opposite direction as + isomer)

What is the structure of a polarimeter?

1. Light source (unpolarised light)

2. Polarising filter (polarised light)

3. Polarised light passes through compartment containing sample

4. Detector determines the angle of rotation of the plane polarised light

What are polarimeters used for?

• To identify which enantiomer is present

• The identify the purity & concentration of the sample

Are racemic mixtures formed in nature?

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

Why is optical isomerism a problem for the drug industry?

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

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

• Separate the enantiomers (difficult & expensive, as they have very similar properties)

• Sell the racemate; it is wasteful because half is inactive

• Design an alternative synthesis to only produce one enantiomer

What are two examples of optically active drugs?

Ibuprofen & thalidomide

Draw the two enantiomers of 2-hydroxypropanoic acid

What is the relationship between two chiral molecules with the same structural formula?

They are non-superimposable mirror images of each other

By considering the mechanism of the reaction of the reduction of butanone, explain why the product has no effect on plane polarised light (6 marks)

• The carbonyl group in butanone is planar. During the reduction, a hydride ion (H^-), from NaBH₄, acts as a nucleophile & attacks the carbonyl carbon. Because the carbonyl group is planar, the H^- ion can attack from either above or below the C=O bond with equal probability

• This forms butan-2-ol, which has a carbon atom bonded to 4 different functional groups, so exists as two chiral forms (enantiomers)

• Since attack occurs equally from either side of the planar carbonyl group, equal amounts of each enantiomer are produced, giving a racemic mixture

• The two enantiomers rotate plane-polarised light by equal amounts in opposite directions, so the effects cancel out & there is no overall rotation of plane-polarised light