3.3.7 Optical isomerism

Question 1

What is a chiral carbon

Answer 1

A chiral (asymetric) carbon is one that has four different groups attached to it. Its possible to arrange the groups in two different groups around the chiral carbon so that the two different molecules are made.

Question 2

What is an enantiomer/ optical isomer?

Answer 2

When four different groups are arranged around a chiral carbon in two different ways that they are mirror images and cannot be superimposed, they are enantiomers

Question 3

How do you draw the optical isomer of a chiral carbon?

Answer 3

1. Locate the chiral carbon - look for the carbon with four different groups attached e.g. H, OH, COOH, and CH₃
2. Draw the isomer - Draw on enantiomer in a tetrahedral shape, dont try to draw the full structure of each group, then draw the mirror image beside it

Question 4

How do optical isomers rotate plane polarised light?

Answer 4

1. Normal light vibrates in all directions. Plane polarised light only vibrates in one direction
2. Optical isomers are optically active - they rotate plane polarised light
3. One enantiomer rotates it in a clockwise direction, and the other rotates it in an anticlockwise direction, equally in opposite directions

Question 5

What is a racemic mixture (racemate)?

Answer 5

A racemic mixture contains equal quantities of each enantiomer of an optically active compound

Question 6

Why don’t racemic mixtures display optical activity?

Answer 6

The two enatiomers cancel each other’s light-rotating effect.

Question 7

Why are racemic mixtures 50% of each enatiomer?

Answer 7

Chemists often react two achiral molecules together and get a racemic mixture of chiral product, this is because when two molecules react there’s an equal chance of froming each of the enatiomers

Question 8

Why do reactions involving planar bonds often produce racemates?

Give an example using propanal with acidified potassium cyanide

Answer 8

Double bonds, such as C=O and C=O, are planar. The products of reactions that happen at the carbonyl group of aldehydes and unsymetrical ketones are often enantiomers present as a racemic mixture

1. The reaction of propanal with KCN involves a CN^- ions attacking a δ+ carbon of propanal’s planar C=O group
2. The CN^- ion can attack from two directions - from above the plane of the molecule, or bellow it
3. Depending on which direction of the nucleophillic attack happens from, one of two enantiomers is formed
4. Because the C=O bond is planar, there’s an equal chance that the nucleophile will attack from either side of these diections. This means that an equal amount of each enantiomer will be formed
5. So when propanal reacts with acidified potassium cyanide, you get a racemic mixture of products

If you start with a symetrical ketone instead, you’ll make a product that doesn’t have a chiral centre so won’t display optical activity