A - Optical Isomerism

Question 1

What are isomers?

Answer 1

Compounds with the same molecular formula but have different molecular structures or a different arrangement of atoms in space.

Question 2

In what three ways can structural isomers differ from each other?

Answer 2

1. Have different functional groups.
2. Have functional groups attached to the main chain at different points.
3. Have a different arrangement of carbon atoms in the skeleton of the molecule.

Question 3

What is stereoisomerism?

Answer 3

Where two or more compounds have the same structural formula but their atoms arranged differently in space.

Question 4

What are the two types of stereoisomerism?

Answer 4

1. E-Z isomerism.

2. Optical isomerism.

Question 5

When does optical isomerism occur?

Answer 5

When there are four different groups attached to a carbon atom. It’s possible to arrange the groups in two different ways around the carbon atom so that two different molecules are made which cannot be superimposed and are not identical.

Question 6

What is a chiral or asymmetric carbon atom?

Answer 6

A carbon atom which has four different groups attached to it.

Question 7

What is another name for optical isomers?

Answer 7

Enantiomers.

Question 8

What does superimposed mean?

Answer 8

That you can put one molecule on top of another and they will match up completely.

Question 9

What’s the word for molecules that do not show optical isomerism and can be superimposed?

Answer 9

Achiral.

Question 10

How would you draw an optical isomer?

Answer 10

1. Locate the chiral centre - the carbon atom with four different groups attached.
2. Draw one enantiomer in a tetrahedral shape.
3. Draw the mirror image beside it to show the other enantiomer.

Question 11

Finish these sentences:

1. Normal light vibrates in …
2. Plane-polarised light only vibrates in …

Answer 11

1. All directions.

2. One direction.

Question 12

Optical isomers are optically active. What does this mean?

Answer 12

They can rotate plane-polarised light.

Question 13

How will the two enantiomers of one molecule rotate plane-polarised light?

Answer 13

One enantiomer will rotate it in a clockwise direction, and the other rotates it in an anti-clockwise direction.

Question 14

What is a racemate or race mix mixture?

Answer 14

A mixture that contains equal quantities of enantiomers.

Question 15

Why are racemic mixtures optically inactive?

Answer 15

The mixture is made up 50:50 of two optically active enantiomers. Their light-rotating effects cancel each other out meaning the racemic mixture is optically inactive.

Question 16

What feature of double bonds means that often reactions taking place at the carbonyl group of aldehydes and unsymmetrical ketones will produce a racemic mixture?

Answer 16

Double bonds are planar (flat) so that the attacking molecule can do so from either above or below the plane of the molecule.

Question 17

Outline and explain the reaction of propanal with acidified potassium cyanide. what type of reaction is this?

Answer 17

This is a nucleophilic addition reaction.

1. The nucleophile (cyanide ion) attacks the slightly positively charged carbon atom of the C=O group in propanal and can do so from two directions, above or below the plane.
2. One of two enantiomers are formed depending on the direction of attack. Because the C=O bond is planar, there is an equal chance that the nucleophile will attack from either direction so that means an equal amount of each enantiomer is formed.
3. Therefore a racemic mixture is formed.