Stereochemistry

Question 1

Molecules that have the same molecular formula but different structural formulae are called?

Answer 1

Isomers

Question 2

Structural isomers occur when?

Answer 2

The atoms are bonded together in a different order in each isomer.

Question 3

Stereoisomers occur when the…

Answer 3

The order of the bonding in the atoms is the same but the spatial arrangement of the atoms is different in each isomer.

Question 4

There are two types of stereoisomer…

Answer 4

Geometric and optical.

Question 5

Give 5 facts about Geometric isomers…

Answer 5

1. Can occur when there is restricted rotation around a carbon-carbon double bond or a carbon-carbon single bond in a cyclic compound
2. Must have two different groups attached to each of the carbon atoms that make up the bond with restricted rotation
3. Can be labelled cis or trans according to whether the substituent groups are on the same side (cis) or on different sides (trans) of the bond with restricted rotation
4. Have differences in physical properties, such as melting point and boiling point
5. Can have differences in chemical properties

Question 6

Give 8 facts about Optical isomers…

Answer 6

1. Occur in compounds in which four different groups are arranged tetrahedrally around a central carbon atom (chiral carbon or chiral centre)
2. Are asymmetric
3. Are non-superimposable mirror images of each other
4. Can be described as enantiomers.
5. Have identical physical properties, except for their effect on planepolarised light
6. Have identical chemical properties, except when in a chiral environment such as that found in biological systems (only one optical isomer is usually present)
7. Rotate plane-polarised light by the same amount but in opposite directions and so are optically active
8. When mixed in equal amounts are optically inactive because the rotational effect of the plane-polarised light cancels out — this is called a racemic mixture

Question 7

What is a racemetric mixture?

Answer 7

A racemic mixture or racemate, is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule.

Question 8

What does it mean when a molecule is chiral?

Answer 8

Means ‘mirror-image, non-superimposable molecules’, and to say that a molecule is chiral is to say that its mirror image (it must have one) is not the same as it self.

Question 9

What is an enantiomer?

Answer 9

A pair of molecules that are mirror images of each other.

Question 10

All enantiomers are chiral objects, but not all chiral objects are? Give an example?

Answer 10

Are enantiomers. For example, you left and right hands are not chemical isomers of each other, but they are chiral.

Question 11

What is plane polarised light?

Answer 11

Only vibrating in a single plane