3c- Stereo chemistry

Question 1

What is an isomer?

Answer 1

Molecules that have the same molecular formula but different structural formulas

Question 2

When do structural isomers occur?

Answer 2

When the atoms are bonded together in a different order in each isomer

Question 3

When do stereoisomers occur?

Answer 3

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

Question 4

What are the two types of stereoisomers?

Answer 4

Geometric and optical

Question 5

When can geometric isomers occur?

Answer 5

When there is restricted rotation around a carbon-carbon double bond or carbon-carbon single bond in a cyclic compound

Question 6

What must geometric isomers have?

Answer 6

Two different groups attached to each of the carbon atoms that make up the bond with restricted rotation

Question 7

What can geometric isomers be labelled as?

Answer 7

Either cis or trans according to whether the substituent groups are on the same side (cis) or different sides (trans) of the bond with restricted rotation

Question 8

Why do trans isomers have higher melting and boiling points than cis isomers?

Answer 8

The molecules in trans isomers are packed tighter together which allows more intermolecular forces to be formed than in cis isomers

Question 9

How may cis and trans isomers vary?

Answer 9

In physical properties like melting and boiling points, and in chemical properties depending on what the compound is

Question 10

When do optical isomers occur?

Answer 10

In compounds in which four different groups are arranged tetrahedrally around a central carbon atom

Question 11

What two things can be said about optical isomers?

Answer 11

• they are asymmetric

- they are non-superimposable mirror images of each other

Question 12

What can optical isomers also be called?

Answer 12

Enantiomers

Question 13

Enantiomers have identical physical properties apart from what?

Answer 13

Their effect on plane-polarised light

Question 14

What do enantiomers do to plane-polarised light

Answer 14

They rotate it by the same amount but in opposite directions, and so are optically active

Question 15

If both enantiomers are mixed in equal amounts what is formed and what happens?

Answer 15

A racemix mixture is formed, this is said to be optically inactive because the rotational effect of the plane-polarised light cancels out

Question 16

Enantiomers have identical chemical properties apart from when?

Answer 16

When in a chiral environment such as that found in biological systems (only one optical isomer is usually present)