Optical Isomerism

Question 1

What are stereoisomers?

Answer 1

• Isomers with the structural formula, but a different spatial arrangement of atoms in space

Question 2

What type of isomerism do optical isomers display?

Answer 2

• Stereoisomerism

Question 3

Why does stereoisomerism occur in optical isomers?

Answer 3

• Occurs as a result of of chirality in molecules, limited to molecules
  with a single chiral centre

Question 4

What is a chiral atom?

Answer 4

• An atom with four different groups attached to it

Question 5

How many different ways can you arrange the groups around a chiral carbon?

Answer 5

• 2 ways

Question 6

What are the two molecules arranged differently around a chiral carbon called?

Answer 6

• Enantiomers

- Optical isomers

Question 7

Are enantiomers mirror images of each other and are they super-imposable?

Answer 7

• Enantiomers are mirror images
• Cannot be superimposed
• If molecules can be superimposed, they’re achiral

Question 8

Define the term chiral

Answer 8

• Asymmetric in such a way that the structure and its mirror image are not superimposable
• E.g left and right hands

Question 9

Define the term superimposable

Answer 9

The ability for an object to be placed over another object, usually in such a way that both will be visible

Question 10

Define the term achiral

Answer 10

• Symmetric in such a way that it can be superimposed on its mirror image

Question 11

Optical isomers are optically active. What does this mean?

Answer 11

• Optical isomers rotate plane polarised light

Question 12

How can you distinguish between optical isomers?

Answer 12

• Optical isomers have the same physical properties, but they rotate the plane of polarised light in opposite directions
• Therefore they can be distinguished using a polarimeter

Question 13

What happens when plane polarised light hits optical isomers?

Answer 13

• Plane polarised light is passed through separate solutions of a pair of enantiomers
• The enantiomers rotate the light in equal and opposite directions

Question 14

What is plane polarised light?

Answer 14

• Normal light vibrates in all directions
• If normal light is passed through a polarising filter it becomes plane polarised
• Meaning, all light is vibrating in the same plane (e.g up and down or side to side)

Question 15

Define the term racemate or racemic mixture

Answer 15

• A racemate contains equal quantities of each enantiomer

Question 16

What is the optical activity of racemates like?

Answer 16

• No observable optical activity

- The effect of the two enantiomers cancel each other out