7. Optical Isomerism

Question 1

What is structural isomerism?

Answer 1

Compounds with the same molecular formula but different structural formula

Question 2

What is stereoisomerism?

Answer 2

Compounds with the same structural formula, but a different arrangement of atoms in a 3D space

Question 3

What can structural isomerism be divided into?

Answer 3

• positional
• chain
• functional group

Question 4

What can stereoisomerism be divided into?

Answer 4

• geometrical

* optical

Question 5

When does optical isomerism occur?

Answer 5

When there are four different atoms or groups bonded to a central carbon atom

Question 6

What does it mean when a carbon atom is bonded to four different groups?

Answer 6

It is asymmetric and so cannot be superimposed on its mirror image

Question 7

What are enantiomers?

Answer 7

When two molecules with an asymmetrical carbon atom cannot be superimposed

Question 8

What are chiral molecules?

Answer 8

Molecules which contain an asymmetrical carbon atom, which is attached to four different groups

Question 9

What is the shape of chiral molecules?

Answer 9

Tetrahedral

Question 10

How may different enantiomers react?

Answer 10

• may react differently with other asymmetric molecules

* will interact differently with plane polarised light

Question 11

What type of molecules will rotate the plane of plane-polarised light?

Answer 11

Chiral molecules

Question 12

What is an optically active substance?

Answer 12

One that can rotate plane-polarised light

Question 13

How will two optical isomers rotate plane polarised light?

Answer 13

Equally, but in opposite directions

Question 14

How can optical isomers be distinguished?

Answer 14

Rotation of plane polarised light in opposite directions

Question 15

How is an isomer that rotates plane-polarised light clockwise named?

Answer 15

Given prefix + or D (dextrorotatory)

Question 16

How is an isomer that rotates plane-polarised light anticlockwise named?

Answer 16

Given prefix - or L (laevorotatory)

Question 17

What does it mean if a sample is tested and produces no rotation of the plane-polarised light?

Answer 17

It is either not chiral or a 50:50 mixture of the two enantiomers (racemate)

Question 18

What is a racemic mixture?

Answer 18

An equimolar mixture mixture of two optical isomers, which is not optically active

Question 19

When will chiral molecules show optical activity?

Answer 19

Only if one isomer is present in greater quantities than the other

Question 20

How can whether a single enantiomer or a racemate will be produced be predicted?

Answer 20

By looking at the mechanism for the reaction

Question 21

When will addition reactions produce a racemate?

Answer 21

Always

Question 22

Why will addition reactions always produce a racemate?

Answer 22

As the electrophile can attack the planar molecule from above or below with equal probability

Question 23

What type of addition reactions produce racemic mixtures?

Answer 23

Both electrophilic and nucleophilic

Question 24

Will substitution reactions produce a racemate? Why is this?

Answer 24

No - if the starting molecule is a single enantiomer, then the attacking species can only attack from one side and a single enantiomer will be produced

Question 25

In a substitution reaction where hydroxide ion replaces. a negative halide, where will it attack?

Answer 25

From behind, the opposite side to the halide, always producing the same enantiomer

Question 26

When will nucleophilic substitution reactions produce a single enantiomer?

Answer 26

When the reacting chiral molecule was also a single enantiomer

Question 27

Do most optical isomers show identical chemical properties in most reactions?

Answer 27

Yes

Question 28

When one optical isomer is an effective drug and the other is inactive, what can be done?

Answer 28

• separate the two isomers
• sell the mixture as a drug
• design alternative synthesis of drug that makes only the required isomer

Question 29

What is the problem with separating two isomers when one is an effective drug and the other isn’t?

Answer 29

This is very difficult as the optical isomers have very similar properties

Question 30

What is the problem with selling a mixture of two isomers as a drug when one is an effective drug and the other isn’t?

Answer 30

Wasteful as half is inactive

Question 31

What is thalidomide?

Answer 31

A drug to treat morning sickness, one enantiomer caused birth defects

Question 32

Which drug has enantiomers which do not act the same?

Answer 32

Ibuprofen