A2 Nomenclature and isomerism

Question 1

What is stereoisomerism

Answer 1

Where compounds have the same structural formulas but differ in the arrangement of the bonds in space

Question 2

What are the two types of stereoisomerism

Answer 2

1) E-Z isomerism
2) Optical isomerism

Question 3

When to optical isomers occur

Answer 3

When there are four different substituents attached to one carbon atom.

Question 4

What are optical isomers

Answer 4

Molecules that are non- superimposable mirror images of one another.

Question 5

How do optical isomers differ

Answer 5

• They differ in the way that they rotate the plane of polarisation of polarised light.
• This can be clockwise ((+)- isomer) or anti-clockwise ((-)-isomer)

Question 6

What are enantiomers

Answer 6

• Enantiomers are optical isomers- they are non-superimposable mirror images of each other.
• They have similar chemical properties but differ in their ability to rotate plane polarised light and in their biological activity.

Question 7

What is the chiral centre of a molecule

Answer 7

The carbon atom bearing four different substituents. It can also be called the chiral carbon/ asymmetric carbon.

Question 8

What is chirality

Answer 8

When a compound and its mirror image are non-superimposable

Question 9

Define optical activity

Answer 9

• The ability of a single optical isomer to rotate the plane of polarisation of plane-polarised monochromatic light.
• An optically active molecule has no planes of symmetry.

Question 10

What does unpolarised (ordinary) light consist of

Answer 10

Vibrating electric and magnetic fields that oscillate in all planes perpendicular to the direction of travel.

Question 11

What is plane- polarised light

Answer 11

Monochromatic light that oscillates only in one plane

Question 12

What is a Polaroid

Answer 12

A filter that cuts out all light vibrations except for those in one plane.

Question 13

What is a polariser

Answer 13

A device that converts unpolarised light to plane-polarised light

Question 14

What is a polarimeter

Answer 14

• A device for measuring an angle of rotation caused by a chemical.
• Plane polarised light passes through the analyser.

Question 15

What is a race mix mixture/racemate

Answer 15

An equimolar mixture of enantiomers of a chiral compound.

Question 16

Is a racemic mixture optically active

Answer 16

No

Question 17

Why isn’t a racemic mixture optically active

Answer 17

A racemic mixture doesn’t rotate the plane of polarisation of plans-polarised light because the two enantiomers cancel out each others light rotating effect.

Question 18

Describe how optical rotation can be measured using a polarimeter

Answer 18

1) Polarised light is passed through two solutions of the same concentration, each containing a different optical isomer of the same substance.
2) One solution will rotate the plane of polarisation through a clockwise angle ((+)-isomer)
3) The other will rotate the plane of polarisation by the same angle anti-clockwise. This is the ((-)-isomer)

Question 19

Compare how chiral molecules are made synthetically and in nature

Answer 19

• Often, when made synthetically, a compound ends up as a mixture of optical isomers.
• However when the same compound is made naturally in living systems, only one of the isomers is formed.

Question 20

Why is it often only one isomer produced when a chiral compound is formed in nature

Answer 20

Many naturally occurring molecules are made using enzyme catalysts which cause one of the isomers to be formed.

Question 21

Why is it that sometimes only one optical isomer of a compound is effective as a drug

Answer 21

• Many drugs work by binding with the active site or receptors of a cell which have specific three dimensional structures.
• This means that sometimes only one optical isomer of a drug will fit and therefore on it one isomer will be effective.

Question 22

Describe the three solutions to the problem that one enantiomer of a drug may be effective whilst the other is not

Answer 22

1) Separate the two isomers- this may be difficult and expensive as optical isomers have very similar properties.
2) Sell the mixture as a drug- this is wasteful as half of it is inactive.
3) Design an alternative synthesis of the drug that makes only the required isomer.