4.1 - Stereoisomerism

Question 1

What are stereoisomers?

Answer 1

Stereoisomers are compounds with the
same structural formula, but with a
different arrangement of atoms in space.

Question 2

How is stereoisomerism different to structural isomerism?

Answer 2

Structural isomers have the same molecular formula but different structural formulas. Stereoisomers have
the same molecular formula and the same structural formula and only differ in the arrangement of the atoms in space.

Question 3

Give the two main types of
stereoisomerism

Answer 3

• Optical isomerism
• E-Z isomerism

Question 4

What is E-Z isomerism?

Answer 4

E-Z isomerism occurs due to the restricted rotation around a carbon double bond. This means that if
both the double-bond carbons have different groups attached to them, then different isomers are produced depending on the arrangement of the
groups around the double bond.

Question 5

Name and define the two isomers E-Z isomerism produces

Answer 5

• E-isomer - the high priority groups on each side of the double bond are diagonally across from each other.
• Z-isomer - the high priority groups on each side of the double bond are either both above or both below the carbon double bond.

Question 6

Draw and name the two isomers produced from but-2-ene

Question 7

Draw and name the two isomers
produced from 1-bromo-1-chloro-2-methylbutane

Answer 7

Question 8

How can you determine the priority of the groups in E-Z isomerism?

Answer 8

Look at the atoms which are directly
bonded to each of the C=C carbon atoms. On each carbon, the atom with the higher atomic number is given the higher priority.

Question 9

Define optical isomerism

Answer 9

Optical isomerism is a type of
stereoisomerism. Optical isomers have the same structure but are non-superimposable mirror images of each other.

Question 10

What is a chiral centre?

Answer 10

A chiral centre is a carbon atom which
has four different groups attached to it. It is often called the chiral or asymmetric carbon.

Question 11

Why is a chiral centre required for optical isomerism?

Answer 11

A chiral centre has four different groups
attached to it and these groups can be
arranged in different ways to give two different molecules known as optical isomers.

Question 11

How is a chiral centre often denoted?

Question 12

Answer 12

Question 13

Define enantiomer

Answer 13

Two different molecules which are
non-superimposable mirror images of
each other and arise from optical
isomerism.

Question 14

Explain the optical activity of optical isomers

Answer 14

Optical isomers are optically active which means they rotate plane polarised light.

Question 15

What is a racemic mixture?

Answer 15

A racemic mixture is a mixture which
contains equal quantities of each
enantiomer of an optically active
compound.

Question 16

How does the optical activity differ for a pair of enantiomers?

Answer 16

The two enantiomers will rotate plane
polarised light in opposite directions.

Question 17

What is another term for a racemic mixture?

Answer 17

A racemate.

Question 18

What effect does a racemic mixture have on plane polarised light?

Answer 18

A racemic mixture has no effect on plane polarised light since there are equal quantities of each enantiomer present. Each enantiomer rotates plane polarised light in opposite directions so they cancel each other out.

Question 19

Draw the two enantiomers of
2-hydroxypropanoic acid

Answer 19

Question 20

Why is optical isomerism a problem for the drug industry?

Answer 20

Often only one enantiomer is effective due to an enzyme’s active site/cell receptors being 3D. A reaction can be modified to produce a single enantiomer but it is difficult and expensive.

Question 21

Why do reactions involving planar bonds often produce racemates?

Answer 21

Planar bonds can be attacked by a nucleophile from either above or below. If the carbon has different groups attached to it then two enantiomers will be produced depending on the direction of attack by the nucleophile on the planar bond. A racemic mixture will be produced as there’s an equal chance of forming each of the enantiomers