Isomerism including Optical Isomerism

Question 1

Structural Isomers

Answer 1

Compounds with the same molecular formula but different structural formula

Question 2

Functional group isomers

Answer 2

Compounds with the same molecular formula but with different functional groups

Question 3

Types of structural isomerism

Answer 3

Chain, position and functional group

Question 4

Types of stereoisomerism

Answer 4

Geometric and optical isomerism

Question 5

Geometric isomerism

Answer 5

Arises due to the non-rotation of the carbon-carbon double bond because the pi bond prevents it

Question 6

Stereoisomers

Answer 6

Compounds with the same structural formula but with atoms/groups arranged differently in space

Question 7

Conditions for geometric isomerism to occur

Answer 7

There should be two different atoms/groups bonded to each unsaturated carbon atom
There should be a carbon-carbon double bond which restricts rotation

Question 8

How do you distinguish between 2 stereoisomers

Answer 8

They have different MP’s / BP’s
They have different infrared spectra
There is a difference in their dipoles

Question 9

Why does one geometric isomer not convert to the other at room temp

Answer 9

The carbon-carbon double bond restricts rotation.
There is not enough energy to break the pi bond at room temperature

Question 10

Z isomers

Answer 10

When the highest priority groups are on the same side of the carbon-carbon double bond

Question 11

E isomers

Answer 11

When the highest priority groups are on opposite sides of the carbon-carbon double bond

Question 12

When do you use E-Z nomenclature instead of cis-trans

Answer 12

When there are 4 different groups around the carbon-carbon double bond

Question 13

Where is optical isomerism found

Answer 13

In molecules containing a chiral centre

Question 14

What is a chiral centre

Answer 14

A carbon atom that is bonded to 4 DIFFERENT atoms or groups of atoms

Question 15

Enantiomers

Answer 15

2 non-superimposible mirror image structures

Question 16

Properties of optical isomers/enantiomers

Answer 16

Same chemical properties
Similar physical properties
Interact with plane polarised light differently

Question 17

An asymmetric carbon atom is ……

Answer 17

chiral and gives rise to optical isomers (enantiomers)

Question 18

Why does ….. show optical isomerism

Answer 18

……shows optical isomerism because it is a chiral molecule. It has a carbon atom that is bonded to 4 different groups

Question 19

How does plane polarised light effect each enantiomer

Answer 19

One enantiomer will rotate the plane of plane polarised light in the clockwise direction

The other will rotate the plane of plane polarised light in the anti-clockwise direction

They roatate the light by equal amounts in opposite directions so there is no overall effect

Question 20

Racemate

Answer 20

A mixture of equal amounts of enantiomers

Question 21

What effect does a racemic mixture have on plane polarised light

Answer 21

No effect as the rotations of the two isomers cancel each other out

Question 22

Explain the term chiral molecule

Answer 22

A chiral molecule is non-superimposable on its mirror image.
A chiral molecule has a carbon atom that is bonded to 4 different groups

Question 23

How can you distinguish between two enantiomers/optical isomers

Answer 23

Use a polarimeter to study their effect on plane polarised light

One isomer will rotate the plane of plane polarised light in a clockwise direction and the other isomer will rotate it in an anticlockwise direction by the same amount

Question 24

Why do racemic mixtures not show optical activity

Answer 24

The rotations of the two isomers cancel each other out

Question 25

Why is only one optical isomer effective as a drug

Answer 25

The receptors in the body are stereospecific
The 2 optical isomers have different 3-dimensional shapes
Only one optical isomer has the correct stereochemistry to bind to the receptor molecule

remember enzymes and specific shaped substrates

Question 26

Explain why a racemic mixture and not a single optical isomer is obtained when propanal reacts with HCN

Answer 26

CN- could attack from either side of the planar carbonyl group with equal probability , giving equal amounts of the two optical isomers.

Question 27

Why is having a mixture of 2 optical isomers not beneficial

Answer 27

You have to separate the mixture and discard the unwanted isomer, so atom economy will be lower

Question 28

why are racemic mixtures optically inactive

Answer 28

The enantiomers cancel out each others effects and the plane of polarised light will not change