Isomerism including Optical Isomerism Flashcards
Structural Isomers
Compounds with the same molecular formula but different structural formula
Functional group isomers
Compounds with the same molecular formula but with different functional groups
Types of structural isomerism
Chain, position and functional group
Types of stereoisomerism
Geometric and optical isomerism
Geometric isomerism
Arises due to the non-rotation of the carbon-carbon double bond because the pi bond prevents it
Stereoisomers
Compounds with the same structural formula but with atoms/groups arranged differently in space
Conditions for geometric isomerism to occur
There should be two different atoms/groups bonded to each unsaturated carbon atom
There should be a carbon-carbon double bond which restricts rotation
How do you distinguish between 2 stereoisomers
They have different MP’s / BP’s
They have different infrared spectra
There is a difference in their dipoles
Why does one geometric isomer not convert to the other at room temp
The carbon-carbon double bond restricts rotation.
There is not enough energy to break the pi bond at room temperature
Z isomers
When the highest priority groups are on the same side of the carbon-carbon double bond
E isomers
When the highest priority groups are on opposite sides of the carbon-carbon double bond
When do you use E-Z nomenclature instead of cis-trans
When there are 4 different groups around the carbon-carbon double bond
Where is optical isomerism found
In molecules containing a chiral centre
What is a chiral centre
A carbon atom that is bonded to 4 DIFFERENT atoms or groups of atoms
Enantiomers
2 non-superimposible mirror image structures
Properties of optical isomers/enantiomers
Same chemical properties
Similar physical properties
Interact with plane polarised light differently
An asymmetric carbon atom is ……
chiral and gives rise to optical isomers (enantiomers)
Why does ….. show optical isomerism
……shows optical isomerism because it is a chiral molecule. It has a carbon atom that is bonded to 4 different groups
How does plane polarised light effect each enantiomer
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
Racemate
A mixture of equal amounts of enantiomers
What effect does a racemic mixture have on plane polarised light
No effect as the rotations of the two isomers cancel each other out
Explain the term chiral molecule
A chiral molecule is non-superimposable on its mirror image.
A chiral molecule has a carbon atom that is bonded to 4 different groups
How can you distinguish between two enantiomers/optical isomers
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
Why do racemic mixtures not show optical activity
The rotations of the two isomers cancel each other out
Why is only one optical isomer effective as a drug
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
Explain why a racemic mixture and not a single optical isomer is obtained when propanal reacts with HCN
CN- could attack from either side of the planar carbonyl group with equal probability , giving equal amounts of the two optical isomers.
Why is having a mixture of 2 optical isomers not beneficial
You have to separate the mixture and discard the unwanted isomer, so atom economy will be lower
why are racemic mixtures optically inactive
The enantiomers cancel out each others effects and the plane of polarised light will not change
