Optical Isomerism Flashcards
What is optical isomerism?
A form of stereoisomerism displayed by molecules that contain an asymmetric carbon atom
What is an aysmmetric carbon atom?
One that has four different atoms or groups bonded to it
What are the two forms that any molecule that has an aysmmetric carbon atom can exist in?
The two forms are non- superimposable mirror images of one another. They are called optical isomers or enantiomers
What are stereoisomers?
Isomers that have the dame same molecular and structural formula but different arrangements in space or the atoms from which they are made
What is the carbon atom bonded to four different atoms or groups called?
A chiral carbon atom and the molecule is said to have a chiral centre
How do we draw the enantiomer of a chiral molecule?
By imagining the reflection of it in a mirror
Pairs of enantiomers have identical physical and chemical properties. What are the two exceptions?
- they interact differently with plane polarised light
* they interact differently with other aysmmetric molecules, especially enzymes
When is light said to be plane polarised?
When passed through a polariser all the waves are absorbed except for those vibrating in one particular plane
What happens when plane polarised light passes through an optically active compound?
The angle of the plain is rotated either clockwise (+) or anticlockwise (-). The degree by which the plane is rotated is called the optical rotation. Optical isomers of the same compound rotate plane polarised light by equal amounts but in opposite directions
What enantiomers are dextrorotatory D- (+) - and which are laevorotatory L-(-)-
Ones that rotate plane polarised light clockwise and dextrorotatory and ones that rotate it anticlockwise are laevorotatory
How can you tell the direction in which a compound rotates plane polarised light?
It has to be determined experimentally using a polarimeter
- the monochromatic light that’s not linear polarised is shined through a polariser
- the monochromatic light now linear polarused now passes through a polarimetry cell with a chiral substance
- the rotated light now passes through an analyser and then is detected by a detector
What is a racemic mixture/ racemate?
When chiral molecules produced during chemical reactions contain a 50:50 mixture of the two enantiomers
When do racemic mixtures occur?
When there is an equal chance of forming each enantiomer during the chemical reaction. We often obtain racemic mixtures when we do addition reactions on a planar double bond (C=C or C=O) because the reaction can occur with equal probability from either side of the plane
What happens to plane polarised light in a racemate?
The enantiomers in a racemate both rotate the plane of polarised light but the rotation effects of each isomers are equal and opposite and therefore cancel out
