7. Optical Isomerism Flashcards
What is structural isomerism?
Compounds with the same molecular formula but different structural formula
What is stereoisomerism?
Compounds with the same structural formula, but a different arrangement of atoms in a 3D space
What can structural isomerism be divided into?
- positional
- chain
- functional group
What can stereoisomerism be divided into?
- geometrical
* optical
When does optical isomerism occur?
When there are four different atoms or groups bonded to a central carbon atom
What does it mean when a carbon atom is bonded to four different groups?
It is asymmetric and so cannot be superimposed on its mirror image
What are enantiomers?
When two molecules with an asymmetrical carbon atom cannot be superimposed
What are chiral molecules?
Molecules which contain an asymmetrical carbon atom, which is attached to four different groups
What is the shape of chiral molecules?
Tetrahedral
How may different enantiomers react?
- may react differently with other asymmetric molecules
* will interact differently with plane polarised light
What type of molecules will rotate the plane of plane-polarised light?
Chiral molecules
What is an optically active substance?
One that can rotate plane-polarised light
How will two optical isomers rotate plane polarised light?
Equally, but in opposite directions
How can optical isomers be distinguished?
Rotation of plane polarised light in opposite directions
How is an isomer that rotates plane-polarised light clockwise named?
Given prefix + or D (dextrorotatory)
How is an isomer that rotates plane-polarised light anticlockwise named?
Given prefix - or L (laevorotatory)
What does it mean if a sample is tested and produces no rotation of the plane-polarised light?
It is either not chiral or a 50:50 mixture of the two enantiomers (racemate)
What is a racemic mixture?
An equimolar mixture mixture of two optical isomers, which is not optically active
When will chiral molecules show optical activity?
Only if one isomer is present in greater quantities than the other
How can whether a single enantiomer or a racemate will be produced be predicted?
By looking at the mechanism for the reaction
When will addition reactions produce a racemate?
Always
Why will addition reactions always produce a racemate?
As the electrophile can attack the planar molecule from above or below with equal probability
What type of addition reactions produce racemic mixtures?
Both electrophilic and nucleophilic
Will substitution reactions produce a racemate? Why is this?
No - if the starting molecule is a single enantiomer, then the attacking species can only attack from one side and a single enantiomer will be produced
In a substitution reaction where hydroxide ion replaces. a negative halide, where will it attack?
From behind, the opposite side to the halide, always producing the same enantiomer
When will nucleophilic substitution reactions produce a single enantiomer?
When the reacting chiral molecule was also a single enantiomer
Do most optical isomers show identical chemical properties in most reactions?
Yes
When one optical isomer is an effective drug and the other is inactive, what can be done?
- separate the two isomers
- sell the mixture as a drug
- design alternative synthesis of drug that makes only the required isomer
What is the problem with separating two isomers when one is an effective drug and the other isn’t?
This is very difficult as the optical isomers have very similar properties
What is the problem with selling a mixture of two isomers as a drug when one is an effective drug and the other isn’t?
Wasteful as half is inactive
What is thalidomide?
A drug to treat morning sickness, one enantiomer caused birth defects
Which drug has enantiomers which do not act the same?
Ibuprofen
