25 - Nomenclature and Isomerism Flashcards
What is stereoisomerism?
When two or more compounds have the same structural formula but a different arrangement of bonds in space
What are the two types of stereoisomerism?
Geometric (E-Z)
Optical
How do optical isomers occur?
When a molecule has a chiral centre on a carbon atom (asymmetric carbon atom), which has four different groups attached. This results in two molecules that are non-superimposable mirror images of one another
How are optical isomers differentiated between?
Using a polarimeter.
How does a polarimeter work?
A light source produces light vibrating in all directions.
A polarising filter only allows through light vibrating in one direction.
The plan polarised light passes through the sample.
If the substance is optically active it rotates the plane polarised light.
The direction of rotation is measured coming towards the observer.
What are enantiomers?
A pair of optical isomers.
What does the direction of light rotation mean in polarimetry in terms of enantiomers?
If the light turned right (clockwise) it is a (+) enantiomer or (d).
If the light turned left (anticlockwise) it is a (-) enantiomer or (l).
How are chiral centres indicated?
*
What is a racemic mixture/ racemate?
A 50:50 mixture of both of the enantiomers, which is not optically active as the effects of the isomers cancel out.
What is one way optical isomers can be formed?
When carbonyl compounds undergo nucleophilic addition, if two different groups bond to the C=O.
What are some issues with optical isomers in drugs?
For some uses only one isomer is required, as it fits in the active site.
What are the problems with separation of enantiomers?
It is complex and expensive.
Non-separation may lead to larger doses needed or dangerous side effects.
What are some solutions to separate enantiomers?
Use stereoselective reactions which give one isomer.
Use a catalyst which gives one isomer.
Use stereoselective enzymes or bacteria.
What is Thalidomide?
A drug which is a chiral molecule and can exist as two enantiomers. It was used to treat anxiety and morning sickness in pregnant women.
What was the issue with Thalidomide?
One of the enantiomers was safe and effective, however the other caused children to be born with birth defects and missing limbs.
