A2 Nomenclature and isomerism Flashcards
What is stereoisomerism
Where compounds have the same structural formulas but differ in the arrangement of the bonds in space
What are the two types of stereoisomerism
1) E-Z isomerism
2) Optical isomerism
When to optical isomers occur
When there are four different substituents attached to one carbon atom.
What are optical isomers
Molecules that are non- superimposable mirror images of one another.
How do optical isomers differ
- They differ in the way that they rotate the plane of polarisation of polarised light.
- This can be clockwise ((+)- isomer) or anti-clockwise ((-)-isomer)
What are enantiomers
- Enantiomers are optical isomers- they are non-superimposable mirror images of each other.
- They have similar chemical properties but differ in their ability to rotate plane polarised light and in their biological activity.
What is the chiral centre of a molecule
The carbon atom bearing four different substituents. It can also be called the chiral carbon/ asymmetric carbon.
What is chirality
When a compound and its mirror image are non-superimposable
Define optical activity
- The ability of a single optical isomer to rotate the plane of polarisation of plane-polarised monochromatic light.
- An optically active molecule has no planes of symmetry.
What does unpolarised (ordinary) light consist of
Vibrating electric and magnetic fields that oscillate in all planes perpendicular to the direction of travel.
What is plane- polarised light
Monochromatic light that oscillates only in one plane
What is a Polaroid
A filter that cuts out all light vibrations except for those in one plane.
What is a polariser
A device that converts unpolarised light to plane-polarised light
What is a polarimeter
- A device for measuring an angle of rotation caused by a chemical.
- Plane polarised light passes through the analyser.
What is a race mix mixture/racemate
An equimolar mixture of enantiomers of a chiral compound.
Is a racemic mixture optically active
No
Why isn’t a racemic mixture optically active
A racemic mixture doesn’t rotate the plane of polarisation of plans-polarised light because the two enantiomers cancel out each others light rotating effect.
Describe how optical rotation can be measured using a polarimeter
1) Polarised light is passed through two solutions of the same concentration, each containing a different optical isomer of the same substance.
2) One solution will rotate the plane of polarisation through a clockwise angle ((+)-isomer)
3) The other will rotate the plane of polarisation by the same angle anti-clockwise. This is the ((-)-isomer)
Compare how chiral molecules are made synthetically and in nature
- Often, when made synthetically, a compound ends up as a mixture of optical isomers.
- However when the same compound is made naturally in living systems, only one of the isomers is formed.
Why is it often only one isomer produced when a chiral compound is formed in nature
Many naturally occurring molecules are made using enzyme catalysts which cause one of the isomers to be formed.
Why is it that sometimes only one optical isomer of a compound is effective as a drug
- Many drugs work by binding with the active site or receptors of a cell which have specific three dimensional structures.
- This means that sometimes only one optical isomer of a drug will fit and therefore on it one isomer will be effective.
Describe the three solutions to the problem that one enantiomer of a drug may be effective whilst the other is not
1) Separate the two isomers- this may be difficult and expensive as optical isomers have very similar properties.
2) Sell the mixture as a drug- this is wasteful as half of it is inactive.
3) Design an alternative synthesis of the drug that makes only the required isomer.
