3.7 Optical Isomerism
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What form of isomerism is optical?
Stereoisomerism
What is optical isomerism?
They have the same molecular formula and same structural formula but a different arrangement of atoms in space
What are significant features of optical isomers?
They are mirror images but are non-superimposable on each other They have a chiral carbon atom
What is a chiral carbon atom?
It has 4 DIFFERENT functional groups attached to it Called the chiral centre
How can the chiral centre be shown?
* will be next to the carbon
What are two optical isomers also called?
Enantiomers
Why is optical isomerism possible?
The isomers are 3D structures Therefore can only be truly shown by 3D representation
What is the difference between white light and plane polarised light?
White light has many vibrations of all light Plane polarised light has all the vibrations cut out but one plane
What do you pass white light through to polarise it?
A Polaroid
How do optical isomers affect plane polarised light?
They affect the rotation of the plane polarised light differently i.e one clockwise and one anti-clockwise
What is the process of testing optical isomers with plane polarised light?
Polarised light is passed through 2 solutions of the same concentration each containing a different optical isomer of the same substance One solution rotates the plane of polarisation through an angle clockwise: + isomer And the other by the same angle anti-clockwise: - isomer
How is optical rotations measured?
Using a polarimeter
What mixture is specific to optical isomers?
A 50/50 mixture of two optical isomers of the same substance is called a racemate or racemic mixture
What is specific about a racemate?
It is not optically active as the effects of the 2 isomers cancel out
How is a racemate formed?
By mixing equal amounts of enantiomeric substances Or Formed when achiral substances are converted into chiral ones
What is the issue of optical isomers in the drug industry?
In many cases only 1 isomer of the two is effective therefore if a racemate has been produced to acquire the isomer it becomes a problem
What can be done in the drug industry if a racemate is present?
Separate the 2 isomers (but this is difficult and expensive as they have similar properties) Sell the mixture of a drug (it is wasteful as half of it will be inactive) Design an alternative synthesis of the drug that makes only the required isomer