Optical Isomerism Flashcards
What property must a carbon atom have for the
molecule to display optical isomerism about that carbon atom?
4 different substituents attached to one carbon atom
What are the similarities and differences between 2 optical isomers?
Same atoms and bonds, but they are non-superimposable mirror images of one another.
Differ in the way they rotate plane polarised light - rotate plane of polarisation by the same angle but in different directions.
What word is used to describe optically active
molecules?
chiral
What are the pair of isomers called?
Enantiomers
What is the chiral centre?
The carbon that has four different substituents attached to it
How is the chiral centre denoted?
C* (star on C)
Give two examples of chiral molecules.
All alpha amino acids, except glycine.
Lactic acid / 2-hydroxypropanoic acid
How is light polarised?
By passing it through a polaroid filter, so oscillations are only in one plane.
What effect does the racemic mixture have on plane polarised light?
None, as the rotation by each enantiomer cancels out to nothing
What effect does the +
isomer have on plane
polarised light?
Rotates plane of polarisation by x0 clockwise
What effect does the -
isomer have on plane
polarised light?
Rotates plane of polarisation by xo anti clockwise
same angle, opposite direction
What is the structure of a polarimeter?
Light source (unpolarised light) → polarising filter (polarised light) → polarised light passes through compartment containing sample → detector determines the angle of rotation of the plane polarised light
What are polarimeters used for?
To identify which enantiomer is present, the purity of the sample, the concentration of the sample etc.
What is the first stage of the synthesis of lactic acid
2-hydroxypropanoic acid) from ethanal? (Equation
(Reagents are KCN and HCl but it is acceptable to write HCN in the balanced eqn as this is the H+ from the HCl and -CN from KCN)
CH3CHO + HCN → CH3CH(OH)CN
Why is the CH3CH(OH)CN molecule formed chiral?
H, CH3, OH and CN groups attached to the central chiral carbon atom - 4 substituents
What is the second stage of the synthesis of lactic acid (2-hydroxypropanoic acid) from ethanal?
Hydrolysis: CH3CH(OH)CN + HCl +2H2O → CH3CH(OH)COOH + NH4Cl
How does this second stage affect the chirality?
Does not affect it - still racemic as chirality not
affected by this stage.
Are racemic mixtures formed in nature? Why?
Not often, as enzyme mechanisms are 3D so only form one enantiomer
Why is optical isomerism a problem for the drug industry?
Sometimes, only one enantiomer is effective due
to enzyme’s active site/cell receptors being 3D.
What are the options to resolve the issue of only one
enantiomer being effective?
- Separate enantiomers - difficult and expensive as have very similar properties
- Sell racemate - wasteful as half is inactive
- Design alternative synthesis to only produce one enantiomer
Examples of optically active drugs?
Ibuprofen, Thalidomide
Why is ibuprofen able to be sold as a racemate, even though the + isomer is needed to treat inflammation?
Sold as 50% racemate.
But body converts 60% of R- isomer to S+ isomer → end up with 80% S+ isomer
