Unit 3: Section 5 - Isomerism and Carbonyl Compounds
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What property must a carbon atom have for the molecule to display optical isomerism about that carbon atom?
4 different substituents attached to 1 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, NOT ALWAYS IDENTICAL in chemical properties Different 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 4 different substituents attached to it
How is the chiral centre denoted?
C*
Give 2 examples of chiral molecules
• All alpha amino acids, excpet glycerine • Lactic acid / 2-hydroxypropanoic acid
How is light polarised?
By passing it through a polaroid filter, so oscillations are only in 1 plane
What effect does the racemic mixture have on plane polarised light?
None, as the rotation by each enantiomer cancels out
What effect does the + isomer have on plane polarised light?
Rotates plane of polarisation clockwise
What effect does the - isomer have on plane polarised light?
Rotates plane of polarisation anti-clockwise
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
What is the first stage of the synthesis of lactic acid (2-hydorxypopanoic acid) from ethanal? And the equation?
Reagents: KCN and HCl 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 the 2nd stage synthesis of lactic acid (2-hydroxypropanoic acid) from ethanal affect the chirality?
Doesn’t affect it - still racemic as chirality is not affected by this stage
Are racemic mixtures formed in nature? Why?
Not often, as enzyme mechanisms are 3D so only form 1 enantiomer
Why is optical isomerism a problem for the drug industry?
Sometimes, only 1 enantiomer is effective due to enzyme’s active site/cell receptors being 3D
What are the options to resolve the issue of only 1 enantiomer being effective?
• Separate enantiomers - difficult and expensive as have very similar properties • Sell racemate - wasteful as half is inactive • Designing alternative synthesis to only produce 1 enantiomer
What are some examples of optically active drugs?
Ibruprofen, Thalidomide
Why is ibruprofen able to be sold as a racemate, even thought the + isomer is needed to treat inflammation?
• Sold as a 50% racemate • The body converts 60% of - isomer to + isomer • End up with 80% + isomer
What is the carbonyl group?
C=O
What is the functional group and general formula for an aldehyde?
RCHO (C double bonded to O, single bond to H and R)