optical isomers Flashcards
what property must a carbon atom have for the molecule to display optical isomerism about the carbon atom
4 different substituents attached to one carbon atom
what property must a carbon atom have for the molecule to display optical isomerism about the 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
NOT IDENTICAL in chemical properties necessarily
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
carbon that has four different substituents attached to it
how is chiral centre denoted
C*
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 x degrees clockwise
what effect does the - isomer have on plane polarised light
rotates plane of polarisation by x degrees anticlockwise
what is the structure of a polarimeter
light source (unpolarised light) –> polarising filter (polarised light) –> polarised light passes though compartment containing sample –> detector determines the angle of rotation of the plane polarised light
what are polarimeters used for
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 from ethanal
reagents are KCN and HCl but can write HCN since H+ from HCl and -CN from HCN
CH3CHO + HCN –> CH3CH(OH)CN
why is the CH3CH(OH)CN molecule formed chiral
H, CH3, OH and CN groups attached to the centre chiral carbon atom - 4 substituents
why is the CH3CH(OH)CN molecule formed chiral
H, CH3, OH and CN groups attached to the centre chiral carbon atom - 4 substituents
what is the second stage of the synthesis of lactic 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
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
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
