optical isomers

Question 1

what property must a carbon atom have for the molecule to display optical isomerism about the carbon atom

Answer 1

4 different substituents attached to one carbon atom

Question 2

what property must a carbon atom have for the molecule to display optical isomerism about the carbon atom

Answer 2

4 different substituents attached to one carbon atom

Question 3

what are the similarities and differences between 2 optical isomers

Answer 3

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

Question 4

what word is used to describe optically active molecules

Answer 4

chiral

Question 5

what are the pair of isomers called

Answer 5

enantiomers

Question 6

what is the chiral centre

Answer 6

carbon that has four different substituents attached to it

Question 7

how is chiral centre denoted

Answer 7

C*

Question 8

how is light polarised

Answer 8

by passing it through a polaroid filter, so oscillations are only in one plane

Question 9

what effect does the racemic mixture have on plane polarised light

Answer 9

none, as the rotation by each enantiomer cancels out to nothing

Question 10

what effect does the + isomer have on plane polarised light

Answer 10

rotates plane of polarisation by x degrees clockwise

Question 11

what effect does the - isomer have on plane polarised light

Answer 11

rotates plane of polarisation by x degrees anticlockwise

Question 12

what is the structure of a polarimeter

Answer 12

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

Question 13

what are polarimeters used for

Answer 13

identify which enantiomer is present, the purity of the sample, the concentration of the sample

Question 14

what is the first stage of the synthesis of lactic acid from ethanal

Answer 14

reagents are KCN and HCl but can write HCN since H+ from HCl and -CN from HCN

CH3CHO + HCN –> CH3CH(OH)CN

Question 15

why is the CH3CH(OH)CN molecule formed chiral

Answer 15

H, CH3, OH and CN groups attached to the centre chiral carbon atom - 4 substituents

Question 16

why is the CH3CH(OH)CN molecule formed chiral

Answer 16

H, CH3, OH and CN groups attached to the centre chiral carbon atom - 4 substituents

Question 17

what is the second stage of the synthesis of lactic acid from ethanal

Answer 17

hydrolysis
CH3CH(OH)CN + HCl + 2H2O –> CH3CH(OH)COOH + NH4Cl

Question 18

how does this second stage affect the chirality

Answer 18

does not affect it - still racemic as chirality not affected by this stage

Question 19

are racemic mixtures formed in nature

Answer 19

not often as enzyme mechanisms are 3D so only form one enantiomer

Question 20

why is optical isomerism a problem for the drug industry

Answer 20

sometimes, only one enantiomer is effective due to enzyme’s active site/cell receptors being 3D

Question 20

why is optical isomerism a problem for the drug industry

Answer 20

sometimes, only one enantiomer is effective due to enzyme’s active site/cell receptors being 3D

Question 21

what are the options to resolve the issue of only one enantiomer being effective

Answer 21

1. separate enantiomers - difficult and expensive as have very similar properties
2. sell racemate - wasteful as half is inactive
3. design alternative synthesis to only produce one enantiomer

Question 22

examples of optically active drugs

Answer 22

ibuprofen, thalidomide