3.3.7 Optical Isomerism (A2)

Question 1

what is the definition of a positional isomer?

Answer 1

same molecular formula
different functional group position

Question 2

what is the definition of a functional group isomer?

Answer 2

same molecular formula
different functional group

Question 3

what are chain isomers?

Answer 3

same molecular formula
different chain length

Question 4

what is a stereoisomer?

Answer 4

same molecular formula
different arrangement of atoms in space

Question 5

what are the two types of stereoisomer?

Answer 5

E/Z
optical

Question 6

when does optical isomerism occur?

Answer 6

when a carbon atom has 4 different substituent groups attached via bonds (chiral)

Question 7

what are the rules when drawing a pair of enantiomers?

Answer 7

draw shapes as a tetrahedral around a chiral carbon
show a mirror as a dotted line and label as ‘imaginary mirror’
draw the image as an exact mirror image with the same groups having dotted and wedge bonds

Question 8

how do optical isomers of the same molecule differ?

Answer 8

how they rotate plane-polarised light

Question 9

what is plane-polarised light?

Answer 9

where light wave moves in only one plane

Question 10

what is unpolarised light?

Answer 10

waves vibrate in all directions

Question 11

how can separate samples of enantiomers be distinguished?

Answer 11

pass through plane polarised light
measure angle and direction light is rotated by
one enantiomers rotates clockwise
other rotates anticlockwise

Question 12

what is a racemic mixture?

Answer 12

optically active products are produced in organic synthesis pathways we get a 50:50 mixture of the +isomer and the -isomer
no optical activity because the effect of the two enantiomers cancels out

Question 13

what is a nucleophile?

Answer 13

electron donor
lone pairs of e-
“love positive charge”

Question 14

what is a cyanide ion?

Answer 14

C triple bond N

Question 15

what type of (x)phile is the cyanide ion?

Answer 15

nucleophile

Question 16

does 2-hydroxylpentanenitrile have a chiral carbon? why?

Answer 16

yes
4 different groups attached to carbon

Question 17

why does the oxygen not re form the double bond in nucleophilic attack (addition)

Answer 17

the carbon is not delta positive enough

Question 18

what between C and ? makes the carbon delta positive?

Answer 18

double bond oxygen

Question 19

what does the carbon have to be for the oxygen to reform the double bond?

Answer 19

delta positive (from at least 1 other group attached) eg. Cl, N etc.

Question 20

what are the similarities and differences between two optical isomers?

Answer 20

similarities - same atoms and bonds, but they are non-superimposable mirror images of one another.
differences - 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 21

what does the hydrolysis of 2-hydroxypentanenitrile produce?

Answer 21

racemic mixture of 2-hydroxypropanoic acid enantiomers

Question 22

why is nucleophillic addition of a nitrile important in chemical synthesis?

Answer 22

increases the number of carbons in the chain
this is difficult to do

Question 23

why is only one enantiomers of lactic acid made in the body naturally?

Answer 23

the enzyme that catalyses the reaction has a very specific active site which can differentiate between enantiomers

Question 24

what is the problem with producing a racemic mixture of a drug?

Answer 24

most drugs work by fitting to a complementary receptor within the body - shape is very important
therefore only one optically active isomer will fit and the other is inactive.

Question 25

what are the solutions for producing a racemic mixture of a drug for the body where only one optically active enantiomer fits to the receptor?

Answer 25

separate the two enantiomers sell as a mixture design a synthesis pathway that only produces one isomer rather than a racemic mixture (usually incorporates enzymes)

Question 26

what is the downside to separating enantiomers?

Answer 26

difficult and too expensive

Question 27

what is the downside to selling drugs as a racemic mixture?

Answer 27

wasteful

Question 28

what happens with ibuprofen?

Answer 28

exists as two enantiomers one is an anti-inflammatory one is inactive (harmless) enzyme used to convert inactive to active enantiomer but is only X% effective *sold as a mixture*

Question 29

what happens with naproxen?

Answer 29

one enantiomer relieves pain from arthritis one enantiomer causes liver failure *synthesis pathway designed to get one enantiomer*

Question 30

what happened with thalidomide?

Answer 30

one enantiomer cured morning sickness one enantiomer caused limb shortening in unborn babies *withdrawn from use*

Question 31

what are the pair of isomers called?

Answer 31

enantiomers

Question 32

what is the chiral centre?

Answer 32

the carbon that has 4 different substituents attached to it

Question 33

how is the chiral centre denoted?

Answer 33

C*

Question 34

how is light polarised?

Answer 34

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

Question 35

what effect does the racemic mixture have on plane polarised light?

Answer 35

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

Question 36

what effect does the + enantiomer have on plane polarised light?

Answer 36

rotates plane of polarisation by x degrees clockwise

Question 37

what effect does the - enantiomer have on plane polarised light?

Answer 37

rotates plane polarised light by x degrees anti-clockwise

Question 38

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

Answer 38

CH3CHO + HCN —> CH3CH(OH)CN

Question 39

what is the second stage of the synthesis of lactic acid from ethanal? how does this affect the chirality?

Answer 39

hydrolysis: CH3CH(OH)CN + HCl + 2H2O —> CH3CH(OH)COOH + NH4Cl does not affect it - still racemic as chirality not affected by this stage.

Question 40

are racemic mixtures formed in nature? why?

Answer 40

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

Question 41

why is optical isomerism a problem for the drug industry?

Answer 41

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