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Organic Chemistry > 3.3.7 Optical Isomerism (A2) > Flashcards

3.3.7 Optical Isomerism (A2) Flashcards

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1
Q

what is the definition of a positional isomer?

A

same molecular formula
different functional group position

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2
Q

what is the definition of a functional group isomer?

A

same molecular formula
different functional group

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3
Q

what are chain isomers?

A

same molecular formula
different chain length

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4
Q

what is a stereoisomer?

A

same molecular formula
different arrangement of atoms in space

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5
Q

what are the two types of stereoisomer?

A

E/Z
optical

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6
Q

when does optical isomerism occur?

A

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

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7
Q

what are the rules when drawing a pair of enantiomers?

A

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

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8
Q

how do optical isomers of the same molecule differ?

A

how they rotate plane-polarised light

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9
Q

what is plane-polarised light?

A

where light wave moves in only one plane

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10
Q

what is unpolarised light?

A

waves vibrate in all directions

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11
Q

how can separate samples of enantiomers be distinguished?

A

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

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12
Q

what is a racemic mixture?

A

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

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13
Q

what is a nucleophile?

A

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

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14
Q

what is a cyanide ion?

A

C triple bond N

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15
Q

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

A

nucleophile

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16
Q

does 2-hydroxylpentanenitrile have a chiral carbon? why?

A

yes
4 different groups attached to carbon

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17
Q

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

A

the carbon is not delta positive enough

18
Q

what between C and ? makes the carbon delta positive?

A

double bond oxygen

19
Q

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

A

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

20
Q

what are the similarities and differences between two optical isomers?

A

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.

21
Q

what does the hydrolysis of 2-hydroxypentanenitrile produce?

A

racemic mixture of 2-hydroxypropanoic acid enantiomers

22
Q

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

A

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

23
Q

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

A

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

24
Q

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

A

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.

25
Q

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?

A

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)

26
Q

what is the downside to separating enantiomers?

A

difficult and too expensive

27
Q

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

A

wasteful

28
Q

what happens with ibuprofen?

A

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

29
Q

what happens with naproxen?

A

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

30
Q

what happened with thalidomide?

A

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

31
Q

what are the pair of isomers called?

A

enantiomers

32
Q

what is the chiral centre?

A

the carbon that has 4 different substituents attached to it

33
Q

how is the chiral centre denoted?

A

C*

34
Q

how is light polarised?

A

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

35
Q

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

A

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

36
Q

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

A

rotates plane of polarisation by x degrees clockwise

37
Q

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

A

rotates plane polarised light by x degrees anti-clockwise

38
Q

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

A

CH3CHO + HCN —> CH3CH(OH)CN

39
Q

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

A

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

40
Q

are racemic mixtures formed in nature? why?

A

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

41
Q

why is optical isomerism a problem for the drug industry?

A

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

Organic Chemistry (16 decks)

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