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Flashcards in Optical Purity Deck (102):
1

How do we measure the purity of enantiomers?

Using light

2

Do enantiomers vary in melting point?

No.

3

Do enantiomers vary in boiling point?

No.

4

Do enantiomers vary in solubility?

No.

5

Do enantiomers vary in their interaction with light?

Yes.

6

What is plane-polarised light?

Light that is passed through very fine slits (known as polaroid)

7

When light passes through fine slits, it becomes plane-polarised. What does this mean?

All wavelengths that don't align with the slits are blocked out.

8

What will occur when you pass light through a solution containing an enantiomer?

It will distort the alignment of the light (it will rotate the waves of polarised light)

9

What can be said about the plane of light when passing through a solution formed with a meso compound?

It has a mirror plane.

10

What does it mean for a compound to have a mirror plane?

One chiral centre will rotate the plane a certain amount, while the mirror image will have an equal and opposite effect.

11

What is observed in the plane of light passed through a solution containing a meso compound?

The effects will cancel each other out, giving no effect.

12

Do meso compounds rotate light?

Technically, meso compounds do rotate light, but the observer won't notice any change.

13

Why doesn't the observer notice a change in the plane of light passed through a solution containing a meso compound?

As the net rotation will be 0.

14

Do achiral molecules rotate light?

Achiral molecules will not rotate light at all.

15

Which is the least important part of equipment in a light rotation practical?

The analyser.

16

Why is the analyser not necessarily important in a light-rotation practical?

Because the observer will notice the change themselves.

17

Can you predict which way a compound will rotate light?

No. You must measure it.

18

Is there a relationship between R and S formations and the direction of the rotation of light?

No.

19

What two factors will effect how much light is rotated for a solution of the same enantiomer?

Concentration Distance travelled in solution

20

How does concentration effect the degree of rotation in a light-rotation practical?

Increasing the concentration will increase the rotation

21

How does distance travelled in solution effect the degree of rotation in a light-rotation practical?

Increasing distance travelled in solution increases the rotation.

22

What is specific rotation?

The degree of rotation in a 1 molar solution, travelling through a 1 centimetre path length.

23

What can be said about the specific rotation of a molecule?

It is a characteristic property of the molecule, much like boiling point or solubility.

24

Are protonated and deprotonated arrangements of a compound the same molecule?

No.

25

As protonated and deprotonated arrangements of the same compound are different molecules, what does this mean for their specific rotation?

They will have a different specific rotation.

26

What is the specific rotation of racemic mixtures?

0°.

27

What are racemic mixtures?

Equal mixtures of two enantiomers.

28

If the specific rotation of a compound is greater than the given reference value, what does this mean?

It is not that compound.

29

What range must the specific rotation of an unknown molecule fall in to possibly be the same as the reference molecule?

It must be equal to or greater than -(reference value) but equal to or lesser than +(reference value)

30

Can you express optical purity without knowing the reference value?

No.

31

Will knowing the specific rotation of a compound give you the D/L configuration?

Yes.

32

Will knowing the specific rotation of a compound give you the R/S configuration?

No.

33

Do diastereomers have the same molecular properties?

No.

34

What is a consequence of diastereomers having different molecular properties?

They can be separated.

35

What is the reference value?

The reference value is always the value of the pure enantiomer.

36

What does L stand for in D/L notation?

Levorotory

37

What does D stand for in D/L notation?

Dextrotory

38

What sign is L in D/L notation?

Negative

39

What sign is D in D/L notation?

Positive.

40

If you have an optical purity value of 85%, what does this mean?

This means you have 85% excess, plus half of the remainder =85+((100-85)/2) =85+(15/2) =85+7.5 =92.5

41

Are diastereomers mirror images?

No.

42

What are the differences between enantiomers?

Only optical purity.

43

Do enantiomers have different solubilities?

No.

44

Do diastereomers have different solubilities?

Yes.

45

Do enantiomers and diastereomers pack into a crystal in the same manner?

No.

46

Which packs more efficiently Ito crystals: diastereomers or enantiomers?

Diastereomers.

47

What three properties do the compounds in racemic mixtures share?

Melting point Boiling point Solubility

48

How can you separate enantiomers?

Change them into diastereomers.

49

What are non-superimposable mirror images?

Enantiomers.

50

In a compound with a plane of symmetry that has more than one stereocentre, is the compound chiral?

No.

51

Why is a compound with a plane of symmetry that has more than one stereocentre, not chiral?

The two stereocentres cancel out.

52

Are compounds with more than one stereoisomer that are non-superimposable considered to be enantiomers of each other?

No.

53

What are compounds with more than one stereoisomer that are non-superimposable considered to be?

Diastereomers.

54

What is the easiest way to remember what shape is "Cis" configuration?

Cis makes a C shape

55

What is the easiest way to remember what shape is "Trans" configuration?

Trans means "across"

56

Do cis and trans isomers act the same chemically?

Cis and trans isomers act very differently chemically.

57

When do you use Cis and trans notation?

If and only if: The branches are on opposite carbons in the same double bond The carbons in the double bond only have one branch each

58

What letter is used to annotate Cis configuration?

Z.

59

What letter is used to annotate Trans configuration?

E

60

In the Cahn-Ingold-Prelog, when does the hybridisation state rule apply?

It only applies when you only have carbon substituents

61

Do enantiomers have the same or different melting point?

They have the same melting point

62

Do enantiomers have the same or different boiling point?

They have the same boiling point

63

Do enantiomers have the same or different solubility?

They have the same solubility.

64

What aspect do enantiomers differ in?

Their ability to rotate plane polarised light

65

What type of molecules rotate light?

Chiral molecules.

66

Do chiral molecules rotate light?

Yes.

67

How does an enantiomer of a molecule rotate light?

To the same extent but in the opposite direction.

68

What direction is levorotory?

Anti clockwise L=Left

69

What direction is dextrorotory?

Clockwise.

70

Is there a link between rotation direction and R/S notation?

No.

71

What is the formula for specific notation?

Observed rotation (degrees)/Path length (decimetres)*concentration(g/mL)

72

What is the symbol for specific rotation?

[α]D

73

What is the notation given to the positive enantiomer (clockwise)?

(+)element name

74

What is the notation given to the negative enantiomer (anticlockwise)?

(-)element name

75

What is a racemic mixture?

An equal mixture of two enantiomers

76

What is a racemic mixture also called?

A racemate.

77

How do we know (using the degree of light rotation) if we have the right compound (and that it's pure)?

The number and direction matches the reference

78

How do we know (using the degree of light rotation) if we have the opposite enantiomer (and that it's pure)?

The number will be the same but it will be in the opposite direction to the reference

79

How do we know (using the degree of light rotation) if we have a racemic mixture?

There will be no rotation of light as they will cancel each other out.

80

In a mixture, how do we know which enantiomer is in excess?

We determine the optical purity percentage.

81

What is optical purity also known as?

Enantiomeric excess (ee)

82

What is the formula for optical purity?

(The specific rotation of the mixture/the specific rotation of the pure enantiomer)*100

83

Having measured the rotation of a solution using a polarimeter, I suspect it is a pure enantiomer. I calculated the specific rotation [a]=-9.2°. The reference value is [a]=-21°. Do I have a pure sample?

No. The numbers don't match, even though the sign does.

84

Having measured the rotation of a solution using a polarimeter, I suspect it is a pure enantiomer. I calculated the specific rotation [a]=-9.2°. The reference value is [a]=-21°. Is my solution predominantly made up of a compound that rotates levorotory or dextrorotory?

My solution is predominantly levorotory (same as the reference)

85

Having measured the rotation of a solution using a polarimeter, I suspect it is a pure enantiomer. I calculated the specific rotation [a]=-9.2°. The reference value is [a]=-21°. Calculate the optical purity of the solution.

(-9.2/-21)*100=43.8% (Or 100-43.8=56.2% is the racemic mixture)

86

Do molecules that are diastereomers of each other have the same or different melting point?

They have a different melting point.

87

Do molecules that are diastereomers of each other have the same or different boiling point?

They have a different boiling point.

88

Do molecules that are diastereomers of each other have the same or different solubility?

They have different solubility.

89

What is resolution?

When we convert a mixture of enantiomers into diastereomers and then separate (resolve) them.

90

What is racemisation?

Taking a pure enantiomer and making it an equal mixture.

91

If the two branches/substituents are on the same side of the double bond, is this Cis or trans?

Cis.

92

Can cis-trans isomers interconvert?

Cis-trans isomers can interconvert but only by breaking and reforming the Pi-bond using heat or light energy.

93

Can double bonds rotate?

No.

94

What are isomers?

Molecules with the same molecular formula.

95

What are the divisions of isomers?

Constitutional isomers Stereoisomers

96

What are constitutional isomers?

Isomers with a different connectivity of atoms.

97

What are stereoisomers?

Isomers with a different orientation in space.

98

What are the divisions of constitutional isomers?

Constitutional isomers do not have divisions.

99

What are the divisions of stereoisomers?

Diastereomers Enantiomers

100

What are diastereomers?

Non-superimposable non-mirror images

101

What are enantiomers?

Non-superimposable mirror images

102

What is a special type of diastereomer?

Cis-trans isomers.