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Chemistry 101 COPY > Optical Purity > Flashcards

Optical Purity Flashcards

1
Q

How do we measure the purity of enantiomers?

A

Using light

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2
Q

Do enantiomers vary in melting point?

A

No.

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3
Q

Do enantiomers vary in boiling point?

A

No.

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4
Q

Do enantiomers vary in solubility?

A

No.

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5
Q

Do enantiomers vary in their interaction with light?

A

Yes.

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6
Q

What is plane-polarised light?

A

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

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7
Q

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

A

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

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8
Q

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

A

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

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9
Q

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

A

It has a mirror plane.

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10
Q

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

A

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

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11
Q

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

A

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

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12
Q

Do meso compounds rotate light?

A

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

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13
Q

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

A

As the net rotation will be 0.

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14
Q

Do achiral molecules rotate light?

A

Achiral molecules will not rotate light at all.

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15
Q

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

A

The analyser.

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16
Q

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

A

Because the observer will notice the change themselves.

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17
Q

Can you predict which way a compound will rotate light?

A

No. You must measure it.

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18
Q

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

A

No.

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19
Q

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

A

Concentration Distance travelled in solution

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20
Q

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

A

Increasing the concentration will increase the rotation

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21
Q

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

A

Increasing distance travelled in solution increases the rotation.

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22
Q

What is specific rotation?

A

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

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23
Q

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

A

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

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24
Q

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

A

No.

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25
Q

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

A

They will have a different specific rotation.

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26
Q

What is the specific rotation of racemic mixtures?

A

0°.

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27
Q

What are racemic mixtures?

A

Equal mixtures of two enantiomers.

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28
Q

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

A

It is not that compound.

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29
Q

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

A

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

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30
Q

Can you express optical purity without knowing the reference value?

A

No.

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31
Q

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

A

Yes.

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32
Q

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

A

No.

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33
Q

Do diastereomers have the same molecular properties?

A

No.

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34
Q

What is a consequence of diastereomers having different molecular properties?

A

They can be separated.

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35
Q

What is the reference value?

A

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

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36
Q

What does L stand for in D/L notation?

A

Levorotory

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37
Q

What does D stand for in D/L notation?

A

Dextrotory

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38
Q

What sign is L in D/L notation?

A

Negative

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39
Q

What sign is D in D/L notation?

A

Positive.

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40
Q

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

A

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

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41
Q

Are diastereomers mirror images?

A

No.

42
Q

What are the differences between enantiomers?

A

Only optical purity.

43
Q

Do enantiomers have different solubilities?

A

No.

44
Q

Do diastereomers have different solubilities?

A

Yes.

45
Q

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

A

No.

46
Q

Which packs more efficiently Ito crystals: diastereomers or enantiomers?

A

Diastereomers.

47
Q

What three properties do the compounds in racemic mixtures share?

A

Melting point Boiling point Solubility

48
Q

How can you separate enantiomers?

A

Change them into diastereomers.

49
Q

What are non-superimposable mirror images?

A

Enantiomers.

50
Q

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

A

No.

51
Q

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

A

The two stereocentres cancel out.

52
Q

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

A

No.

53
Q

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

A

Diastereomers.

54
Q

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

A

Cis makes a C shape

55
Q

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

A

Trans means “across”

56
Q

Do cis and trans isomers act the same chemically?

A

Cis and trans isomers act very differently chemically.

57
Q

When do you use Cis and trans notation?

A

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
Q

What letter is used to annotate Cis configuration?

A

Z.

59
Q

What letter is used to annotate Trans configuration?

A

E

60
Q

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

A

It only applies when you only have carbon substituents

61
Q

Do enantiomers have the same or different melting point?

A

They have the same melting point

62
Q

Do enantiomers have the same or different boiling point?

A

They have the same boiling point

63
Q

Do enantiomers have the same or different solubility?

A

They have the same solubility.

64
Q

What aspect do enantiomers differ in?

A

Their ability to rotate plane polarised light

65
Q

What type of molecules rotate light?

A

Chiral molecules.

66
Q

Do chiral molecules rotate light?

A

Yes.

67
Q

How does an enantiomer of a molecule rotate light?

A

To the same extent but in the opposite direction.

68
Q

What direction is levorotory?

A

Anti clockwise L=Left

69
Q

What direction is dextrorotory?

A

Clockwise.

70
Q

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

A

No.

71
Q

What is the formula for specific notation?

A

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

72
Q

What is the symbol for specific rotation?

A

[α]D

73
Q

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

A

(+)element name

74
Q

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

A

(-)element name

75
Q

What is a racemic mixture?

A

An equal mixture of two enantiomers

76
Q

What is a racemic mixture also called?

A

A racemate.

77
Q

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

A

The number and direction matches the reference

78
Q

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

A

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

79
Q

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

A

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

80
Q

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

A

We determine the optical purity percentage.

81
Q

What is optical purity also known as?

A

Enantiomeric excess (ee)

82
Q

What is the formula for optical purity?

A

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

83
Q

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?

A

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

84
Q

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?

A

My solution is predominantly levorotory (same as the reference)

85
Q

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.

A

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

86
Q

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

A

They have a different melting point.

87
Q

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

A

They have a different boiling point.

88
Q

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

A

They have different solubility.

89
Q

What is resolution?

A

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

90
Q

What is racemisation?

A

Taking a pure enantiomer and making it an equal mixture.

91
Q

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

A

Cis.

92
Q

Can cis-trans isomers interconvert?

A

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

93
Q

Can double bonds rotate?

A

No.

94
Q

What are isomers?

A

Molecules with the same molecular formula.

95
Q

What are the divisions of isomers?

A

Constitutional isomers Stereoisomers

96
Q

What are constitutional isomers?

A

Isomers with a different connectivity of atoms.

97
Q

What are stereoisomers?

A

Isomers with a different orientation in space.

98
Q

What are the divisions of constitutional isomers?

A

Constitutional isomers do not have divisions.

99
Q

What are the divisions of stereoisomers?

A

Diastereomers Enantiomers

100
Q

What are diastereomers?

A

Non-superimposable non-mirror images

101
Q

What are enantiomers?

A

Non-superimposable mirror images

102
Q

What is a special type of diastereomer?

A

Cis-trans isomers.

Chemistry 101 COPY (32 decks)

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