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Chemistry for Biologists > Lecture 9 and 11. Basic Mechanisms and Stereochemistry > Flashcards

Lecture 9 and 11. Basic Mechanisms and Stereochemistry Flashcards

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1
Q

What is a saturated carbon ?

A

Has bonds to four different atoms

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2
Q

What happens in a nucleophilic substitution reaction ?

A

One of the atoms or groups bound to a carbon is replaced by another

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3
Q

What are the two types of mechanisms for nucleophilic substitution reactions ?

A
  1. Stepwise

2. Simultaneous

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4
Q

What does Sn1 mechanism mean ?

A

Stepwise reaction

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5
Q

What are the kinetics of a stepwise mechanism ?

A

Unimolecular

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6
Q

What is meant by unimolecular ?

A

An elementary reaction in which the rearrangement of a single molecule produces one or more molecules of product

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7
Q

What does the stepwise mechanism rate depend on ?

A

The electrophile only

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8
Q

What is the rate equation for a stepwise mechanism ?

A

r = k[R - X]

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9
Q

What type of reaction is the stepwise mechanism ?

A

1st order reaction

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10
Q

What is a first order reaction ?

A

Concentration depends on only one reactant

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11
Q

Would you describe a stepwise mechanism a nucleophilic or electrophilic reaction ?

A

Nucleophilic

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12
Q

What is the rate limiting step of a stepwise mechanism ?

A

Slow, reversible dissociation of leaving group to form carbocation

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13
Q

What has no effect on the rate of the stepwise process ?

A

Adding more nucleophiles

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14
Q

In a stepwise mechanism, what is the formation of the cation followed by ?

A

Relatively fast attack by nucleophile

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15
Q

How can we show the Gibbs free energy change over the course of a reaction ?

A

A reaction coodinate diagram

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16
Q

What is a transition state ?

A

They are not stable species and cannot be isolated

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17
Q

What type of kinetics are simultaneous ?

A

Bimolecular

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18
Q

What is a bimolecular kinetic ?

A

The chemical combination of two molecular entities in a reaction that is either reversible or irreversible

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19
Q

What is the rate equation for a simultaneous mechanism ?

A

r = k[R - X][Nu]

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20
Q

What are simultaneous mechanisms rates dependent on ?

A

Electrophile and nucleophiles

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21
Q

What type of reactions are simultaneous mechanisms ?

A

2nd order

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22
Q

What features in the rate limiting step of the simultaneous mechanism ?

A

Nucleophiles and electrophiles

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23
Q

What does not feature in the simultaneous mechanism ?

A

Intermediate species

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24
Q

What angle does the nucleophile attack at in a simultaneous reaction ?

A

180

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25
Q

What do stepwise reactions proceed via ?

A

Carbocation intermediates

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26
Q

What will speed up a stepwise mechanism ?

A

Structural features which stabilise carbocations

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27
Q

What do factors that stabilise a chemical species also stabilise ?

A

The transition states that lead to and from the stabilisation of the chemical species

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28
Q

What does the stabilisation of the cation also lower ?

A

The activation energy

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29
Q

What are tertiary carbocations well - stabilised by ?

A

Hyperconjugation

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30
Q

What is hyperconjugation ?

A

The donation of electron density from aligned C-H or C-C bonds

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31
Q

When does a stepwise mechanism become more favorable ?

A

More groups binding to the reacting carbon

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32
Q

What is a simultaneous reaction sensitive to ?

A

Steric hindrance

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33
Q

What is steric hindrance ?

A

Other atoms getting in the way

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34
Q

What does the increasing steric hindrance block in a simultaneous reaction ?

A

The approach of the nucleophile at 180 degrees

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35
Q

What is the only mechanism for tertiary centres ?

A

Stepwise mechanism

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36
Q

What is the only mechanism for substitution at methyl and primary centres ?

A

Simultaneous mechanisms

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37
Q

What can secondary electrophiles react by ?

A

Either mechanism

38
Q

What is the rate of reaction sensitive to ?

A

The reactivity of the nucleophile

39
Q

What are better leaving groups ?

A

Those with lower pKas

40
Q

How would you convert HO- to a better leaving group ?

A

Protonation with strong acid

41
Q

When does nucleophilicity increase ?

A

As the attacking atom comes from further down the periodic table

42
Q

What do wedged bonds show ?

A

Substituents that stick out from the paper

43
Q

What do hashed bonds show ?

A

If they stick into the paper

44
Q

What is a chiral molecule ?

A

If it cannot be superimposed on its mirror image

45
Q

What is meant by achiral ?

A

Not chiral

46
Q

What happens in a simultaneous reaction, when the nucleophile approaches at 180 degrees to the leaving group ?

A

The stereochemistry of the reaction centre is inverted

47
Q

What happens in a stepwise mechanism, when the planar carbocation intermediate is attacked from either side ?

A

A 50:50 mixture of both enantiomers (a racemic mixture)

48
Q

What is a enantiomer ?

A

Mirror image form

49
Q

What is an example of a natural enantiomer ?

A

Alanine

50
Q

Are enantiomers any different from there original form ?

A

No

51
Q

How can enantiomer’s be differentiated ?

A

By their ability to rotate plane polarised light

52
Q

How can enantiomer’s be differentiated ?

A

By their ability to rotate plane polarised light

53
Q

How can enantiomer’s be differentiated ?

A

By their ability to rotate plane polarised light

54
Q

What is optical rotation ?

A

The ability to rotate plane polarised light

55
Q

What is meant by dextrorotatory ?

A

If a chiral compound rotates plane polarised light to the right

56
Q

What is meant by laevorotatory ?

A

If a chiral compound rotates plane polarised light to the left

57
Q

What are the rules in Cahn-Ingold-Prelog ?

A
  1. Assign a priority to each substituent at the chiral centre using atomic number
  2. Orient the molecules so that the lowest priority group is at the back
  3. Are the substituents linked by clockwise or anticlockwise rotation
58
Q

What is meant by clockwise ?

A

R - enantiomer (rectus)

59
Q

What is meant by anticlockwise ?

A

S- enantiomer - sinister

60
Q

What can stabilise the carbocation using the alignment of C-H or C-C on adjacent carbons ?

A

Hyperconjugation

61
Q

What can C=C double bonds be stabilised by ?

A

Conjugation

62
Q

What is delocalisation ?

A

Spreading out of charge which is a stabilising effect

63
Q

What is a consequence of pi bond stabilisation of carbocations ?

A

Carbocations from two isomeric species can be the same

64
Q

What are electrons in pi bonds higher in energy than ?

A

Electrons in sigma bonds

65
Q

What are electrons in pi bonds higher in energy than ?

A

Electrons in sigma bonds

66
Q

As electrons in pi bonds are higher in energy than those in sigma bonds, what is the result of this ?

A

They are more reactive

67
Q

What do alkenes react with ?

A

Electrophilic species

68
Q

What is regioselectivity ?

A

Preference of chemical bonding or breaking in one direction over all possible other directions

69
Q

What does the electrophilic addition of HBr start with ?

A

The protonation of the alkene resulting in a carbocation intermediate

70
Q

Where can protonation occur on an alkene ?

A

Either fae

71
Q

Where can nucleophilic attack on a carbocation occur ?

A

On either face

72
Q

How can we define the prochiral faces of the alkene or carbocation ?

A

Cahn-Ingold-Prelog

73
Q

Does adding more hydroxide to a stepwise mechanism speed it up ?

A

No but it can cause an elimination reaction to begin

74
Q

What is an E2 reaction ?

A

An elimination, simultaneous reaction

75
Q

What isn’t hydroxide strong enough to do ?

A

To deprotonate a C-H to form a carbanion

76
Q

Why does an E2 reaction proceed in the presence of hydroxide ?

A

Because there is a leaving group able to accept the electrons, allowing a double bond to form

77
Q

What is an E1 reaction ?

A

An elimination reaction proceeding via a carbocation intermediate - stepwise

78
Q

Why doesn’t a Sn1 reaction in the presence of H2SO4 ?

A

Its conjugate base is a terrible nucleophile

79
Q

What do strong bases favour ?

A

Deprotonation, making elimination more likely

80
Q

At nucleophilic dependent elimination reactions, where does it preferentially attack ?

A

At the Hydrogen base

81
Q

At nucleophilic dependent substitution reactions, where does it preferentially attack ?

A

At the carbon nucleophile

82
Q

What does neutral ethanol attack preferentially by ?

A

Sn1

83
Q

What does charged ethoxide attack preferentially by ?

A

E2

84
Q

What else can effect whether a reaction is substitution or elimination ?

A

The size of the nucleophile/base

85
Q

What does nucleophilic substitution require ?

A

The nucleophile to push towards a four coordinate carbon atom - more crowded than deprotonation

86
Q

What is a trans molecule ?

A

On opposite sides

87
Q

What is a cis molecule ?

A

On same sides

88
Q

What letter do we use for trans molecules ?

A

E - entegegen

89
Q

What letter do we use for cis molecules ?

A

Z- zusammen

90
Q

What is the Re face ?

A

Clockwise

91
Q

What is the Si face ?

A

Anticlockwise

Chemistry for Biologists (8 decks)

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