Brainscape's Knowledge GenomeTM


Top Flashcards (Certified)
All Flashcards

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

Lecture 9 and 11. Basic Mechanisms and Stereochemistry Flashcards

1
Q

What is a saturated carbon ?

A

Has bonds to four different atoms

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What happens in a nucleophilic substitution reaction ?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

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

A
  1. Stepwise

2. Simultaneous

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What does Sn1 mechanism mean ?

A

Stepwise reaction

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What are the kinetics of a stepwise mechanism ?

A

Unimolecular

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What does the stepwise mechanism rate depend on ?

A

The electrophile only

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What is the rate equation for a stepwise mechanism ?

A

r = k[R - X]

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What type of reaction is the stepwise mechanism ?

A

1st order reaction

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What is a first order reaction ?

A

Concentration depends on only one reactant

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

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

A

Nucleophilic

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What is the rate limiting step of a stepwise mechanism ?

A

Slow, reversible dissociation of leaving group to form carbocation

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

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

A

Adding more nucleophiles

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

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

A

Relatively fast attack by nucleophile

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

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

A

A reaction coodinate diagram

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What is a transition state ?

A

They are not stable species and cannot be isolated

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

What type of kinetics are simultaneous ?

A

Bimolecular

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

What is a bimolecular kinetic ?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

What is the rate equation for a simultaneous mechanism ?

A

r = k[R - X][Nu]

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

What are simultaneous mechanisms rates dependent on ?

A

Electrophile and nucleophiles

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

What type of reactions are simultaneous mechanisms ?

A

2nd order

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

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

A

Nucleophiles and electrophiles

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

What does not feature in the simultaneous mechanism ?

A

Intermediate species

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

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

A

180

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
What do stepwise reactions proceed via ?
Carbocation intermediates
26
What will speed up a stepwise mechanism ?
Structural features which stabilise carbocations
27
What do factors that stabilise a chemical species also stabilise ?
The transition states that lead to and from the stabilisation of the chemical species
28
What does the stabilisation of the cation also lower ?
The activation energy
29
What are tertiary carbocations well - stabilised by ?
Hyperconjugation
30
What is hyperconjugation ?
The donation of electron density from aligned C-H or C-C bonds
31
When does a stepwise mechanism become more favorable ?
More groups binding to the reacting carbon
32
What is a simultaneous reaction sensitive to ?
Steric hindrance
33
What is steric hindrance ?
Other atoms getting in the way
34
What does the increasing steric hindrance block in a simultaneous reaction ?
The approach of the nucleophile at 180 degrees
35
What is the only mechanism for tertiary centres ?
Stepwise mechanism
36
What is the only mechanism for substitution at methyl and primary centres ?
Simultaneous mechanisms
37
What can secondary electrophiles react by ?
Either mechanism
38
What is the rate of reaction sensitive to ?
The reactivity of the nucleophile
39
What are better leaving groups ?
Those with lower pKas
40
How would you convert HO- to a better leaving group ?
Protonation with strong acid
41
When does nucleophilicity increase ?
As the attacking atom comes from further down the periodic table
42
What do wedged bonds show ?
Substituents that stick out from the paper
43
What do hashed bonds show ?
If they stick into the paper
44
What is a chiral molecule ?
If it cannot be superimposed on its mirror image
45
What is meant by achiral ?
Not chiral
46
What happens in a simultaneous reaction, when the nucleophile approaches at 180 degrees to the leaving group ?
The stereochemistry of the reaction centre is inverted
47
What happens in a stepwise mechanism, when the planar carbocation intermediate is attacked from either side ?
A 50:50 mixture of both enantiomers (a racemic mixture)
48
What is a enantiomer ?
Mirror image form
49
What is an example of a natural enantiomer ?
Alanine
50
Are enantiomers any different from there original form ?
No
51
How can enantiomer's be differentiated ?
By their ability to rotate plane polarised light
52
How can enantiomer's be differentiated ?
By their ability to rotate plane polarised light
53
How can enantiomer's be differentiated ?
By their ability to rotate plane polarised light
54
What is optical rotation ?
The ability to rotate plane polarised light
55
What is meant by dextrorotatory ?
If a chiral compound rotates plane polarised light to the right
56
What is meant by laevorotatory ?
If a chiral compound rotates plane polarised light to the left
57
What are the rules in Cahn-Ingold-Prelog ?
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
What is meant by clockwise ?
R - enantiomer (rectus)
59
What is meant by anticlockwise ?
S- enantiomer - sinister
60
What can stabilise the carbocation using the alignment of C-H or C-C on adjacent carbons ?
Hyperconjugation
61
What can C=C double bonds be stabilised by ?
Conjugation
62
What is delocalisation ?
Spreading out of charge which is a stabilising effect
63
What is a consequence of pi bond stabilisation of carbocations ?
Carbocations from two isomeric species can be the same
64
What are electrons in pi bonds higher in energy than ?
Electrons in sigma bonds
65
What are electrons in pi bonds higher in energy than ?
Electrons in sigma bonds
66
As electrons in pi bonds are higher in energy than those in sigma bonds, what is the result of this ?
They are more reactive
67
What do alkenes react with ?
Electrophilic species
68
What is regioselectivity ?
Preference of chemical bonding or breaking in one direction over all possible other directions
69
What does the electrophilic addition of HBr start with ?
The protonation of the alkene resulting in a carbocation intermediate
70
Where can protonation occur on an alkene ?
Either fae
71
Where can nucleophilic attack on a carbocation occur ?
On either face
72
How can we define the prochiral faces of the alkene or carbocation ?
Cahn-Ingold-Prelog
73
Does adding more hydroxide to a stepwise mechanism speed it up ?
No but it can cause an elimination reaction to begin
74
What is an E2 reaction ?
An elimination, simultaneous reaction
75
What isn't hydroxide strong enough to do ?
To deprotonate a C-H to form a carbanion
76
Why does an E2 reaction proceed in the presence of hydroxide ?
Because there is a leaving group able to accept the electrons, allowing a double bond to form
77
What is an E1 reaction ?
An elimination reaction proceeding via a carbocation intermediate - stepwise
78
Why doesn't a Sn1 reaction in the presence of H2SO4 ?
Its conjugate base is a terrible nucleophile
79
What do strong bases favour ?
Deprotonation, making elimination more likely
80
At nucleophilic dependent elimination reactions, where does it preferentially attack ?
At the Hydrogen base
81
At nucleophilic dependent substitution reactions, where does it preferentially attack ?
At the carbon nucleophile
82
What does neutral ethanol attack preferentially by ?
Sn1
83
What does charged ethoxide attack preferentially by ?
E2
84
What else can effect whether a reaction is substitution or elimination ?
The size of the nucleophile/base
85
What does nucleophilic substitution require ?
The nucleophile to push towards a four coordinate carbon atom - more crowded than deprotonation
86
What is a trans molecule ?
On opposite sides
87
What is a cis molecule ?
On same sides
88
What letter do we use for trans molecules ?
E - entegegen
89
What letter do we use for cis molecules ?
Z- zusammen
90
What is the Re face ?
Clockwise
91
What is the Si face ?
Anticlockwise

Chemistry for Biologists (8 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2025 Bold Learning Solutions. Terms and Conditions