Nucleophilic Substitution- SN2 Flashcards
(36 cards)
1
Q
Where does the substitution take place
A
- At saturated (sp3 hybridised) carbons
2
Q
What are the two types of nucleophilic subsititution
A
- SN1
2. SN2
3
Q
What is the part that breaks off called
A
- The leaving group
4
Q
What is the basic kinetics of a SN2 reaction
A
- Bimolecular rate-limiting step
- Both the nucleophile and electrophile are involved in the same step
- The reaction rate is proportional to concentrations of both nucleophile and electrophile
- 1st order for each
5
Q
Write the rate equation of a SN2 reaction
A
- Rate= k[nucleophile][electrophile]
6
Q
Describe what happens in the reaction of an SN2 reaction
A
- Simultaneous attack of nucleophile and loss of leaving group
- Nucleophile forms new bond with carbon atom at the same time as the C-X bond is broken
7
Q
Describe the basic mechanism of an SN2 reaction
A
- Curly arrow from nucleophile to carbon atom
- Another arrow from C-X bond to X (Leaving group)
- All in one step
8
Q
Describe the transition state in an SN2 reaction
A
- Carbon bonded to 3 R-groups all in the same plane
- Partially bonded to Y and X 180 degrees apart- dotted lines
- Trigonal bipyramidal transition state
- delta- charge on nucleophile
- delta - charge on leaving group
- In square brackets with dagger symbol
9
Q
Describe the orbital requirements for SN2 reaction
A
- The HOMO on the nucleophile must interact with the LUMO of the electrophile
- The lone pair on the nucleophile is added to the antibonding C-X orbital breaking the bond
10
Q
How do you get maximum orbital overlap
A
- When the nucleophile approaches at 180 degrees from the electrophile
- It attaches at the opposite side to where the electrophile was- inversion of configuration
11
Q
Describe the reaction profile for SN2 reactions
A
- 1 step = 1 transition state
- Free energy over reaction coordinate
- Peak at transition state
- Reactants start above products if thermodynamically favourable
- Activation energy is difference between reactants and transition state
- Delta G= difference between reactants and products, has to be negative to be thermodynamically favourable
12
Q
What factors effect the rate of SN2
A
- Electrophile structure
- Leaving group
- Nucleophile
- Solvent
13
Q
Describe how the electrophile structure affects the rate of SN2 reaction
A
- Larger substituents = slower reactions
2. Adjacent pi system= faster rate
14
Q
Why does increasing the size of substituents lead to slower reactions
A
- Affected by the sterics around the reacting centre of the electrophile
- Larger size of groups= greater steric interactions= higher energy transition state
- The nucleophile must be able to interact with the sigma* c-x orbital
- Can’t if too much hindrance
15
Q
Why does steric hindrance increase leading to the transition state
A
- The electrophile is tetrahedral- bond angles 109 degrees
- Only 4 things need to be around the carbon
- The transition state is trigonal bipyramidal-5 things around carbon
16
Q
Rate the order of rate of reaction of a primary, secondary and tertiary alkyl bromide
A
- Primary>secondary>tertiary
2. No reaction for tertiary
17
Q
What are the electronic effects of alkyl groups bonded to the carbon of C-X bond on the rate of reaction
A
- Alkyl groups bonded to the carbon exert a positive inductive effect which reduces the partial positive charge on the carbon
- The nucleophile attacks teh partially positive carbon.
- The greater the number of alkyl groups on the carbon atom the weaker the partial positive charge on the carbon and the slower the rate of attack by nucleophile
18
Q
Describe the stereochemistry of SN2 reactions
A
- SN2 results in the inversion of configuration at a stereocenter containing the leaving group within the electrophile
19
Q
What is the name for the inversion of configuration in SN2
A
- Walden inversion
20
Q
Why deoes SN2 reactions show this stereochemistry
A
- Orbitals require nucleophilic attack 180 degrees from leaving group
- Nucleophile attaches on opposite side
21
Q
Describe the rates of SN2 substitutions on alkyl rings
A
- Slow on small alkyl rings
- Increases from 3 carbon to 4 carbon to 5 carbon
- Decreases at 6 carbon ring
22
Q
Why is SN2 slow on small alkyl rings
A
- Ring strain in the transition state
23
Q
How do rings adopt a minimum energy conformation
A
- In 3d
24
Q
Describe the structure in cyclopropane
A
- The 3 carbons must lie in plane
- Has high torsional strain- eclipsed C-H bonds on adjacent carbons
- High ring strain- bond angle compressed from 109.5 to 60 degrees
25
Describe the reactivity of cyclopropane
1. Has poor overlap of sp3 hybrid orbitals in sigma bonds
2. C-C bonds are bent so has unusual reactivity
3. Weaker than normal c-c bond
4. More reactive than expected
26
Describe the structure in cyclobutane
1. Planar strucutre would have high ring strain- 90 degrees- and high torsional strain
2. The torsional strain is minimised through ring puckering
3. This slightly increases ring angle strain but reduces torsional strain considerably
27
Describe the structure of cyclopentane
1. Planar structure would have low ring strain (108) but high torsional strain
2. Ring puckers to minimise energy
3. Forms envelope conformation- 105 degrees
4. Relief in torsional strain compensates for ring strain increase
28
Describe the structure of cycohexane
1. Planar would have high ring strain 120 and high torsional strain
2. Torsional strain is minimised by puckering
3. Forms chair conformation- free of ring and torsional strain
4. All carbons adopt tetrahedral geometry
5. all C-H bonds are staggered
29
What is torsional strain
1. Repulsion of bonding electrons
2. Pairs of electrons in different C-H bonds repel each other
3. When they get close to one another the repulsion is great and this raises the energy of the conformation
30
What is ring/angle strain
1. Arises when C-C-C bond angle in cycloalkanes is different from normal tetrahedral angle- 109.5
2. 109.5 is preferred for sp3 hybridised carbon atoms
31
What determines how reactive an electrophile a alkyl ring is
1. The deviation from TS angle of 120 degrees
32
Describe the chair conformation of cyclohexane
1. Substituents exist in two different environments- axial or equatorial
2. Axial- alternate pointing up or down
3. Equatorial- parallel to C-C bonds in ring
33
What is a ring flip
1. Bond rotation allows interconversion of axial and equatorial substituents
2. Carbon atoms that are down flip to up
3. Axial hydrogens become equatorial
4. Equatorial hydrogens become axial
34
What does t-Bu do to a ring
1. It is a conformational lock
2. Always equatorial
3. No ring-flip is possible
35
Which is the preferred position of a leaving group in cyclohexane
1. Axial leaving group has a less sterically hindered approach of nucleophile
2. Faster rate
36
How does an adjacent pi system increase the rate of SN2 reactions
1. It stabilises the transition state of forming/breaking bonds
2. Conjugation with pi system
