Nucleophilic Substitution- SN2

Question 1

Where does the substitution take place

Answer 1

1. At saturated (sp3 hybridised) carbons

Question 2

What are the two types of nucleophilic subsititution

Answer 2

1. SN1

2. SN2

Question 3

What is the part that breaks off called

Answer 3

1. The leaving group

Question 4

What is the basic kinetics of a SN2 reaction

Answer 4

1. Bimolecular rate-limiting step
2. Both the nucleophile and electrophile are involved in the same step
3. The reaction rate is proportional to concentrations of both nucleophile and electrophile
4. 1st order for each

Question 5

Write the rate equation of a SN2 reaction

Answer 5

1. Rate= k[nucleophile][electrophile]

Question 6

Describe what happens in the reaction of an SN2 reaction

Answer 6

1. Simultaneous attack of nucleophile and loss of leaving group
2. Nucleophile forms new bond with carbon atom at the same time as the C-X bond is broken

Question 7

Describe the basic mechanism of an SN2 reaction

Answer 7

1. Curly arrow from nucleophile to carbon atom
2. Another arrow from C-X bond to X (Leaving group)
3. All in one step

Question 8

Describe the transition state in an SN2 reaction

Answer 8

1. Carbon bonded to 3 R-groups all in the same plane
2. Partially bonded to Y and X 180 degrees apart- dotted lines
3. Trigonal bipyramidal transition state
4. delta- charge on nucleophile
5. delta - charge on leaving group
6. In square brackets with dagger symbol

Question 9

Describe the orbital requirements for SN2 reaction

Answer 9

1. The HOMO on the nucleophile must interact with the LUMO of the electrophile
2. The lone pair on the nucleophile is added to the antibonding C-X orbital breaking the bond

Question 10

How do you get maximum orbital overlap

Answer 10

1. When the nucleophile approaches at 180 degrees from the electrophile
2. It attaches at the opposite side to where the electrophile was- inversion of configuration

Question 11

Describe the reaction profile for SN2 reactions

Answer 11

1. 1 step = 1 transition state
2. Free energy over reaction coordinate
3. Peak at transition state
4. Reactants start above products if thermodynamically favourable
5. Activation energy is difference between reactants and transition state
6. Delta G= difference between reactants and products, has to be negative to be thermodynamically favourable

Question 12

What factors effect the rate of SN2

Answer 12

1. Electrophile structure
2. Leaving group
3. Nucleophile
4. Solvent

Question 13

Describe how the electrophile structure affects the rate of SN2 reaction

Answer 13

1. Larger substituents = slower reactions

2. Adjacent pi system= faster rate

Question 14

Why does increasing the size of substituents lead to slower reactions

Answer 14

1. Affected by the sterics around the reacting centre of the electrophile
2. Larger size of groups= greater steric interactions= higher energy transition state
3. The nucleophile must be able to interact with the sigma* c-x orbital
4. Can’t if too much hindrance

Question 15

Why does steric hindrance increase leading to the transition state

Answer 15

1. The electrophile is tetrahedral- bond angles 109 degrees
2. Only 4 things need to be around the carbon
3. The transition state is trigonal bipyramidal-5 things around carbon

Question 16

Rate the order of rate of reaction of a primary, secondary and tertiary alkyl bromide

Answer 16

1. Primary>secondary>tertiary

2. No reaction for tertiary

Question 17

What are the electronic effects of alkyl groups bonded to the carbon of C-X bond on the rate of reaction

Answer 17

1. Alkyl groups bonded to the carbon exert a positive inductive effect which reduces the partial positive charge on the carbon
2. The nucleophile attacks teh partially positive carbon.
3. 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

Question 18

Describe the stereochemistry of SN2 reactions

Answer 18

1. SN2 results in the inversion of configuration at a stereocenter containing the leaving group within the electrophile

Question 19

What is the name for the inversion of configuration in SN2

Answer 19

1. Walden inversion

Question 20

Why deoes SN2 reactions show this stereochemistry

Answer 20

1. Orbitals require nucleophilic attack 180 degrees from leaving group
2. Nucleophile attaches on opposite side

Question 21

Describe the rates of SN2 substitutions on alkyl rings

Answer 21

1. Slow on small alkyl rings
2. Increases from 3 carbon to 4 carbon to 5 carbon
3. Decreases at 6 carbon ring

Question 22

Why is SN2 slow on small alkyl rings

Answer 22

1. Ring strain in the transition state

Question 23

How do rings adopt a minimum energy conformation

Answer 23

1. In 3d

Question 24

Describe the structure in cyclopropane

Answer 24

1. The 3 carbons must lie in plane
2. Has high torsional strain- eclipsed C-H bonds on adjacent carbons
3. High ring strain- bond angle compressed from 109.5 to 60 degrees

Question 25

Describe the reactivity of cyclopropane

Answer 25

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

Question 26

Describe the structure in cyclobutane

Answer 26

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

Question 27

Describe the structure of cyclopentane

Answer 27

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

Question 28

Describe the structure of cycohexane

Answer 28

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

Question 29

What is torsional strain

Answer 29

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

Question 30

What is ring/angle strain

Answer 30

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

Question 31

What determines how reactive an electrophile a alkyl ring is

Answer 31

1. The deviation from TS angle of 120 degrees

Question 32

Describe the chair conformation of cyclohexane

Answer 32

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

Question 33

What is a ring flip

Answer 33

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

Question 34

What does t-Bu do to a ring

Answer 34

1. It is a conformational lock
2. Always equatorial
3. No ring-flip is possible

Question 35

Which is the preferred position of a leaving group in cyclohexane

Answer 35

1. Axial leaving group has a less sterically hindered approach of nucleophile
2. Faster rate

Question 36

How does an adjacent pi system increase the rate of SN2 reactions

Answer 36

1. It stabilises the transition state of forming/breaking bonds
2. Conjugation with pi system