Nucleophilic Substitution-SN1

Question 1

Describe the basic kinetics of SN1 reaction

Answer 1

1. Reaction rate is only proportional to the concentration of the electrophile
2. Unimolecular- overall 1st order
3. Only electrophile involved in rate-limiting step

Question 2

Describe the basic mechanism for SN1 reaction

Answer 2

1. 2 Step mechanism
2. Leaving group leaves to form a carbocation intermediate (Rate limiting)
3. Nucleophile attacks the carbocation intermediate

Question 3

Describe the reaction profile for an SN1 reaction

Answer 3

1. Free energy over reaction coordinate
2. Two peaks for each transition state
3. Transition state 1 is higher energy
4. Carbocation intermediate is in between the peaks of transition state
5. Products lower than reactants if thermodynamically favourable
6. Transition state 1 has much longer activation energy as from reactants
7. TS2 is from the carbocation intermediate

Question 4

What affects the reaction rates of SN1 reactions

Answer 4

1. Electrophile structure
2. Leaving group
3. solvent

Question 5

What is a requirement for SN1 electrophile structure

Answer 5

1. Only substrates that can form stable carbocation intermediates can react via SN1 mechanism

Question 6

What things influence the stability of the carbocation intermediates

Answer 6

1. Alkyl groups
2. Hyperconjugation
3. Conjugation

Question 7

Describe the structure of a carbocation

Answer 7

1. Planar, sp2 hybridised
2. Contain empty p-orbital
3. Trigonal planar as this:
4. This provides filled sp2 hybrid orbitals that are of the lowest possible energy
5. Filled sp3- hybrid orbitals that are as far apart as possible

Question 8

Describe the difference in stability between methyl, primary, secondary and tertiary carbocations

Answer 8

1. methyl

Question 9

Why do tertiary carbocations have a higher stability

Answer 9

1. Hyperconjugation
2. They are stabilised by weak donation of a sigma-bond electrons from groups on the carbons adjacent to the empty p-orbital
3. Other types of bond can also provide stabilisation e.g. sigma c-c and sigma c-Si
4. H-atoms bonded to cationic centre cannot provide stabilisation as there is no orbital overlap
5. C-H bonds at 90 degrees to the p orbital so cannot overlap

Question 10

How does conjugation affect carbocation stability

Answer 10

1. Carbocations are stabilised by conjugation with pi-bonds or a lone-pair of electrons
2. Allows the positive charge to be delocalised over multiple atoms
3. P-orbitals overlap so electron density can be delocalised over the whole system
4. In benzylic cations there is a lot of stabilisation as all the p-orbitals can overlap so a lot of electrons are delocalised over the whole system
5. Positive mesomeric effects

Question 11

How does the size of alkyl groups in tertiary electrophiles affect the rate of SN1 reactions

Answer 11

1. Increasing the size, increases the rate
2. Steric acceleration
3. When the electrophile is converted to carbocation there is release of steric strain as alkyl groups are further apart
4. Larger groups therefore lead to greater release of steric strain so faster forming of product.

Question 12

What is the Hammond Postulate

Answer 12

1. A transition state linking two points on a potential energy surface resembles the nearest stable species
2. Use carbocation intermediate to rationalise the stability of TS
3. The more stable the carbocation, the lower the energy TS, and so lower Ea
4. The TS is what is important but you can’t see it so pretend it looks closer to intermediate

Question 13

Describe the stereochemistry of SN1 reactions

Answer 13

1. Results in loss of stereochemical information on the reacting centre of the electrophile
2. Enantiomerically enriched electrophile leads to formation of a racemic product (50:50 mixture of enantiomers)

Question 14

Describe why SN1 reactions produce racemic products

Answer 14

1. SN1 proceeds via a carbocation intermediate
2. Carbocation is planar and therefore sp2 hybridised
3. No stereocenter possible on the planar carbon atom
4. Nucleophile can approach from above or below the plane of the cation
5. The product is therefore racemic

Question 15

What is an allylic system

Answer 15

1. Contains C=C adjacent to reacting carbon?

Question 16

Describe why there is regioselectivity for allylic systems undergoing SN1

Answer 16

1. Leads to two regioisomers
2. Intermediate allylic cation is a resonance hybrid
3. Can undergo nucleophilic attacks in two positions that bear a partial positive charge

Question 17

What position is the nucleophile more likely to attack in an allylic carbocation

Answer 17

1. The least sterically hindered end of the allylic carbocation e.g. primary

Question 18

Why may the expected product not be observed

e.g. CH(CH3)3CH2OH + HBR –> CHBr(CH3)2CH2CH3

Answer 18

1. Cation rearrangements

2. Intermediate carbocation may undergo rearrangement to form a more stable species

Question 19

Describe how carbocation can rearrange and 2 types of shift

Answer 19

1. Groups that migrate must be involved in hyperconjugation before shifting
2. 1,2-alkyl shift- moving methyl group to positive charge to form a positive charge somewhere else in more stable position
3. 1,2-hydride shift- moving hydrogen to positive charge to create positive charge somewhere else in more stable position

Question 20

What is the name of a 1,2-alkyl shift

Answer 20

1. Wagner-Meerwein rearrangement