Organics - substitution + elimination

Question 1

Describle Sn1 mechanism

Answer 1

1. Protonation of Leaving group (if they bad)
2. Dissociation
3. bond with nucleophile

Question 2

Describle Sn2 mechanism

Answer 2

1. TsCl if leaving group is bad
2. Back side attack

Question 3

Sterochem in Sn1

Answer 3

single -> racemic

100% single to 50%/50% (S) / (R)

Question 4

Sterochem in Sn2

Answer 4

Single to single

(S) to (R) or vice versa

due to backside attack

Question 5

slow step in Sn1

species in rds

Answer 5

Dissociation

only RX in rds

Question 6

slow step in Sn2

species in rds

Answer 6

Back side attack

both RX and Y

Question 7

prefered class in Sn1 and Sn2

+ reason

Answer 7

Sn1 : higher class
(more stable carbocation due to inductive effect)

Sn2 : lower class
(less steric hinderance = more access to carbon)

Question 8

Why backside attack is more favourable in Sn2

Answer 8

Top/side = insufficent overlap

front = destructive interference (antiphase)

Question 9

Describle energy -progress diagram of Sn1

Answer 9

1. two humps
2. first for dissociation
3. second for nucleophile capture
4. intermediate = R+ and X-

Question 10

Describle the energy diagram of Sn2

Answer 10

1. one hump
2. bond breaking and bond making simultaneously happen in backside attack
3. transistion state = RXY

Question 11

which mech. benefits from

Inc. nucleophilicity

Answer 11

Sn2

as it is in rds

Question 12

which mech. benefits from

good leaving group

Answer 12

Sn1 and Sn2

Question 13

how does Sn1 deals with bad leaving group

Answer 13

Use protic solvent to protonate leaving group

Question 14

how does Sn2 deals with bad leaving group

Answer 14

Use TsCl to bond with hydroxide leaving group

resonance structure of Tosylate is more stable

Question 15

Which mech. benefits from

non-protic polar solvent

protic polar solvent

+ reason

Answer 15

Sn1 - protic
(stabilises both the anion + carbocation)

Sn2 - non-protic
(stabilises the metal ion from interfering with the nucleophile)

Question 16

which mech. benefits from allylic carbon

Answer 16

Allylic carbon - next to C=C

Sn1 + Sn2

Sn1 - delocalised +ve charge

Sn2 - extended p-orbital overlap

Question 17

When does methyl shift occurs (in nucleophilic substitution)

and what happens?

Answer 17

1. beta fat ass causing steric hinderance
2. due to triple subbed

what happens?
1. Usual dissociation
2. methyl shifts from beta to alpha
3. more stable carbocation
4. Nu bonds to beta instead as +ve charge shifts