FPS10: Factors affecting SN1/SN2/E1/E2

SN1 products

racemic mixture + LG-

SN2 products

Walden inverted product + LG-

E1 product

typically forms a more stable E-alkene (but can be Z-)

E2 product

stereospecific alkene depending on starting diastereomer (only E- or only Z-)

Factors affecting SN1 vs. SN2 pathway

1. nature of carbocation

2. steric effects

3. nature of Nu

4. leaving group

5. solvent effects

carbocation stability order

tertiary > secondary > primary

tertiary carbocation

carbocation w three carbon atoms directly bonded to the positively charged carbon

secondary carbocation

carbocation w two carbon atoms directly bonded to the positively charged carbon

primary carbocation

carbocation w one carbon atom directly bonded to the positively charged carbon

why is the tertiary carbocation the most stable?

due to hyperconjugation

hyperconjugation

stabilisation of carbocation by overlap between sigma bonds and an empty p orbital, allowing e- density donation into positively charged carbon i.e. sigma bond e- sharing e- density into positive C

resonance

stabilisation of carbocation when e- density is delocalised over multiple atoms through pi-bonds or lps, rather than being localised on 1 atom i.e. sharing + charge across molecule

the more stable the carbocation...

...the more likely it'll proceed via. the SN1 RXN mechanism

primary carbocations will never...

...proceed via. SN1, only SN2

SN2 steric implication

SN2 reqs. a backside attack so bulky substituents hinder Nu approach

substrate order for SN2

primary substrate > secondary substrate > tertiary substrate

primary substrate

2 H's and 1 C bound to alpha-carbon

secondary substrate

1 H bound and 2 Cs to alpha-carbon

tertiary substrate

no H's and 3 Cs bound to alpha-carbon (bulky substituents instead)

alpha-carbon

carbon bound to leaving group

why do SN2 RXNs favour primary substrates?

as they're not v. bulky since they're bound to 2 H's. they therefore don't hinder Nu backside attack

the nature of the nucleophile is only...

...only important for SN2 bc their rate depends on the Nu, unlike SN1 where it's only dependent on substrate!

what do stronger Nu's do?

increase SN2 rate

what makes a stronger Nu?

1. basicity

2. negative charge

3. larger atom size

how does a larger atom make a stronger Nu?

bc e- more shielded from nucleus so weaker attractions and more e- available to react

what does a stronger Nu do?

pushes RXN to SN2

what does a weaker Nu do?

pushes RXN to SN1

main concept to get a stronger Nu?

e- availability. the more available, the better the Nu!

what makes a good LG?

1. forms stable anions

2. have weak C-LG bonds

3. higher EN so e- in bond attracted to LG so it can leave w them

4. well solvated anion

which RXN pathway do strong LGs favour

both but is essential for SN1 bc can't proceed to step 2 w/o LG departure

the departure of the LG is the...

...rate-determining step in both SN1 and SN2

which RXN pathway do polar protic solvents favour?

SN1 bc they favour formation of carbocations (which are only formed in SN1!)

which RXN pathway do polar aprotic solvents favour?

SN2 as they dissolve Nu's w/o reducing nucleophilicity and don't overly solvate Nu's, therefore leaving them more "free" and reactive

changing solvents can...

...switch SN1 and SN2 pathways

polar protic solvent

solvent containing O-H or N-H bonds that can donate a proton, strongly solvating ions through H-bonding (stabilises Nu but hinders ability to attack substrate. also favours carbocation formation for SN1)

polar aprotic solvent

solvent that lacks O-H and N-H bonds and cannot donate a proton, leaving Nu's relatively unsolvated, so more "free" and more reactive

what are E1 RXNs favoured by?

[similar to SN1]

1. favoured by stable carbocations (tertiary)

2. favoured by polar, ionising solvents e.g. polar protic

what are E2 RXNs favoured by?

[similar to SN2]

1. favoured by high base concentrations

2. favoured by strong base concentrations

[diff to SN2]

1. less sensitive to steric hindrance bc the base isn't attacking a C bound to multiple substituents like it is in SN2, it's attacking a more accessible H!

all Nu's can act as...

...bases and vice versa

what type of substrates/bases favour elimination?

strong, bulky ones

what do polar protic solvents do to Nu's?

solvate them, stabilising them but reducing nucleophilicity