1. E1 Reactions

2. E1: Elimination, Unimolecular
The E1 reaction proceeds via a two-step mechanism: the bond to the leaving group breaks first before the  bond is formed.
The slow step is unimolecular, involving only the C-LG

3. Four-way Rate Competition
SN1
SN2 E1 E2

4. E1: Elimination, Unimolecular
As with SN1 the C–L s bond breaks first in a slow equilibrium; this is the rate limiting step
The B–H s bond and the C=C p bond form in a rapid second step

5. E1: Elimination, Unimolecular
E1 free energy diagram - maps DE as reaction progresses
Two ‡s
Highest EA gives
the rate limiting step
Carbocation
intermediate
Most E1 reactions are endothermic as a strong
base is not used

6. E1: Elimination, Unimolecular
SN1 and E1 proceed through the same intermediate and rate limiting step.
Once the carbocation is formed, the EA for the cation to bind to a nucleophile or undergo elimination are small by comparison.
In most cases SN1 and E1 occur together.

7. E1: Elimination, Unimolecular
Some exceptions.
The key is E1. If there is no b-hydrogen for the base to react with, SN1 only
If the base is not strong enough to react with a b-hydrogen (see E2), SN1 only

8. Factor 1: Structure of R-X/LG
As with SN1, the rate of E1 increases with as the number of R groups on the carbon with the LG increases:
E1 is never observed for 1o substrates:

9. E1 Rate Determining Step
Like an SN1 reaction, The E1 reaction proceeds via a two-step mechanism: the bond to the leaving group breaks first before the -bond is formed.
Only the carbon substrate is involved in the rate limiting step, thus a unimolecular rate law
Rate (E1) = kE1[C-LG]

10. Factor 2: Strength of the Base:
Due to the observation that the base does not participate in the rate limiting step (Step [1]), the B: has no effect on the E1 process
As with SN1, weak bases, even the solvent can react.
The solvent or weak base will react with a b-hydrogen (Step [2]) forming the alkene product. The EA for this step is small.

11. Factor 3: Leaving Group Ability
A leaving group must leave in the rate-determining step of an SN2, SN1, E2, or E1 reaction.
The identity of the leaving group has an effect on the rate of each reaction.
A good leaving group is necessary for the reaction to be exothermic (and spontaneous) via a –DH
Leaving group ability strongly affects E1 reactions

12. Factor 3: Leaving Group Ability
Overall, E1 is similar to SN2, SN1 and E2 with regard to leaving group ability:
Are never LGs!

13. Factor 4: Solvent Effects
Observation: SN1 reactions are most rapid in polar protic solvents, likewise E1 will proceed rapidly as well.
Polar protic solvents facilitate the separation of ions in the rate limiting step:

14. Factor 5: Heat
When substitution and elimination reactions are both favored under a specific set of conditions, it is often possible to influence the outcome by changing the temperature under which the reactions take place.
All of these reactions have an EA that needs to be surmounted.
Heat will accelerate the rate of all reactions; the object is not to overheat to allow higher EA reaction pathways to compete
E1 reactions are more strongly accelerated by heat than SN1

15. Factor 5: Heat

16. Factor 5: Heat This temperature effect is due to entropy.
∆S °rxn is more positive for an elimination reaction than for a substitution reaction.

17. Factor 6a: Regioselectivity of E1
Zaitsev’s rule applies to E1 reactions also.
E1 reactions, like E2 are regioselective, favoring formation of the more substituted, more stable alkene.

18. Factor 6b: Stereochemistry of E1
The E1 reaction is regioselective, but not stereoselective Because the carbocation intermediate has a finite lifetime, there is time for free rotation about the s-bond so that any H-atom can be brought in line with the empty p-orbital of the carbocation:

19. Factor 6b: Stereochemistry of E1
The E1 reaction is regioselective, but not stereoselective
Insert eq 8-22, pg 23 here
26_p440_Karty1_CH08
Jmk: I changed “Stereoselectivity” to “Stereochemistry”
Jmk2: I added the second bullet.

20. Factor 6b: Stereochemistry of E1
Because the carbocation intermediate has a finite lifetime, this allows for free rotation about the s-bond so that any H-atom can be brought in line with the empty p-orbital of the carbocation:

21. Summary E1 SN1 SN2 E1 E2 Optimize SN2 rate:
Factor 1: CH3>1o>2o; never 3o
Factor 2: Strong, small Nu:
Factor 3: Good LG-weak CB
Factor 4: Polar aprotic solvent
Factor 5: DS = 0
Factor 6: Stereospecific
Optimize SN1 rate:
Factor 1: 3o >2o; never 1o, CH3
Factor 2: Any Nu:
Factor 3: Good LG-weak CB
Factor 4: Polar protic solvent
Factor 5: DS = 0
Factor 6: Non-stereospecific
SN2
SN1 E1 E2
Optimize E2 rate:
Factor 1: 3o >2o>>1o
Factor 2: Strong Base:
Factor 3: Good LG-weak CB
Factor 4: Polar aprotic solvent
Factor 5: +DS, T increase rate
Factor 6: Stereospecific and regiospecific
Optimize E1 rate:
Factor 1: 3o >2o; never 1o, CH3
Factor 2: Any Base:
Factor 3: Good LG-weak CB
Factor 4: Polar protic solvent
Factor 5: +DS, T increase rate
Factor 6: Regiospecific only