E1 Reactions

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

Four-way Rate Competition SN1 SN2 E1 E2

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

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

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.

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

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:

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]

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.

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

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

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:

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

Factor 5: Heat

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

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.

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:

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.

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:

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