E2 vs E1 Reactions
The competition between E2 and E1 are very similar to that of SN2 and SN1 reactions, though the prediction is
MUCH easier.
Table 1: Comparison between E1 and E2 Reactions
Note: Regular bases (not bulky ones) are assumed in most of the reactions. In this case, regioselectivity of
E2/E1 favors Zaitsev products. If bulky bases are used instead, Hoffman products will be favored!
E2 E1
Reaction order 2 1
Rate law Rate = k[Substrate][base] Rate = k[Substrate]
Stereospecificity Anti-coplanar elimination No stereospecificity
Stereoselectivity When cis/trans or E/Z are possible, both will favor the alkene with less steric interaction.
Regioselectivity Regular base: Zaitsev product(s) Zaitsev product(s)
Bulky base: Hoffman product(s)
# of main steps 1 (concerted step involves both proton 2 (Loss of LG first to create C+
in mechanism transfer and loss of LG) intermediate and then proton transfer)
Base Strong base generally needed More common for weak base
Solvent Not as sensitive to solvent Commonly in polar protic solvent
Leaving group Both need good leaving group (for alcohol, activation of leaving group needed)
Structure of Less sensitive to structure of substrate 2°or 3°substrate needed for meaningful
substrate because β-H is easily accessible by base E1 reaction
Other possible Activation of LG for alcohol is needed as 1. Activation of LG for alcohol is needed
steps pre-requisite, either through proton transfer as pre-requisite;
or tosylation 2. C+ rearrangement is also a possibility
Up to two steps possible in E2 mechanism So up to 4 steps in E1 mechanism
Activation of LG concerted step of Activation Loss of LG C+
proton transfer + Loss of LG rearrangement Proton transfer
Sample
mechanism #1:
E2 of alkyl
halides
Notice the stereoselectivity! The trans alkene is the major product; there is also regioselectivity
here: the one with less substituent is even less significant than the cis-alkene.
Sample
mechanism #2:
E2 of alcohol
(note the
additional step)
Once again, there are both stereoselectivity and regioselectivity for the E2 products. Only the
major product of alkene is shown here.
Sample
mechanism #3:
E1 of alkyl
halide (simplest)
Sample Too large. See mechanism after table.
mechanism #4: Note: anytime you do E1, you must consider if C+ rearrangement a legitimate possibility!! If
E1 of alcohol rearrangement generate a more stable C+, you must consider C+ rearrangement!
(complicated)
Rate determining Commonly the concerted step of proton Commonly the loss of leaving group to
step in transfer and loss of leaving group; create the high energy C+ intermediate;
mechanism activation of OH group, if needed, proton transfers and C+ rearrangement and
commonly has low Ea and is fast commonly have low Ea and are fast
Ea rational E2 does not involve higher energy C+ E1 does involve higher energy C+
intermediate, so in general it has lower Ea intermediate, so in general it has higher Ea
and is faster. and is slower
The only scenario that E1 is faster than E2
is when weak bases are involved in 3°
substrate elimination
Sample E1 mechanism (more complicated): The following demonstrates different E1 products due to
carbocation rearrangements.
Predicting E1 vs E2 is super easy: focus on the strength of the base! Other factors are minor. Structure of
substrate, though not as important in E2, is something to be considered for E1 reaction.
Table 2: Predicting E1 vs E2
Structure Strong Base Weak base
of substrate Any Solvent Protic solvent
1° E2 E2 (slow)
2° E2 E1 (slow)
3° E2 E1
Table 3. Factors that impact E2 and E1 reaction rates
Factors Impact on E2 rate Impact on E1 rate
Concentration of substrate Concentration doubles, rate doubles Concentration doubles, rate doubles
Concentration of base Concentration doubles, rate doubles No or minimum impact
Strength of leaving group Better leaving group, faster rate Better leaving group, faster rate
Strength of base Stronger base, faster rate No or minimum impact
Temperature Higher temperature, higher rate Higher temperature, higher rate
Structure of substrate Not sensitive to structure of substrate The more substituents at α and β C,
the faster the rate;
Solvent Not as sensitive to solvent as SN2; Aprotic solvent decreases rate and
aprotic solvent increases the rate protic solvent increases rate
