Organic Reactions & Mechanisms (CHEM 220) · Module 6 of 9
E1 Elimination & Substitution vs. Elimination
The stepwise elimination pathway, and how to predict which of the four mechanisms actually wins.
The E1 Mechanism
- E1 (elimination unimolecular) proceeds through the same carbocation intermediate as SN1: the leaving group departs first (the slow step), then a base removes a beta-hydrogen to form the alkene (a fast step).
- Because E1 shares its rate-determining step with SN1, E1 and SN1 often occur together as competing pathways under the same conditions.
Why do E1 and SN1 reactions often occur as competing side reactions of each other?
Solution. Both mechanisms proceed through the same carbocation intermediate, formed in the same rate-determining ionization step; once formed, the carbocation can either be attacked by a nucleophile (SN1) or lose a beta-hydrogen to a base (E1).
Carbocation Stability in E1
- Like SN1, E1 favors substrates that form stable carbocations — tertiary > secondary, with primary essentially not undergoing E1 — and can also involve carbocation rearrangements.
- E1 also generally favors the more stable, more substituted (Zaitsev) alkene product, since there's no anti-periplanar geometric constraint as strict as in E2.
Why doesn't E1 have as strict a geometric requirement as E2 does?
Solution. In E1, the carbocation is already fully formed and can rotate freely before the base removes a beta-hydrogen, so there's no need for a specific anti-periplanar alignment between the base's target hydrogen and a leaving group that has already departed.
Predicting Substitution vs. Elimination
- Strong, bulky, or hindered bases, like tert-butoxide, favor elimination (E2) over substitution, since they struggle to attack a carbon (SN2) but readily grab an accessible proton.
- Weak nucleophiles/bases with tertiary substrates favor SN1/E1, often as a mixture; strong nucleophiles that are also weak/small bases favor SN2 with unhindered substrates.
Between a small, strong nucleophile and a bulky, strong base, which is more likely to favor elimination over substitution with a secondary substrate?
Solution. The bulky, strong base — its size makes backside attack (SN2) difficult, but it can still easily grab a proton to eliminate, favoring the E2 pathway.
Temperature's Effect
- Higher temperature generally favors elimination over substitution, because elimination reactions have a more favorable (more positive) entropy change — forming an alkene plus the conjugate acid of the base from one starting material — and the TΔS term becomes more significant at higher temperature.
How does increasing reaction temperature generally affect the balance between substitution and elimination?
Solution. Higher temperatures tend to favor elimination reactions, since they have a more favorable entropy change than substitution reactions, and the TΔS term becomes more significant at higher temperature, favoring the pathway with the greater entropy increase.
Check yourself
2 of this module's 5 quiz questions. Pick your answers, then check them in the app, with an explanation for every question.
1. What intermediate do E1 and SN1 share?
2. What carbocation stability order does E1 favor?