Choosing between E1 and E2

Key Questions

• What are the differences between E1 and E2 reactions?

  E1 reactions are unimolecular whereas E2 reactions are bimolecular. By extension,

  #R_("E"1) = k["LG"^-]#, and

  #R_("E"2) = k["LG"^-]["NUC"]#

  where the notations stand for leaving group and nucleophile.

  As a consequence of the preceding, E2 reactions usually proceed with a strong nucleophile (e.g. base), whereas E1 reactions are fine with a weaker nucleophile (e.g. base).

  Mechanistically, E2 reactions are concerted (and occur faster), whereas E1 reactions are stepwise (and occur slower and at a higher energy cost, generally).

  Due to E1's mechanistic behavior, carbocation rearrangements can occur in the intermediate, such that the positive charge is relocated on the most stable carbon. Factors influencing this include hyperconjugation and resonance. As a result of this, E1 usually won't occur on primary substrates.

  Moreover, when competing with SN1, E1's rate is more sensitive and will dominate when supplied with heat.

  If you'd like to see mechanisms, ask!

  Al E. · 4 · Mar 23 2018
• Do both elimination reactions, E1 and E2, follow Zaitsev’s rule?

  Answer:

  #"E1"# eliminations follow Zaitsev's rule. With two exceptions, #"E2"# reactions also follow Zaitsev's Rule.

  Explanation:

  (1) Certain cyclohexane derivatives.

  Some cyclohexane derivatives, such as menthyl chloride, form the Hofmann product instead:

  
  (from en.wikipedia.org)

  Formation of the Zaitsev product requires antiperiplanar elimination at the 2-position, but the isopropyl group – not the proton – is in the antiperiplanar location.

  
  (from en.wikipedia.org)

  But there is an antiperiplanar hydrogen at the 6-position, so elimination occurs there and produces only the Hofmann product.

  (2) Bulky bases

  
  (from www.masterorganicchemistry.com)

  A bulky base attacks the least sterically hindered proton to form the Hofmann product.

  Ernest Z. · 4 · Jul 14 2015
• What do E1 and E2 reactions have in common?

  E1 and E2 are both elimination reactions.

  Both consist of a step where the leaving group leaves, and one where a proton is abstracted to form a #pi# bond.

  Both can generally occur in polar protic solvent (there are extreme exceptions), and both are technically always in competition with nucleophilic substitution.

  If you want mechanisms, just ask!

  Al E. · 2 · Mar 23 2018

• What are the differences between E1 and E2 reactions?
• What do E1 and E2 reactions have in common?
• Do both elimination reactions, E1 and E2, follow Zaitsev’s rule?
• Which elimination reactions, E1 or E2, depend on the concentration of the substrate?
• Is there a formation of carbocation in both elimination reactions, E1 and E2?
• Do E1 reactions require a strong base, like E2 reactions?
• Why does the rate of the E1 reaction depend only on the substrate?
• Is there a requirement for the stereochemistry of the starting material in E1 reactions? Why?
• What does the rate of the E2 reactions depend on?
• Why does the stereochemistry of the hydrogen to be removed must be anti to that of the leaving group for E2 reactions?
• Why can cyclohexane rings cause some complications with elimination reactions?
• What is the most likely mechanism and the product from the reaction between 2-chloro-2-methylpentane and sodium ethoxide in ethanol?
• The major product which results when 2-chloro-2-methylpentane is heated in ethanol is an ether. What is the mechanism by which this ether forms?
• Based on Saytzeff's rule, select the most stable alkene: 1-methylcyclohexene, 3-methylcyclohexene or 4-methylcyclohexene?
• What product results from the action of #"sodium ethoxide"# on #"tertiary butyl chloride"#?