E1 and E2 eliminations are reaction pathways that convert alkyl halides or protonated alcohols into alkenes by removing a leaving group and a beta hydrogen. They matter because they explain how chemists build carbon carbon double bonds and predict which alkene product will form. These mechanisms often compete with substitution reactions, so recognizing the conditions helps predict the major product.
A small change in base strength, solvent, temperature, or substrate structure can switch the reaction outcome.
Key Facts
- E2 rate law: rate = k[substrate][base]
- E1 rate law: rate = k[substrate]
- E2 is one concerted step: C-H bond breaks, C-LG bond breaks, and C=C bond forms at the same time.
- E1 is two main steps: leaving group leaves to form a carbocation, then a base removes a beta H to form an alkene.
- Zaitsev's rule: the more substituted alkene is usually the major product, especially for E1 and many E2 reactions with small bases.
- E2 requires an anti-periplanar beta H and leaving group arrangement for best orbital overlap.
Vocabulary
- Elimination reaction
- A reaction that removes atoms or groups from adjacent carbons to form a pi bond.
- Beta hydrogen
- A hydrogen atom attached to a carbon next to the carbon bearing the leaving group.
- Leaving group
- An atom or group that departs with an electron pair during a reaction, such as Br-, I-, or water from a protonated alcohol.
- Carbocation
- A positively charged carbon intermediate formed in E1 reactions after the leaving group departs.
- Zaitsev product
- The more substituted alkene product that is often favored because it is usually more stable.
Common Mistakes to Avoid
- Calling every elimination E2, which is wrong because weak bases in polar protic solvent with tertiary substrates often favor E1 through a carbocation.
- Forgetting the anti-periplanar requirement in E2, which is wrong because the beta C-H bond and C-LG bond must align properly for concerted pi bond formation.
- Using only Zaitsev's rule without checking the base, which is wrong because bulky bases can favor the less substituted Hofmann alkene.
- Ignoring substitution competition, which is wrong because strong nucleophiles, weak bases, low temperature, and unhindered substrates can shift products toward SN1 or SN2 instead of elimination.
Practice Questions
- 1 2-bromobutane reacts with sodium ethoxide in ethanol at heat. Draw the possible alkene products and identify the major product using Zaitsev's rule.
- 2 For tert-butyl bromide in water, write the E1 rate law and explain why doubling the water concentration does not double the elimination rate.
- 3 A reaction of a cyclohexyl bromide gives little E2 product until the ring flips. Explain how the anti-periplanar requirement controls whether elimination can occur.