E2 elimination
In E2 elimination, a base removes a hydrogen while the leaving group departs from the next carbon, forming a double bond in one step. Rate = k[substrate][base]. The H and the leaving group must be anti to each other. The more substituted alkene usually dominates (Zaitsev's rule).
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Analogy
A zipper opening in one smooth pull: the base pulls one end, and the other end comes away at the same time.
Example
Major: but-2-ene (more substituted)Minor: but-1-eneOne step, anti arrangementRate = k[RBr][base]
In practice
Heating 2-bromobutane with strong base in ethanol gives mostly but-2-ene rather than but-1-ene.
Practice
Flashcards
- What does E2 need geometrically?The H and leaving group anti to each other.
Questions people ask
How do I decide between substitution and elimination?
Strong, bulky bases and heat favour elimination. Good nucleophiles that are weak bases, such as iodide, favour substitution. Tertiary halides with strong base give E2, since SN2 is blocked. Higher temperature always pushes the balance toward elimination because it releases more molecules and gains entropy.
What is Zaitsev's rule?
Zaitsev's rule says elimination gives mainly the most substituted alkene, which is the most stable. With 2-bromobutane, but-2-ene beats but-1-ene. A bulky base such as tert-butoxide can break the rule and give the less substituted alkene, called the Hofmann product.