Elimination Reaction
Definition and meaning of Elimination Reaction in chemistry.
Elimination is a reaction in which two substituents are removed from adjacent (or the same) carbon atoms of a molecule, typically generating a double or triple bond, often with loss of a small molecule such as water or a hydrogen halide.
In more detail
Elimination reactions run through two main mechanistic pathways: E1, a two-step process via a carbocation intermediate favored by polar protic solvents and weak bases, and E2, a concerted one-step process requiring a strong base and antiperiplanar geometry between the leaving group and the departing hydrogen. Both compete with substitution reactions (SN1/SN2) at the same substrate, and reaction conditions (base strength, solvent, temperature, substrate structure) determine which pathway dominates. When more than one alkene product is possible, Zaitsev's rule predicts the more substituted (more stable) alkene as the major product, unless a bulky base favors the Hofmann (less substituted) product.
Key facts
| Field | Organic Chemistry |
|---|---|
| Main mechanisms | E1, E2, E1cb |
| Common leaving groups lost | H2O, HX (hydrogen halide) |
| Regiochemistry rule | Zaitsev's rule favors the more substituted alkene |
Treating 2-bromopropane with a strong base such as potassium hydroxide in ethanol removes HBr in an E2 elimination, producing propene and water.
Frequently asked questions
How does elimination differ from substitution?
Substitution replaces a leaving group with a new nucleophile, keeping the carbon skeleton's saturation the same, while elimination removes two groups entirely to form a new pi bond (or ring), reducing the number of substituents on the carbon skeleton.
What is the key difference between E1 and E2?
E1 proceeds in two steps through a carbocation intermediate and its rate depends only on substrate concentration, whereas E2 is a single concerted step requiring a strong base, with rate depending on both substrate and base concentration.