Alkyl Halides - Elimination Reactions | Organic Chemistry 1

Further reactions of alkyl halides are studied in this chapter: elimination reactions (E reactions), competition between E1 and E2, competition between SN and E reactions

Alkyl Halide Reactions

Substitution reactions (SN reactions):


β Elimination reactions (E reactions):


In both reactions alkyl halide acts as an electrophile, reacting with an electron-rich reagent. In a substitution, the nucleophile attacks the carbon atom bearing the good leaving group, while in an elimination, the base removes a proton to form a π bond, and 2 carbons are involved in the reaction


Zaitsev rule:

In a β elimination reaction, the major product is the alkene with the most substituted double bond. The major product is called Zaitsev Product

E2 Reactions

E2 reactions: 

Nucleophilic eliminations which proceed via a concerted mechanism ⇒ E2 reactions are bimolecular with simultaneous bond-making and bond-breaking steps. The kinetic rate involves 2 components: the base and the electrophile. Therefore the E2 reaction is favored by strong bases


E2 reactions occur when H and X atoms are oriented on opposite sides of the molecules. This geometry is called anti periplanar and is preferred over syn periplanar geometry


E1 Reactions

E1 reactions: 

Nucleophilic eliminations which proceed via an intermediate carbocation ⇒ E1 reactions are unimolecular with a bond-breaking step following by a bond-making step. The kinetic rate only involves the starting material. Because the base does not appear in the rate equation, weak bases favor E1 reactions


In the first step, the leaving group comes off to form a planar carbocation, then in the second step, a β proton is removed by the base to give the alkene. Due to this 2-step mechanism, E1 reactions do not require anti periplanar geometry. The first step is slower and therefore determines the rate: it is the rate-determining step. The major product will be the Zaitsev product

E2 vs. E1

E2 mechanism:

1 step

Rate = k [RX] [Base]  ⇒  second-order kinetics

anti periplanar arrangement of H and X

favored by strong bases

E1 mechanism:

2 steps

Rate = k [RX]  ⇒  first-order kinetics

planar intermediate carbocation

favored by weak bases

SN1, SN2, E1, E2 Competition

Substitution reactions compete with β elimination reactions. The structure of alkyl halides and/or nucleophiles determines the type of reaction. Strong nucleophiles that are weak bases favor substitution over elimination. Bulky, non-nucleophilic bases favor elimination over substitution

  • Primary alkyl halides:
    - strong nucleophile ⇒ SN2
    - strong bulky base ⇒ E2

  • Secondary alkyl halides:
    - strong base and nucleophile ⇒ SN2 + E2
    - strong bulky base ⇒ E2
    - weak base and nucleophile ⇒ SN1 + E1

  • Tertiary alkyl halides:
    - weak base and nucleophile ⇒ SN1 + E1
    - strong base ⇒ E2


Common strong bulky bases: