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
 


Mechanism:

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
 

Mechanism:

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: