And the E2 reaction

Effect of the Base The nature of the base is the single most important factor in determining whether an elimination will go by the El or E2 mechanism. If a strong base is present, the rate of the bimolecular reaction will be greater than the rate of ionization, and the E2 reaction will predominate (perhaps accompanied by the SN2). 

Effect of the Substrate For both the El and the E2 reactions, the order of reactivity is... 

Both the Sn2 reaction in Chapter 11 (Section 11.2) and the E2 reaction discussed before follow second-order kinetics (see Chapter 7, Section 7.11.2) and are bimolecular. The E2 reaction involves a collision of the base with the P-hydrogen atom to initiate the elimination. In effect, collision of the base with the P-hydrogen will initiate the elimination sequence that expels the leaving group. Similarly, collision of a nucleophile with the alkyl hahde will kick out the leaving group for an Sn2. 

Even secondary halides are of little use in the Williamson reaction. Alkoxides are strong bases (high affinity for a hydrogen Is orbital) and the E2 reaction is a prominent side reaction when a secondary halide is used (Fig. 7.107). 

The Sn2 reaction and the E2 reaction are also competitive processes. In the Sn2 reaction, a nucleophile attacks the substrate at the tear of a carbon—leaving group bond. In the E12 reaction, it is the adjacent hydrogen that is removed along with the leaving group (Rg. 7.74). 

If the nucleophile is a strong base, tertiary haloalkanes react by either an S l or E2 mechanism, and the E2 reaction is favored. 

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