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When a nucleophile and an alkyl halide react, nucleophilic substitution and β-elimination reactions compete to generate products.

The following factors can influence the mechanisms competing against each other:

  • Structure of the substrate
  • Structure and basicity of the nucleophile
  • Temperature conditions
  • Solvent (protic vs. aprotic)

Thus, depending upon the relative rate of the unimolecular or bimolecular reaction, a mixture of products is generally obtained.

To predict the major and minor products use the following table:

Predicting products of substitution vs. elimination reactions of alkyl halides
Methyl halide 1° halide 2° halide 3° halide
Bimolecular reactions predominate Unimolecular reactions predominate
SN2 products are highly favored SN2 products favored.

E2 product dominates in the presence of a hindered strong base.

Both SN1 and E1 products are disfavored as the 1° carbocations are relatively unstable.
SN2 products are favored in the presence of weak bases.

E2 products are favored in the presence of strong bases.

Both SN1 and E1 products are rarely observed and occur only in the presence of weak nucleophiles or weak bases in polar protic solvents.
Due to steric hindrance, SN2 products are disfavored.

E2 products are favored only in the presence of strong bases.

Both SN1 and E1 products predominate in the presence of a weak nucleophile or a weak base in a polar protic solvent.

With increased temperature, E1 products are favored.
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