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:

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
S_N2 products are highly favored	S_N2 products favored. E2 product dominates in the presence of a hindered strong base. Both S_N1 and E1 products are disfavored as the 1° carbocations are relatively unstable.	S_N2 products are favored in the presence of weak bases. E2 products are favored in the presence of strong bases. Both S_N1 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, S_N2 products are disfavored. E2 products are favored only in the presence of strong bases. Both S_N1 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.