Overview

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
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.

Procedure

When a nucleophile and an alkyl halide react, a competition between the nucleophilic substitution and β-elimination takes place, dictating the reaction outcome.

An alkyl halide has two reactive sites. An attack on the electrophilic α-carbon results in a substitution reaction via an SN1 or an SN2 mechanism.

Alternatively, the nucleophile acts as a base and deprotonates the β-hydrogen, leading to an elimination reaction via an E1 or an E2 mechanism.

The reaction outcome is influenced by parameters like the relative steric hindrance of the substrate, the nucleophile’s size and basicity, temperature, and the type of solvent.

Depending upon the relative rate of the unimolecular or bimolecular reactions, a mixture of products is generally obtained. To predict the major and minor products, consider the following.

In general, β-eliminations are favored over substitution reactions with increased temperature, more basic and hindered nucleophiles, and increased substitution of the substrate.

Primary halides, being the least sterically hindered, exclusively undergo bimolecular reactions. Higher concentrations of strong nucleophiles or bases further accelerate the SN2 or E2 reaction, irrespective of the solvent.

A strong unhindered nucleophile attacks the α-carbon faster, favoring an SN2 product. Conversely, a strong hindered base has less access to the α-carbon, thus favoring an E2 product.

Primary halides, however, disfavor unimolecular reactions, as primary carbocations are relatively unstable.

In secondary halides, as increased branching hinders SN2 reactions, E2 products are favored, especially with strong bases. Conversely, the use of weak bases increases the likelihood of SN2 products.

Unimolecular reactions are rarely observed with secondary halides. However, they may proceed at a moderate pace in the presence of a weak nucleophile or base in polar protic solvents.

In tertiary halides, increased steric hindrance disfavors the SN2 mechanism.  With strong bases, the E2 reaction is favored, while both unimolecular reactions are predominant in the presence of weak bases or nucleophiles.

However, with increased temperature and the possibility of generating highly substituted alkenes, the E1 mechanism dominates over SN1.