Full Answer Section

Similarities

• All four reactions are substitution or elimination reactions of alkyl halides.
• All four reactions require a good leaving group.
• All four reactions can be affected by the substrate, nucleophile/base, solvent, and temperature.

Differences

SN1 vs SN2:

• SN1 is a unimolecular reaction, while SN2 is a bimolecular reaction.
• SN1 requires a weak nucleophile, while SN2 requires a strong nucleophile.
• SN1 occurs in a step-wise mechanism, while SN2 occurs in a concerted mechanism.
• SN1 inverts the configuration at the stereocenter, while SN2 retains the configuration at the stereocenter.

E1 vs E2:

• E1 is a unimolecular reaction, while E2 is a bimolecular reaction.
• E1 requires a weak base, while E2 requires a strong base.
• E1 occurs in a step-wise mechanism, while E2 occurs in a concerted mechanism.
• E1 forms a mixture of products, including Z and E alkenes, while E2 forms Z-alkene as the major product.

Competition Between SN1 and E1

SN1 and E1 compete with each other when the substrate is primary or tertiary and the leaving group is good. The reaction that occurs predominantly depends on the solvent and the base. In polar protic solvents, SN1 is favored, while in polar aprotic solvents, E1 is favored. Strong bases favor E1, while weak bases favor SN1.

Competition Between SN2 and E2

SN2 and E2 compete with each other when the substrate is primary or secondary and the leaving group is good. The reaction that occurs predominantly depends on the steric hindrance of the substrate and the nucleophile. Strong bases favor E2, while weak bases favor SN2. Bulky nucleophiles favor SN2, while unhindered nucleophiles favor E2.

Rate Expressions

The rate expressions for the four reactions are as follows:

• SN1: Rate = k[alkyl halide]
• SN2: Rate = k[alkyl halide][nucleophile]
• E1: Rate = k[alkyl halide]
• E2: Rate = k[alkyl halide][base]

Concerted vs Step-Wise Reactions

Concerted reactions occur in one step, while step-wise reactions occur in two or more steps. SN2 and E2 are concerted reactions, while SN1 and E1 are step-wise reactions.

Stereospecific Reactions

Stereospecific reactions produce a single stereoisomer as the product. SN1 and SN2 are stereospecific reactions. SN1 inverts the configuration at the stereocenter, while SN2 retains the configuration at the stereocenter. E1 and E2 are not stereospecific reactions, as they produce a mixture of stereoisomers.

Factors That Influence the Reaction Pathway

The following factors influence the reaction pathway that will lead to the major product formed:

• Substrate: The substitution pattern and leaving group ability of the substrate can affect the reaction pathway. Primary alkyl halides are more likely to undergo SN1 and E1 reactions, while secondary alkyl halides are more likely to undergo SN2 and E2 reactions. Tertiary alkyl halides can undergo all four reactions. Good leaving groups, such as I- and Br-, favor SN1, SN2, and E2 reactions. Poor leaving groups, such as Cl-, favor E1 reactions.
• Nucleophile/base strength and size: The strength and size of the nucleophile/base can also affect the reaction pathway. Strong nucleophiles favor SN2 reactions. Strong bases favor E2 reactions. Bul