What is the Difference Between SN2 and E2 Reactions?

The SN2 and E2 reactions are both types of elimination reactions in organic chemistry, but they have distinct differences:

1. Reaction Mechanism: SN2 reactions involve a concerted, bimolecular nucleophilic substitution, where the nucleophile attacks the substrate from the opposite side of the leaving group, resulting in the expulsion of the leaving group and the formation of a new bond. On the other hand, E2 reactions involve a concerted, bimolecular elimination, where the base abstracts a hydrogen from one of the carbon atoms adjacent to the leaving group, forming an antiperiplanar configuration and eliminating both the leaving group and the base.
2. Requirements: SN2 reactions require a good nucleophile, while E2 reactions require a strong base. The nucleophile or base strength determines which mechanism is favored. A good nucleophile that is a weak base favors SN2, while a weak nucleophile that is a strong base favors E2.
3. Substrate Preference: SN2 reactions occur in primary and secondary substrates, while E2 reactions typically occur in tertiary substrates or under neutral conditions with poor nucleophiles/weak bases.
4. Leaving Group Requirements: Both reactions require good leaving groups, but the presence of antiperiplanar β-hydrogens is essential for E2 reactions to occur. If there are no antiperiplanar β-hydrogens, the SN2 reaction dominates.
5. Branching Effect: α and β branching can block the nucleophile's access to the substrate, reducing the proportion of SN2 relative to E2. However, E2 reactions can still occur even with extensive branching because they rely on the β-hydrogens, which are more accessible than the σ* C-LG antibond.

In summary, SN2 reactions involve a nucleophilic substitution mechanism and favor good nucleophiles, while E2 reactions involve a bimolecular elimination mechanism and favor strong bases. The substrate preferences, leaving group requirements, and branching effect also differentiate these two types of reactions.

Comparative Table: SN2 vs E2 Reactions

Below is a table comparing the differences between SN2 and E2 reactions:

SN2 reactions typically occur with primary or secondary alkyl halides, while E2 reactions occur with secondary or tertiary alkyl halides. SN2 reactions are favored by good nucleophiles (relatively weaker bases), whereas E2 reactions are favored by strong bases. The stereospecificity of SN2 reactions is determined by Bederson's rule, which states that the priority of inversion is given by the product stability > leaving group ability. On the other hand, E2 reactions follow Zaitsev's rule, which states that the more substituted double bond is favored over the less substituted double bond.