SN2 reactions are a type of nucleophilic substitution reaction in organic chemistry. In this reaction, a nucleophile (a molecule with a lone pair of electrons) attacks an electrophilic carbon atom, resulting in the displacement of a leaving group (usually a halide or tosylate). The reaction proceeds through a single transition state, resulting in a single product.

The reaction begins with the nucleophile attacking the electrophilic carbon atom, forming a covalent bond. This bond is formed by the donation of a lone pair of electrons from the nucleophile to the electrophilic carbon atom. The leaving group is then displaced from the carbon atom, resulting in the formation of a new covalent bond.

The reaction proceeds through a single transition state, which is characterized by a partial bond between the nucleophile and the electrophilic carbon atom. This transition state is relatively unstable, and the reaction is usually completed in a few milliseconds.

The reaction is favored by polar protic solvents, such as water, and is usually not favored in non-polar solvents, such as hexane. The reaction is also favored by strong nucleophiles, such as hydroxide ions, and is usually not favored by weak nucleophiles, such as ammonia.

The reaction is also affected by the steric hindrance of the leaving group. If the leaving group is bulky, the reaction is less likely to occur. Additionally, the reaction is favored by primary and secondary alkyl halides, and is usually not favored by tertiary alkyl halides.

SN2 reactions are important in organic chemistry, as they are used to synthesize a variety of compounds. They are also used in the synthesis of pharmaceuticals, agrochemicals, and other industrial products.