SN2 mechanism

Overview:

The general form of the S_N2 mechanism is as follows:



nuc: = nucleophile
X = leaving group (usually halide or tosylate)

The S_N2 reaction involves displacement of a leaving group (usually a halide or a tosylate), by a nucleophile. This reaction works the best with methyl and primary halides because bulky alkyl groups block the backside attack of the nucleophile, but the reaction does work with secondary halides (although it is usually accompanied by elimination), and will not react at all with tertiary halides. In the following example, the hydroxide ion is acting as the nucleophile and bromine is the leaving group:

Because of the backside attack of the nucleophile, inversion of configuration occurs.

Solvents: Protic solvents such as water and alcohols stabilize the nucleophile so much that it won't react. Therefore, a good polar aprotic solvent is required such as ethers and ketones and halogenated hydrocarbons.

Nucleophiles: A good nucleophile is required since it is involved in the rate-determining step.

Leaving groups: A good leaving group is required, such as a halide or a tosylate, since it is involved in the rate-determining step