Overview:
The general
form of the SN2 mechanism is as follows:
nuc:
= nucleophile
X = leaving group (usually halide or tosylate)
The SN2
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.
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