Substitution reaction
Mechanisms

Nucleophilic substitutions can proceed by two different mechanisms, unimolecular nucleophilic substitution (S 1) and bimolecular nucleophilic substitution (S 2). The two reactions are named according tho their rate law, with S 1 having a first-order rate law, and S 2 having a second-order.

The S 1 mechanism has two steps. In the first step, the leaving group departs, forming a carbocation (C). In the second step, the nucleophilic reagent (Nuc:) attaches to the carbocation and forms a covalent sigma bond. If the substrate has a chiral carbon, this mechanism can result in either inversion of the stereochemistry or retention of configuration. Usually, both occur without preference. The result is racemization.

The stability of a carbocation (C) depends on how many other carbon atoms are bonded to it. This results in S 1 reactions usually occurring on atoms with at least two carbons bonded to them. A more detailed explanation of this can be found in the main SN1 reaction page.

The S 2 mechanism has just one step. The attack of the reagent and the expulsion of the leaving group happen simultaneously. This mechanism always results in inversion of configuration. If the substrate that is under nucleophilic attack is chiral, the reaction will therefore lead to an inversion of its stereochemistry, called a Walden inversion.

S 2 attack may occur if the backside route of attack is not sterically hindered by substituents on the substrate. Therefore, this mechanism usually occurs at an unhindered primary carbon center. If there is steric crowding on the substrate near the leaving group, such as at a tertiary carbon center, the substitution will involve an S 1 rather than an S 2.

Other types of nucleophilic substitution include, nucleophilic acyl substitution, and nucleophilic aromatic substitution. Acyl substitution occurs when a nucleophile attacks a carbon that is doubly bonded to one oxygen and singly bonded to another oxygen (can be N or S or a halogen), called an acyl group. The nucleophile attacks the carbon causing the double bond to break into a single bond. The double can then reform, kicking off the leaving group in the process.

Aromatic substitution occurs on compounds with systems of double bonds connected in rings. See aromatic compounds for more.
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