Editing Substitution reaction (section)

==Nucleophilic substitution==
{{Main|Nucleophilic substitution}}
In organic (and inorganic) chemistry, [[nucleophilic substitution]] is a fundamental class of reactions in which a [[nucleophile]] selectively bonds with or attacks the positive or partially positive charge on an atom or a group of atoms. As it does so, it replaces a weaker nucleophile, which then becomes a [[leaving group]]; the remaining positive or partially positive atom becomes an [[electrophile]]. The whole molecular entity of which the electrophile and the leaving group are part is usually called the [[Substrate (chemistry)|substrate]].<ref name=":0" />

The most general form for the reaction may be given as 
:<chem>Nuc\mathbf{:}- + R-LG -> R-Nuc{} + LG\mathbf{:}-</chem>
where {{chem2|R\sLG}} indicates the substrate. The [[electron pair]] (''':''') from the nucleophile (Nuc:) attacks the substrate ({{chem2|R\sLG}}), forming a new covalent bond {{chem2|Nuc\sR\sLG}}. The prior state of charge is restored when the leaving group (LG) departs with an electron pair. The principal product in this case is {{chem2|R\sNuc}}. In such reactions, the nucleophile is usually electrically neutral or negatively charged, whereas the substrate is typically neutral or positively charged.

An example of nucleophilic substitution is the hydrolysis of an [[alkyl]] bromide, {{chem2|R\sBr}}, under basic conditions, where the ''attacking'' nucleophile is the base {{chem2|OH-}} and the leaving group is {{chem2|Br-}}:
:<chem>R-Br + OH- -> R-OH + Br-</chem>
Nucleophilic substitution reactions are commonplace in organic chemistry, and they can be broadly categorized as taking place at a carbon of a saturated [[aliphatic compound]] carbon or (less often) at an aromatic or other unsaturated carbon center.<ref name=":0" />

=== Mechanisms ===
{{Main|SN1 reaction|SN2 reaction|Nucleophilic acyl substitution}}

Nucleophilic substitutions can proceed by two different mechanisms, unimolecular nucleophilic substitution ([[SN1 reaction|S<sub>N</sub>1]]) and bimolecular nucleophilic substitution ([[SN2 reaction|S<sub>N</sub>2]]). The two reactions are named according tho their [[Rate equation|rate law]], with S<sub>N</sub>1 having a first-order rate law, and S<sub>N</sub>2 having a second-order.<ref name=":03">{{Cite book |last=Bruice |first=Paula Yurkanis |title=Organic Chemistry |publisher=Pearson Education Inc. |year=2011 |isbn=978-0-321-66313-9 |edition=6th |location=1900 E. Lake Ave., Glenview, IL 60025 |pages=332-365 |language=English}}</ref>
[[File:SN1 Reaction Mechanism.jpg|thumb|S<sub>N</sub>1 reaction mechanism occurring through two steps]]
The S<sub>N</sub>1 mechanism has two steps. In the first step, the leaving group departs, forming a [[carbocation]] (C<sup>+</sup>). In the second step, the nucleophilic reagent (Nuc:) attaches to the carbocation and forms a covalent sigma bond. If the substrate has a [[Chirality|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<sup>+</sup>) depends on how many other carbon atoms are bonded to it. This results in S<sub>N</sub>1 reactions usually occurring on atoms with at least two carbons bonded to them.<ref name=":03" /> A more detailed explanation of this can be found in the main [[SN1 reaction]] page.
[[File:SN2 reaction mechanism.png|thumb|S<sub>N</sub>2 reaction mechanism]]
The S<sub>N</sub>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<sub>N</sub>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<sub>N</sub>1 rather than an S<sub>N</sub>2.<ref name=":03" />
[[File:General Scheme for Base Catalyzed Nucleophilc Acyl Substitution.png|thumb|Nucleophilic acyl substitution mechanism]]
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|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 compound|aromatic compounds]] for more.