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In chemistry, nucleophilic substitution is a class of substitution reaction in which an electron-rich nucleophile attacks a molecule and replaces a group or atom, called the leaving group. It is a fundamental class of reaction in organic chemistry, where the reaction occurs at a carbon centre, but nucleophilic substitutions are also well known in inorganic covalent compounds too. If we ignore formal charges, the most general form for the reaction may be given as Nuc: + R-LG → R-Nuc + LG: The electron pair (:) from the nucleophile (Nuc) attacks the substrate (R-LG) forming a new bond, while the leaving group (LG) departs with an electron pair. The principal product in this case is R-Nuc. The nucleophile may be 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, R-Br, under alkaline conditions, where the attacking nucleophile is the OH− and the leaving group is Br-. R-Br + OH− → R-OH + Br− Nucleophilic substitution reactions are commonplace in organic chemistry, and they can be broadly categorised as taking place at an aliphatic (saturated) carbon or at (less often) an aromatic or other unsaturated carbon centre.
S In 1935, Edward D. Hughes and Sir Christopher Ingold studied nucleophilic substitution reactions of alkyl halides and related compounds. They proposed that there were two main mechanisms at work, both of them competing with each other. The two main mechanisms are the SN1 reaction and the SN2 reaction. S stands for chemical substitution, N stands for nucleophilic, and the number represents the kinetic order of the reaction. Nucleophilic substitution reactions There are many reactions in organic chemistry that involve this type of mechanism. Common examples include R-X → R-H using LiAlH4 (SN2) R-Br + OH− → R-OH + Br− (SN2) or R-Br + H2O → R-OH + HBr (SN1) R-Br + OR'− → R-OR' + Br− (SN2) Other mechanisms Besides SN1 and SN2, other mechanisms are known, although they are less common. The SNi mechanism is observed in reactions of thionyl chloride with alcohols, and it is similar to SN1 except that the nucleophile is delivered from the same side as the leaving group. Nucleophilic substitutions can be accompanied by an allylic rearrangement as seen in reactions such as the Ferrier rearrangement. This type of mechanism is called an SN1' or SN2' reaction (depending on the kinetics). With allylic halides or sulfonates, for example, the nucleophile may attack at the γ unsaturated carbon in place of the carbon bearing the leaving group. This may be seen in the reaction of 1-chloro-2-butene with sodium hydroxide to give a mixture of 2-buten-1-ol and 1-buten-3-ol: CH3CH=CH-CH2-Cl → CH3CH=CH-CH2-OH + CH3CH(OH)-CH=CH2 Nucleophilic substitution at unsaturated carbon centres Nucleophilic substitution via the SN1 or SN2 mechanism does not generally occur with vinyl or aryl halides or related compounds. Under certain conditions nucleophilic substitutions may occur, via other mechanisms such as those described in the nucleophilic aromatic substitution article. When the substitution occurs at the carbonyl group, the acyl group may undergo nucleophilic acyl substitution. This is the normal mode of substitution with carboxylic acid derivatives such as acyl chlorides, esters and amides. See also Notes | ||||||||
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