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Concept
Nucleophilic substitution
Summary
In chemistry, a nucleophilic substitution is a class of chemical reactions in which an electron-rich chemical species (known as a nucleophile) replaces a functional group within another electron-deficient molecule (known as the electrophile). The molecule that contains the electrophile and the leaving functional group is called the substrate.R. A. Rossi, R. H. de Rossi, Aromatic Substitution by the SRN1 Mechanism, ACS Monograph Series No. 178, American Chemical Society, 1983. .
The most general form of the reaction may be given as the following:
The electron pair (:) from the nucleophile (Nuc) attacks the substrate () and bonds with it. Simultaneously, the leaving group (LG) departs with an electron pair. The principal product in this case is . 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 basic conditions, where the attacking nucleophile is hydroxyl () and the leaving group is bromide ().
R-Br + OH- -> R-OH + Br-
Nucleophilic substitution reactions are common in organic chemistry. Nucleophiles often attack a saturated aliphatic carbon. Less often, they may attack an aromatic or unsaturated carbon.
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 were the SN1 reaction and the SN2 reaction, where S stands for substitution, N stands for nucleophilic, and the number represents the kinetic order of the reaction.
In the SN2 reaction, the addition of the nucleophile and the elimination of leaving group take place simultaneously (i.e. a concerted reaction). SN2 occurs when the central carbon atom is easily accessible to the nucleophile.
In SN2 reactions, there are a few conditions that affect the rate of the reaction.
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