A substitution reaction (also known as single displacement reaction or single substitution reaction) is a chemical reaction during which one functional group in a compound is replaced by another functional group. Substitution reactions are of prime importance in organic chemistry. Substitution reactions in organic chemistry are classified either as electrophilic or nucleophilic depending upon the reagent involved, whether a reactive intermediate involved in the reaction is a carbocation, a carbanion or a free radical, and whether the substrate is aliphatic or aromatic. Detailed understanding of a reaction type helps to predict the product outcome in a reaction. It also is helpful for optimizing a reaction with regard to variables such as temperature and choice of solvent.
A good example of a substitution reaction is halogenation. When chlorine gas (Cl<sub>2</sub>) is irradiated, some of the molecules are split into two chlorine radicals (Cl•), whose free electrons are strongly nucleophilic. One of them breaks a C–H covalent bond in CH<sub>4</sub> and grabs the hydrogen atom to form the electrically neutral HCl. The other radical reforms a covalent bond with the CH<sub>3</sub>• to form CH<sub>3</sub>Cl (methyl chloride).
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Substitution reaction : chlorination of methane
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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.
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 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.
Substituted compounds
Substituted compounds are compounds where one or more hydrogen atoms have been replaced with something else such as an alkyl, hydroxy, or halogen. More can be found on the substituted compounds page.
Inorganic and organometallic chemistry
While it is common to discuss substitution reactions in the context of organic chemistry, the reaction is generic and applies to a wide range of compounds. Ligands in coordination complexes are susceptible to substitution. Both associative and dissociative mechanisms have been observed.
Associative substitution, for example, is typically applied to organometallic and coordination complexes, but resembles the Sn2 mechanism in organic chemistry. The opposite pathway is dissociative substitution, being analogous to the Sn1 pathway.
Examples of associative mechanisms are commonly found in the chemistry of 16e square planar metal complexes, e.g. Vaska's complex and tetrachloroplatinate. The rate law is governed by the Eigen–Wilkins Mechanism.
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Dissociative substitution resembles the S<sub>N</sub>1 mechanism in organic chemistry. This pathway can be well described by the cis effect, or the labilization of CO ligands in the cis position. Complexes that undergo dissociative substitution are often coordinatively saturated and often have octahedral molecular geometry. The entropy of activation is characteristically positive for these reactions, which indicates that the disorder of the reacting system increases in the rate-determining step. Dissociative pathways are characterized by a rate determining step that involves release of a ligand from the coordination sphere of the metal undergoing substitution. The concentration of the substituting nucleophile has no influence on this rate, and an intermediate of reduced coordination number can be detected. The reaction can be described with k<sub>1</sub>, k<sub>−1</sub> and k<sub>2</sub>, which are the rate constants of their corresponding intermediate reaction steps:
:<chem>L_\mathit{n}M-L <=>[-\mathrm L, k_1][+\mathrm L, k_{-1}] L_\mathit{n}M-\Box ->[+\mathrm L', k_2] L_\mathit{n}M-L'</chem>
Normally the rate determining step is the dissociation of L from the complex, and [L'] does not affect the rate of reaction, leading to the simple rate equation:
:<chem> Rate = {\mathit k_1 [L_{\mathit{nM-L]}</chem>
Further reading
- Imyanitov, Naum S. (1993). "Is This Reaction a Substitution, Oxidation-Reduction, or Transfer?". J. Chem. Educ. 70 (1): 14–16. Bibcode:1993JChEd..70...14I. doi:10.1021/ed070p14.