A substitution reaction (also known as single displacement reaction or single substitution reaction) is a chemical reaction during which one
A good example of a substitution reaction is
|chlorination of methane by chlorine|
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.
The most general form for the reaction may be given as
where R−LG indicates the substrate. The electron pair (:) from the nucleophile (Nuc:) attacks the substrate (R−LG), forming a new covalent bond Nuc−R−LG. The prior state of charge is restored when the leaving group (LG) departs with an electron pair. The principal product in this case is R−Nuc. 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
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.
Nucleophilic substitutions on aliphatic carbon centers can proceed by two different mechanisms, unimolecular nucleophilic substitution (SN1) and bimolecular nucleophilic substitution (SN2).
The SN1 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 SN2 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.
SN2 attack may occur if the backside route of attack is not
When the substrate is an
Electrophiles are involved in electrophilic substitution reactions, particularly in electrophilic aromatic substitutions.
In this example, the benzene ring's electron resonance structure is attacked by an electrophile E+. The resonating bond is broken and a carbocation resonating structure results. Finally a proton is kicked out and a new aromatic compound is formed.
|Electrophilic aromatic substitution|
Electrophilic reactions to other unsaturated compounds than
A radical substitution reaction involves radicals. An example is the Hunsdiecker reaction.
Coupling reactions are a class of metal-catalyzed reactions involving an organometallic compound RM and an organic halide R′X that together react to form a compound of the type R-R′ with formation of a new carbon–carbon bond. Examples include the Heck reaction, Ullmann reaction, and Wurtz–Fittig reaction. Many variations exist.
Substituted compounds are
|Examples of substituted benzene compounds|
|compound||general formula||general structure|
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
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:
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