Substitution reaction

In a substitution reaction, a functional group in a particular chemical compound is replaced by another group.^[1][2] In organic chemistry, the electrophilic and nucleophilic substitution reactions are of prime importance. Organic substitution reactions are classified in several main organic reaction types depending on whether the reagent that brings about the substitution is considered an electrophile or a nucleophile, whether a reactive intermediate involved in the reaction is a carbocation, a carbanion or a free radical or 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 the photochemical chlorination of methane forming methyl chloride.

chlorination of methane by chlorine

Nucleophilic substitution

Nucleophilic substitution happens when the reagent is a nucleophile, which means, an atom or molecule with free electrons.

A nucleophile reacts with an aliphatic substrate in a nucleophilic aliphatic substitution reaction. These substitutions can be produced 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. In the second step, the nucleophilic reagent attacks the carbocation and forms a sigma bond. This mechanism can result in either inversion or retention of configuration. An SN₂ reaction 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.

When the substrate is an aromatic compound the reaction type is nucleophilic aromatic substitution. Carboxylic acid derivatives react with nucleophiles in nucleophilic acyl substitution. This kind of reaction can be useful in preparing compouds

Electrophilic substitution

Electrophiles are involved in electrophilic substitution reactions and particularly in electrophilic aromatic substitutions.

Electrophilic aromatic substitution

Electrophilic reactions to other unsaturated compounds than arenes generally lead to electrophilic addition rather than substitution.

Radical substitution

A radical substitution reaction involves radicals. An example is the Hunsdiecker reaction.

Organometallic substitution

Coupling reactions are a class of metal-catalyzed reactions involving an organometallic compound RM and an organic halide R'X that together react to a compound of the type R-R' with formation of a new carbon-carbon bond. Examples are the Heck reaction and the Ullmann reaction. Many variations exist.^[3]

Substituted compounds

Substituted compounds are chemical compounds where one or more hydrogen atoms of a core structure have been replaced with a functional group like alkyl, hydroxy, or halogen.

For example benzene is a simple aromatic ring and substituted benzenes are a heterogeneous group of chemicals with a wide spectrum of uses and properties:

compound	general formula	general structure
Benzene	C6H6
Toluene	C6H5-CH3
o-Xylene	C6H4(-CH3)2
Mesitylene	C6H3(-CH3)3
Phenol	C6H5-OH
Just a few substituted benzene compounds

References

1. ^ March, Jerry (1985), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (3rd ed.), New York: Wiley, ISBN 0-471-85472-7 
2. ^ Imyanitov, Naum S. (1993). "Is This Reaction a Substitution, Oxidation-Reduction, or Transfer?". J. Chem. Educ. 70 (1): 14–16. doi:10.1021/ed070p14. 
3. ^ Elschenbroich, C.; Salzer, A. (1992). Organometallics: A Concise Introduction (2nd ed.). Weinheim: Wiley-VCH. ISBN 3-527-28165-7.