Substitution reaction

In a substitution reaction, a functional group in a particular chemical compound is replaced by another group ^[1]. 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:

Nucleophilic substitutions

These kind of substitution reactions happen 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.
• 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 compounds

The Nucleophilic substitutions can be produced by two different mechanisms:

• Monomolecular nucleophilic substitution (SN1): In this case the reaction proceeds in stages, the compounds first dissociate in their ions and then this ions react between them. It's produced by carbocations.
• Bimolecular nucleophilic substitution (SN2): In this case the reaction proceeds in only one stage. The attack of the reagent and the expulsion of the leaving group happen simultaneously.

Electrophilic substitutions

• Electrophiles are involved in electrophilic substitution reactions and particularly in electrophilic aromatic substitutions:

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

Radical substitutions

A radical substitution reaction involves radicals.

Substituted compounds

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

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:

References

External links

• Unimolecular Nucleophilic Substitution does not Exist! / N.S.Imyanitov. SciTecLibrary