In a substitution reaction, a functional group in a particular chemical compound is replaced by another group.

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:

CH₄ + Cl₂ → CH₃Cl + HCl (in the presence of sunlight)

Nucleophilic substitutions

• 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

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

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:

• Benzene C₆H₆
• Toluene C₆H₅-CH₃
• Xylene C₆H₄(-CH₃)₂
• Mesitylene C₆H₃(-CH₃)₃
• Phenol C₆H₅-OH
• Aniline C₆H₅-NH₂
• Chlorobenzene C₆H₅-Cl
• Nitrobenzene C₆H₅-NO₂
• Benzoic acid C₆H₅-COOH
• Picric acid C₆H₂(-OH)(-NO₂)₃
• Trinitrotoluene C₆H₂(-CH₃)(-NO₂)₃
• Salicylic acid C₆H₄(-OH)(-COOH)
• Acetylsalicylic acid C₆H₄(-O-C(=O)-CH₃)(-COOH)
• Paracetamol C₆H₄(-NH-C(=O)-CH₃)(-OH)
• Phenacetin C₆H₄(-NH-C(=O)-CH₃)(-O-CH₂-CH₃)

Further reading

• Is This Reaction a Substitution, Oxidation-Reduction, or Transfer? / N.S.Imyanitov. J. Chem. Educ. 1993, 70(1), 14 – 16.

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