Wiktionary
n. (context chemistry English) a reaction in which one functional group is replaced with another
Wikipedia
Substitution reaction (also known as single displacement reaction or single replacement reaction) is a chemical reaction during which one functional group in a chemical 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. There are other classifications as well that are mentioned below.
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 halogenation. When chlorine gas (Cl-Cl) is irradiated, some of the molecules are split into two chlorine radicals (Cl) whose free electrons are strongly nucleophilic. One of them breaks a weak C-H covalent bond and grabs the liberated proton to form the electrically neutral H-Cl. The other radical reforms a covalent bond with the CHto form CHCl ( methyl chloride).
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chlorination of methane by chlorine