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How does a substitution reaction replace one atom or group in an organic molecule?
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reaction mechanism
Possible energy diagram for the dissociation of a covalent molecule, E–N, into its ions E+ and N− (see text).
A substitution reaction is a chemical process in which one atom, ion, or group of atoms in a molecule is replaced by another. These reactions are fundamental to organic chemistry, allowing for the transformation of one compound into another by swapping functional groups.12
Substitution reactions are generally classified based on the nature of the substituent involved:
- Nucleophilic Substitution: The substituent is electron-rich and provides an electron pair to bond with the substrate. This can occur via a unimolecular mechanism, where the rate-determining step involves the formation of a carbonium ion intermediate through the separation of a leaving group. Alternatively, it may occur through a bimolecular process where the nucleophile and substrate interact simultaneously.23
- Electrophilic Substitution: The substituent is electron-deficient, and the electron pair for bonding originates from the substrate itself. This is common in aromatic systems, often beginning with the formation of a pi complex before proceeding through a carbonium ion intermediate.24
- Free Radical Substitution: These involve the reactions of free radicals with suitable substrates.2
The specific pathway depends on the substrate's structure, the nature of the reagent, and reaction conditions.534