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How does a substitution reaction replace an atom or group in an organic molecule?
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chemical reaction
Log burning in a fire. Burning wood is an example of a chemical reaction in which wood in the presence of heat and oxygen is transformed into carbon dioxide, water vapor, and ash.
A substitution reaction occurs when an atom, ion, or group of atoms in a molecule is replaced by another atom, ion, or group. These reactions are categorized into three primary classes based on the nature of the substituent involved:12
- Nucleophilic substitution: An electron-rich substituent (nucleophile) provides an electron pair to bond with the substrate molecule, displacing the original group.1
- Electrophilic substitution: An electron-deficient substituent (electrophile) bonds with the substrate, which provides the necessary electron pair.1
- Radical substitution: Involves the reaction of free radicals with a substrate.1
For example, in the conversion of chloromethane to methanol, a hydroxide ion displaces a chlorine atom. Similarly, benzyl bromide can be converted to benzyl alcohol when the bromine atom is replaced by a hydroxyl group from sodium hydroxide. In more complex systems, such as aromatic rings or unsaturated carbon centers, the process may involve multi-stage mechanisms, including the addition of a reagent followed by the expulsion of an anionic group.1234