A bimolecular process formally involving the exchange of a bond (or bonds) between similar interacting chemical species so that the bonding affiliations in the products are identical (or closely similar) to those in the reactants. For example:
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(The term has its origin in inorganic chemistry with a different meaning, but this older usage is not applicable in physical organic chemistry.)
Source:
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1139 (https://doi.org/10.1351/pac199466051077)