What does cycloaddition mean?
Cycloaddition is a noun denoting a fundamental class of chemical reactions in which two or more unsaturated molecules — or separate parts of one molecule — join together to form a new ring. In a cycloaddition, the π bonds of the reacting partners are reorganized into σ bonds that close the ring, and the process is classified by notation such as [4+2] or [2+2], which counts how many atoms each component contributes to the product. The best-known example is the Diels–Alder reaction, in which a conjugated diene and a dienophile combine to give a six-membered ring, a transformation so reliable and versatile that it earned Otto Diels and Kurt Alder the 1950 Nobel Prize in Chemistry. Cycloadditions are governed by the Woodward–Hoffmann rules, which predict whether thermal or photochemical conditions will allow the reaction to proceed. The term is confined to chemistry and appears almost exclusively in academic papers, textbooks, and technical discussions of synthesis.
nounA chemical reaction in which two or more unsaturated molecules (or parts of the same molecule) combine to form a cyclic product, with a net reduction in bond multiplicity.
- In chemistry: a pericyclic reaction in which two or more unsaturated molecules or fragments combine to form a ring, classified by the number of electrons involved (e.g. [4+2], [2+2]).
"The Diels–Alder reaction is the most celebrated example of a cycloaddition in organic synthesis."
"The synthesis relied on an intramolecular [2+2] cycloaddition to construct the strained four-membered ring."
"Thermal cycloadditions obey the Woodward–Hoffmann rules, proceeding suprafacially when 4n+2 electrons are involved."
Plural is standard when referring to multiple instances or types of the reaction, especially in research literature.
"The review surveyed photochemical cycloadditions across three decades of literature."
The Diels–Alder cycloaddition won Otto Diels and Kurt Alder the 1950 Nobel Prize in Chemistry — and it builds rings so cleanly that chemists still call it 'the' cycloaddition.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of cycloaddition
The word combines 'cyclo-', from Greek kyklos meaning 'circle' or 'wheel', with 'addition', from Latin additio, derived from addere, 'to add'. It arose as an organic chemistry term in the early twentieth century as chemists systematized ring-forming reactions, becoming standard after the Diels–Alder reaction was described in 1928 and the field of pericyclic reactions matured mid-century. Its components are shared with words like cycle, cyclone, encyclopedia, and addition.
How cycloaddition is actually used
Strictly a technical term used in physical organic chemistry; not encountered in general conversation. Often written with bracket notation such as [4+2] or [2+2] to indicate how many atoms of each reacting component enter the ring. The reverse process is called cycloreversion or a retro-cycloaddition.
Easily confused with cycloaddition
Addition is any reaction joining atoms or groups to a molecule, whereas cycloaddition specifically forms a ring from two or more π systems.