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Bonding changes along the Diels-Alder reaction between butadiene 1 and ethylene 2 and related non-polar Diels- Alder reactions have been analysed using the Bonding Evolution Theory (BET). The Electron Localization Function (ELF) analysis of these synchronous single bond-formation processes indicates that C-C bond formation takes place by the C-to-C coupling of two pseudoradical centers formed along the reaction. The present study permits the establishment of two significant findings: i) the breaking of the C=C double bonds in butadiene 1 and ethylene 2 and the formation of the new C-C single bonds in cycloadduct are non-concerted due to the changed in electron density required for the formation of the pseudoradical centers, and ii) the symmetric changes in electron density along these cycloadditions do not have a cyclic movement. These behaviours, which are opposite to the “concerted and close curve bonding changes” proposed by R. B. Woodward and R. Hoffmann for pericyclic reactions, allow refuting this mechanism for Diels-Alder reactions.