Fig. 5.

An example mass spectrum of online ESI-MS analysis of the inverse electron-demand Diels-Alder and subsequent retro Diels-Alder reaction. The reagents trans-cyclooctene (1) and tetrazine (2) are observed as protonated molecules at m/z 227, and m/z 246, respectively, while the reaction product 4,5-dihydropyridazine (4) is observed as a doubly charged ion [4+2H]²⁺, at m/z 222.6. The mass-to-charge range m/z 320–500 is presented in the inset with smaller y-axis scale. In the inset the reaction related ions [4+H–C₄H₁₀N₂O₂]⁺ at m/z 326 and [4 + H]⁺ at m/z 444 are seen. In addition, ions [4+H–H₂]⁺ at m/z 442 and [3 + H]⁺ at m/z 472 are seen, which we propose also to be reaction related ions. See Sections S6 – S8 for arguments, which support these assignments.