Structure reactivity relationship in the accelerated formation of 2,3-diarylquinoxalines in the microdroplets of a nebuliser.

Competition experiments in which 1,2-phenylenediamine, CH(NH), condenses with equimolar quantities of benzil, (CHCO), and a 3,3'- or 4,4'-disubstituted benzil (XCHCO) (X = F, Cl, Br, CH or CHO) to form a mixture of 2,3-diphenylquinoxaline and the corresponding 2,3-diarylquinoxaline (Ar = XCH) in the microdroplets produced in a nebuliser allow a Hammett relationship with a ρ value of 1.8 to be developed for this accelerated condensation in the nebuliser. This structure reactivity relationship reveals that an appreciable amount of negative charge builds up on the carbon of the carbonyl group of the benzil during the rate-limiting step of the reaction, thus confirming that this process involves nucleophilic addition of the 1,2-phenylenediamine to the benzil.