ABSTRACT
The diazines react with alkyl halides to give mono-quaternary salts, though somewhat less readily than comparable pyridines. Dialkylation cannot be achieved with simple alkyl halides, however the very much more reactive trialkyloxonium tetrafluoroborates do convert all three systems into di-quaternary salts. Chlorination of 2-methylpyrazine occurs under such mild conditions that it is almost certain that an addition/elimination sequence is involved, rather than a classical aromatic electrophilic substitution. The diazines are generally resistant to oxidative attack at ring carbon, though alkaline oxidising agents can bring about degradation via intermediates produced by initial nucleophilic addition. The diazines are very susceptible to nucleophilic addition: pyrimidine, for example, is decomposed when heated with aqueous alkali by a process which involves hydroxide addition as a first step, and it is converted into pyrazole by reaction with hydrazine. The Chichibabin reaction can be carried out under the usual conditions in a few cases, but is much less general than for pyridines.
