p-Nitrobenzenediazonium tetrafluoroborate dissolved in dimethylsulfoxide (DMSO) at 50" forms p-nitrophenol in 88-90% yield. The phenolic oxygen atom originates exclusively from the oxygen atom of DMSO as demonstrated by the use of 180-labelled DMSO. The first-order rate of dediazoniation is the same under Nz as it is in the presence of air. The rate is little influcnccd by the addition of benzene or iodobenzenc. However, the products formed in the presence of thcsc additives are significantly different. UV. spectra and thc reactivity of diazonium salt solutions in DMSO when mixed with reagents in aqueous solution demonstrate that a relatively stable chargc-transfer complex is formed between the diazonium ion and DMSO. The product analyses and the kinetic and spectral results of dediazoniation in DMSO with and without additives are consistent with a mechanism in which the rate-limiting step is the formation of a 9-nitrophcnyl radical from the charge-transfer complex. 9-Nitrophenol and the products with benzene and iodobenzene are formed in subsequent fast competition steps. In the presence of small amounts of pyridine the dediazoniation is much faster and follows a different kinetic law. Pyridine effectively competes with DMSO in the reaction with diazonium ions.

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