Dediazoniation of 4‐Nitrobenzenediazonium Ions in Acidic MeOH/H2O Mixtures: Role of Acidity and MeOH Concentration on the Formation of Transient Diazo Ethers that Initiate Homolytic Dediazoniation

Abstract

AbstractWe have investigated the dediazoniation of 4‐nitrobenzenediazonium (4NBD) ions in MeOH/H2O mixtures under acidic conditions at 50 °C employing a combination of spectrophotometric and chromatographic techniques. The kinetic behaviour is quite complex; in the absence of MeOH, the dediazoniations follow first‐order kinetics with a half‐life t1/2 = 1383 min, but the addition of small concentrations of MeOH lead to more rapid but non‐first‐order kinetics, suggestive of a radical mechanism, with t1/2 ≈ 6 min at 10 % MeOH. Further increases in the MeOH concentration slow down the rate of dediazoniation and reactions progressively revert to first‐order behaviour, with clean first‐order kinetics at percentages of MeOH higher than 90 % (t1/2 ≈ 77 min). The analyses of the reaction mixtures by HPLC indicate up to four dediazoniation products depending on the particular experimental conditions. These are 4‐nitrophenol (ArOH), 4‐nitroanisole (ArOMe), nitrobenzene (ArH), and 4,4′‐dinitrobiphenyl (DNB), this last product being detected only at MeOH percentages in the range 0.5–15 %. In the absence of MeOH, ArOH is the only product and formed in quantitative yield; however, at only 20 % MeOH, ArOH is down to less than 10 % and the reduction product, ArH, is obtained in more than 90 %. Upon increasing the MeOH content further, the formation of ArOMe becomes competitive and, at 99 % MeOH, both ArH (60 %) and ArOMe (40 %) are obtained. The observed rate constants, kobsd., or half‐lives, t1/2, are the same regardless of whether they are determined by monitoring product formation (by a well‐established product derivatization protocol followed by HPLC measurements) or reactant decomposition (obtained UV spectrophotometrically). At any given MeOH content, the plot of kobsd. or t1/2 values against the acidity (defined as –log [HCl]) is S‐shaped, the inflexion point depending upon the MeOH concentration. The complex behaviour can be accounted for by two competitive mechanisms, and whichever is dominant depends upon the acidity and the composition of the aqueous methanol solvent. Furthermore, the acid‐dependence of the switch between the homolytic and heterolytic mechanisms suggests the homolytic dediazoniation proceeds via transient diazo ethers. Thus, the MeOH molecules react with ArN2+ to yield an O‐coupling adduct in a highly unstable Z configuration, i.e. a (Z)‐diazo ether. Subsequently, this undergoes homolytic fragmentation which initiates a radical process. In the polar alternative, the methanol simply solvates the ArN2+ ions, allowing them to undergo thermal heterolysis, i.e. a DN + AN mechanism via the aryl cation. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)