Formation and Decomposition of Diazo Ether under Acidic Conditions. Effects of Ethanol Concentration, Acidity, and Temperature on the Ethanolysis of 4-Methylbenzenediazonium Ions

We investigated the effects of solvent composition, acidity, and temperature on the dediazoniation of 4-methylbenzenediazonium (4MBD) ions in EtOH/H2O mixtures by employing a combination of spectrometric and chromatographic techniques. First-order behavior is found in all solvent composition ranges. HPLC Analyses of the reaction mixtures indicate that three main dediazoniation products are formed depending on the particular experimental conditions. These are 4-cresol (ArOH), 4-phenetole (ArOEt), and toluene (ArH). A1 acidities (defined as -log [HCl]) <2, the main dediazoniation products are the substitution products ArOH and ArOEt but upon decreasing the acidity, the reduction product ArH becomes predominant at the expenses of ArOH and ArOEt, indicating that a turnover in the reaction mechanism takes place under acidic conditions. At any given EtOH content, the plot of k(obs) values against acidity is S-shaped, the inflexion point depending upon the EtOH concentration and the temperature. Similar S-shaped variations are found when plotting the dediazoniation-product distribution against the acidity. The acid dependence of the switch between the homolytic and heterolytic mechanisms suggests that the homolytic dediazoniation proceeds via transient diazo ether, and this complex kinetic behaviour can be rationalized by assuiming two competitive mechanism in which the EtOH molecules capture ArN2+ to yield a highly unstable (Z)-adduct which undergoes homolytic fragmentation initiating a radical process (Scheme). Analyses of the effects of temperature on the equilibrium constant for the formation of the diazo ether and on the rate of splitting of the diazo ether allowed the estimation of relevant thermodynamic parameters for the formation of diazo ethers derived from methylbenzenediazonium ions under acidic conditions.