Methanolysis of 4-bromobenzenediazonium ions. Effects of acidity, [MeOH] and temperature on the formation and decomposition of diazo ethers that initiate homolytic dediazoniation

Abstract

We have investigated the effects of solvent composition, acidity and temperature on the dediazoniation of 4-bromobenzenediazonium (4BrBD) ions in MeOH-H(2)O mixtures by employing a combination of spectrometric and chromatographic techniques. The kinetic behaviour is quite complex; in the absence of MeOH, dediazoniations follow first-order kinetics with a half-life t(1/2) approximately 3000 min (T = 45 degrees C), but addition of small concentrations of MeOH lead to more rapid but non-first-order kinetics, suggestive of a radical mechanism, with t(1/2) approximately 125 min at 25% MeOH. Further increases in the MeOH concentration slow down the rate of dediazoniation and reactions progressively revert to first-order behaviour, and at percentages of MeOH higher than 90%, t(1/2) approximately 1080 min. Analyses of reaction mixtures by HPLC indicate that three main dediazoniation products are formed depending on the particular experimental conditions. These are 4-bromophenol (ArOH), 4-bromoanisole (ArOMe), and bromobenzene (ArH). At acidities (defined as -log[HCl]) < 2, the main dediazoniation products are the substitution products ArOH and ArOMe but, upon decreasing the acidity, the reduction product ArH becomes predominant at the expense of ArOH and ArOMe, indicating that a turnover in the reaction mechanism takes place under acidic conditions. At any given MeOH content, the plot of k(obs) or t(1/2) values against acidity is S-shaped, the inflexion point depending upon the MeOH 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 the homolytic dediazoniation proceeds via transient diazo ethers. The complex kinetic behaviour can be rationalized by assuming two competitive mechanisms: (i) the spontaneous heterolytic dediazoniation of 4BrBD, and (ii) an O-coupling mechanism in which the MeOH molecules capture ArN(2)(+) to yield a highly unstable Z-adduct which undergoes homolytic fragmentation initiating a radical process. 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, for the first time, estimation of relevant thermodynamic parameters for the formation of diazo ethers under acidic conditions.

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

“…1 can be derived where k HET and k HOM are the rate constants for the spontaneous thermal heterolytic decomposition of ArN þ 2 and the decomposition of the diazo ether, respectively, with K standing for the equilibrium constant for the diazo ether formation shown in the Scheme. [5] and confirming that increasing concentrations of EtOH have opposing effects upon the formation of the diazo ether in this concentration range: it is favored modestly by higher concentrations of EtOH through the mass-action effect, but strongly opposed by the medium effect on the equilibrium constant.…”

“…Formation of the diazo ether is exothermic yet has a positive standard free energy of formation at any of the temperatures investigated. The Gibbs free energy DG is higher for 70% EtOH/H 2 O than for 35% EtOH/H 2 O, highlighting the opposing effects of the concentration of EtOH, as it was previously found in solvolytic dediazoniations [4] [5].…”

“…An increase in the temperature does not seem to have a significant effect on K both at low and high percentages of EtOH, in contrast with the behavior found for arenediazonium ions with electron-withdrawing substituents such as 4BrBD [5]. The analysis of the variation of the K values with the temperature allows to gain some insight into the energetics of diazo ether formation (Scheme).…”

Spectroscopic studies of arenediazonium ion stability in surfactant solutions

“…In nonaqueous solvents, products associated with free radicals, for example, biphenyl derivatives and replacement of N 2 + by H are often observed, especially when electron-withdrawing groups are present on the arenediazonium ion [4,[17][18][19][20][21]. In this context heterolytic dediazoniation has been reported to occur with ortho-, meta-and para-methylbenzenediazonium (2MBD, 3MBD, 4MBD) and para-nitrobenzenediazonium (4NBD) ions in an aqueous medium [10,22], while other authors interpreted their results as showing evidence of heterolytic and homolytic processes during the thermal and photochemical dediazoniation of several arenediazonium ions in trifluoroethanol and ethanol [19][20][21]. However, in our latest article, we demonstrated that, at moderate pH, water can act as nucleophile giving rise to a diazohydroxide, which eventually can decompose to give radical products [23].…”

Kinetics and mechanism of the reaction between 3‐methylbenzenediazonium ions and catechol

“…The stability of arenediazonium salts is also modulated by the nature and position of substituents in the aromatic ring . In aqueous solution, electron‐withdrawing substituents make arenediazonium ions to be more stable than those with electron‐releasing groups but make them more prone to react through radical pathways …”