A Product Analytical Study of the Thermal and Photolytic Decomposition of Some Arenediazonium Salts in Solution

Abstract

Products of thermal and photochemical reactions of eleven arenediazonium tetrafluoroborates in various solvents have been analyzed. All compounds in most solvents undergo unimolecular heterolysis to give singlet aryl cations which are captured by solvent. This mechanism is dominant for arenediazonium ions without electron-withdrawing substituents in all solvents, and the only reaction observed in water. Additionally, appreciable yields of fluoroarenes are obtained by fluoride abstraction by the aryl cation from fluorinated solvents and from tetrafluoroborate in fluorinated solvents. Yields from photochemical processes are very similar to those from thermal reactions indicating that the main reactions proceed through common or very similar intermediates. Aryl cations formed from ion-paired diazonium ions may react with the counterion, but fragmentation of dissociated diazonium ions leads only to solvent-derived product. Some arenediazonium ions in some solvents undergo an alternative radical reaction leading principally to hydrodediazoniation. It is proposed that this reaction involves initial rate-limiting electron transfer from ethanol to the arenediazonium ion followed rapidly by homolysis of the resultant aryldiazenyl radical. Within the same solvent cage, the aryl radical then either abstracts an α-hydrogen from the ethanol radical cation generated in the first step to give the reduction product and protonated acetaldehyde, or combines with it at the oxygen to give a protonated aryl ethyl ether.

pH Effects on Ethanolysis of Some Arenediazonium Ions: Evidence for Homolytic Dediazoniation Proceeding through Formation of Transient Diazo Ethers

“…[1Ϫ3,10Ϫ14] The source of electrons in solvolytic dediazoniations is thought to be the solvent, [12,14,15] but in contrast to other dediazoniations, no obvious reductants are employed in those experiments to initiate the radical process, so the mechanism and nature of the initiation step remain a matter of debate, [2,3,13,14] because what is lacking in these cases is a single and still convincing piece of evidence in favor, or against, the different proposed mechanistic alternatives. [10,16] Bunnet et al [10,17] postulated that, in alkaline methanol solutions, the source of radicals is the homolytic decomposition of a (Z)-diazo methyl ether, which is formed rapidly, yielding the aryl radical, N 2 and the methoxy radical, while under acidic conditions they favor initiation by direct electron transfer from MeOH to the arenediazonium ion.…”

“…[10,17] Recent solvolytic investigations also appear to favor direct electron transfer from the solvent (EtOH) to initiate the radical reaction, and alternative radical mechanisms proceeding through the formation of intermediate aryl diazo ethers have been deemed less likely. [13,14] There exist, however, a variety of reactions in which nucleophiles add to diazonium ions at N β to give (Z) adducts as kinetically controlled products. These (Z) adducts may undergo subsequent isomerization to the thermodynamically more stable (E) isomers, which in some instances can be isolated, [18] or may eventually give rise to homolytic rupture of the bonds providing the initiation of a radical process.…”

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

“…The experimental basis for our proposal was the finding of S-shaped variations in both k obs and product formation with acidity for 4-methylbenzenediazonium (4MBD) [4] and 4-nitrobenzenediazonium (4NBD) ions [4], and the results allowed identification of the dual role played by the ROH molecules as nucleophiles that i) simply solvate the diazonium ions (allowing them to undergo thermal heterolytic decomposition), and ii) react directly with the arenediazonium ions to yield O-coupling adducts in a highly unstable (Z)-configuration, i.e., (Z)-diazo ethers. On the second route, the (Z)-diazo ethers then undergo homolytic fragmentation initiating radical processes [1] [3] [8] [9] [11 -13]. Moreover, the kinetic investigation of the methanolysis of 4NBD and 4BrBD under acidic conditions provided an excellent example of the influence of solvent composition on the change from a heterolytic mechanism to a homolytic one [4] [5].…”

“…The 4MBD was chosen as a model arenediazonium ion because it is well recognized that substituents in the aromatic ring of diazonium ions have distinct mechanistic effects [8] [17] or antioxidants such as ascorbic acid [18] and methyl gallate [19]; for both nucleophiles, the adducts were detected experimentally and, in some instances, isolated [20]. As we will show, the results obtained for the ethanolysis of 4MBD provide further support for a radical-initiation mechanism via formation of a transient diazo ether, ArÀN¼NÀOEt, which undergoes homolytic fragmentation yielding an aryl radical (Ar . )…”

Effects of Micellar Aggregates on the Kinetics and Mechanism of the Reaction between 4‐Nitrobenzenediazonium Ions and Some Amino Acids

“…Previous kinetic and product-distribution studies on the spontaneous decomposition of 4NBD in acidic aqueous solution (0.01m HCl) show that the reaction follows a D N þ A N mechanism, i.e., the reaction proceeds through the rate-limiting formation of a highly reactive aryl cation which takes up any nucleophile available in the solvation shell [32]. Because p/s acceptor substituents in 4-position destabilize the aryl cation more than they destabilize the parent arenediazonium ion [32] [37], the thermal decomposition of 4NBD is very slow, with half-life values t 1/2 % 5.5 h at 608, 4-nitrophenol being the major product of dediazoniation [32] [38]. At 308, t 1/2 is ca.…”