Dediazoniations of Arenediazonium Ions in Homogeneous Solutions. Part XV. Products of dediazoniation of p-chlorobenzenediazonium tetrafluoroborate in weakly alkaline aqueous solutions

The products of decomposition of solutions of p-chlorobenzenediazonium tetrafluoroborate in aqueous buffer solutions (pH 9.0–10.3; ionic strength 0.1–0.5) at 20.0° have been analyzed quantitatively. Up to eleven low molecular weight compounds could be identified besides the major product, the complex polymeric diazo tar. The distribution of products is influenced by trace amounts of oxygen as well as by p-chlorophenol and the radical trapping reagent iodoacetic acid. Mechanisms of formation of the products are discussed.     

Dediazoniations of Arenediazonium Ions. Part XXI. Dediazoniation of Arenediazonium Ions Complexed with Crown Ethers

The evaluation of the dediazoniation kinetics of various m- and p-substituted benzenediazonium tetrafluoroborates in 1,2-dichloroethane at 50° in the presence of 18-crown-6, 21-crown-7 and dicyclohexano-24-crown-8 demonstrates that the rate constant for the dediazoniation within the complex (k₂) is smallest, and the equilibrium constant for complex formation (K) is largest for the complexes with 21-crown-7 (cf. Scheme 1). The logarithms of the equilibrium constants (K) for complex formation with each of the crown ethers studied correlate well with Hammett's substituent constants, σ, to give reaction constants ρ = 1.18–1.38. A linear correlation between the logarithms of the rate constants for the dediazoniation within the complex with those of the dediazoniation rate constants of uncomplexed diazonium ions (log k₂ vs. log.k₁), found for most substituted diazonium salts, indicates that the dediazoniation mechanism of the complexed diazonium ions is not significantly different from that of the free ions. For very electrophilic diazonium ions (p-Cl, m-CN), k₂ was much larger than expected on the basis of the linear log k₂ vs. log k₁ relationship. Analysis of the dediazoniation products showed that this was due to a change in mechanism from heterolytic to homolytic dediazoniation. The complexation rate of diazonium salts by crown ethers (k_c) is practically diffusion controlled and does not change much with the size of the crown ether. The decomplexation rate (k_d), however, is significantly lower for complexes with 21-crown-7, than for those with 18-crown-6 and dicyclohexano-24-crown-8, and is therefore the reason for the variations in the equilibrium constant (K) and thus for the fact that complexes of arenediazonium salts with 21-crown-7 are the most stable. The amounts of the N_α-N_β rearrangement, as well as those of the exchange of the ¹⁵N-labelled diazonio group with external nitrogen during dediazoniation of p-toluenediazonium salt were independent of the addition of crown ethers. A dediazoniation mechanism involving a charge transfer, as well as an insertion-type diazonium ion-crown ether complex is proposed. In this mechanism, dediazoniation of the insertion complex does not take place directly, but through the charge-transfer complex.     

Dediazoniation of Arenediazonium Ions in Homogeneous Solutions. Part XVI. Kinetics and mechanisms of dediazoniation of p-chlorobenzenediazonium tetrafluoroborate in weakly alkaline aqueous solutions under nitrogen gas

The kinetics of reactions of p-chlorobenzenediazonium ions in aqueous buffer solutions (pH 9.0–10.6) under N₂ (< 5 ppb of O₂) have been measured between 20 and 50°C. The formation of trans-diazotate is first-order with respect to the concentration of hydroxyl ions and to the equilibrium concentration of diazonium ions, if the diazonium ion?cis-diazotate equilibrium is considered as a fast prior equilibrium. This indicates that the p-chlorobenzenediazonium ion, in contrast to all previous investigations with the p-nitrobenzenediazonium ion and benzenediazonium ions carrying similar substituents with a ?M effect, rearranges from the cis- to the trans-configuration as diazohydroxide and not as diazotate. The formation of trans-diazotate is catalyzed by carbonate and inhibited by hydrogen carbonate ions; mechanisms of these catalyses are discussed, and the solvent isotope effect K_H2O/K_D2O measured by an ¹H-NMR. technique reported. The kinetics of the dediazoniations can be analyzed as a mixture of two reactions, a relatively fast first reaction, reaction A, which is responsible for about 5% of the total reaction, and a second reaction F. Both are first-order with respect to diazonium ion; reaction A is also first-order in hydroxyl ions. There are some indications that reaction A corresponds to the hydrolysis of the diazonium ion to give eventually amine and nitrite ions. Reaction F shows a complex dependence on hydroxyl ions; it is related to the homolytic dediazoniation.     

Dediazoniation of Arenediazonium Ions in Homogeneous Solutions. Part XVII. Kinetics and mechanisms of dediazoniation of p-chlorobenzenediazonium tetrafluoroborate in weakly alkaline aqueous solutions in the presence of oxygen

The kinetics of dediazoniation of p-chlorobenzenediazonium tetrafluoroborate have been studied in buffer solutions in the pH-range 9.0–10.0, ionic strength I = 0.10, at 20.0° in glass and polytetrafluoroethylene vessels. The presence of oxygen (<5 ppb of O₂, 60 to 100 ppb of O₂, air, > 99% of O₂) has a decisive influence on the rate and kinetic order of the dediazoniation. Iodoacetic acid inhibits the reaction, whereas p-chlorophenol has a catalytic effect, and in air and >99% of O₂ it acts as an autocatalyst. The reaction is subject to general-base catalysis by water, hydroxyl ions, hydrogen carbonate and carbonate ions. The kinetic results are interpreted in conjunction with data concerning the reaction products [2] and a ¹⁵N-CIDNP. investigation of a related system [3]. Specific radical chain mechanisms are consistent with the results.     

Dediazoniation of Arenediazonium Ions in Homogeneous Solution. Part XII. Solvent effects in competitive heterolytic and homolytic dediazoniations

In heterolytic dediazoniations arenediazonium salts form aryl cations. The reaction rates are relatively slow; they depend only to a small extent on the solvent. It is shown that the solvents in which the heterolytic dediazoniation mechanism is predominant have a low nucleophilicity, whereas in solvents of high nucleophilicity homolysis of arenediazonium salts, i.e. the formation of aryl radicals and related intermediates, is favoured. Under comparable conditions, homolytic rates are faster than the corresponding rates of heterolysis. Homolysis is strongly enhanced by addition of nucleophiles which form relatively stable radicals by electron transfer. The ability of additives to catalyze homolysis of arenediazonium salts can be explained using the concept of a nucleofugic

1 In the original proposal [32] we used the word nucleofugal. In keeping with a forthcoming proposal on nomenclature in physical organic chemistry by Commission III.2 (Physical Organic Chemistry) of IUPAC we now use the word nucleofugic.

homolytic leaving group.     

Dediazoniation of Arenediazonium Ions in Homogeneous Solution. Part IX. Spectral Evidence for a Homolytic Mechanism Involving Pyridine Complexes

The mechanism of dediazoniation of arenediazonium tetrafluoroborates in 2,2,2-trifluoroethanol (TFE) is strongly dependent on the concentration of added pyridine. The added base complexes with the diazonium ion and diverts it to a homolytic pathway. Complex formation is indicated by the disappearance of the \documentclass{article}\pagestyle{empty}\begin{document}$\raise1pt\hbox{---} \mathop {\rm N}\limits^ \oplus \equiv {\rm N}\raise1pt\hbox{---}$\end{document} stretching vibration and appearance of a new band at about 1640–1690 cm^?1 ascribed to the \documentclass{article}\pagestyle{empty}\begin{document}$\raise1pt\hbox{---} {\rm N}\raise1pt\hbox{=\kern-3.45pt=} {\rm N}\raise1pt\hbox{---} \mathop {\rm N}\limits^ \oplus {\rm C}_5 {\rm H}_5$\end{document} system. UV. and NMR. results support this conclusion. Chemically induced dynamic nuclear polarization (CIDNP) experiments clearly implicate a radical-pair as an important intermediate in the decomposition of these complexes.     

Dediazoniations of Arene Diazonium Ions in Homogeneous Solution. Part VI: Solvolyses in 2,2,2-trifluoroethanol/water mixtures and exclusion of a potential aryne mechanism

The rates and products of dediazoniation of benzenediazonium tetrafluoroborate in 2,2,2-trifluoroethanol (TFE)/water mixtures has been determined. The results are not consistent with a mechanism in which TFE and water enter the rate-determining part of the reaction as nucleophiles. The influence of the solvent composition on product ratios and rates is explained as a solvent effect, the formation of a (solvated) aryl cation being the rate-determining part of the reaction. The magnitude of the preexponential factor of the Arrhenius equation is consistent with this interpretation. Since the solvolysis of p -chlorobenzenediazonium tetrafluoroborate in TFE yields no detectable m-products, an aryne-like mechanism is excluded.     

Dediazoniations of Arene Diazonium Ions in Homogeneous Solution. Part III: Heterolytic arylations in 2,2,2-trifluoroethanol - an SN1-or an SN2-reaction?

Benzenediazonium tetrafluoroborate in 2,2,2-trifluoroethanol decomposes to give fluorobenzene and phenyl 2,2,2-trifluoroethyl ether. In the presence of benzene, toluene, trifluoromethyl-benzene or anisole, the respective biphenyl derivatives are formed in addition to fluorobenzene and the ether. The distribution of the isomeric substituted biphenyls is consistent with an electrophilic substitution. No homolytic products (diazo tars, benzene) are formed. The reaction kinetics clearly show that ether formation and aryl-dediazoniations are of second-order type, i.e. that trifluoroethanol and the benzene derivatives mentioned above are rate-determining factors. It is shown that these results exclude the S_N1-mechanism which is usually assumed for heterolytic dediazoniations; free aryl cations are therefore not involved in these reactions. An S_N2-like mechanism seems to be the most likely, but one involving an encounter complex containing the dissociated benzenediazonium ion is also consistent with the experimental data.     

Dediazoniation of Arenediazonium Ions. Part XIX. Effect of thiocyanate ion in the reactions of 2,4,6-trimethylbenzenediazonium tetrafluoroborate in 2,2,2-trifluoroethanol

The dediazoniation of 2,4,6-trimethylbenzenediazonium tetrafluoroborate ( 1 ) in 2,2,2-trifluoroethanol (TFE) was studied in the presence of potassium thiocyanate. The effect of added salt on the dediazoniation rate, the N_α-N_β rearrangement (Eqn. 2), the exchange of the ¹⁵N-labelled diazo group with molecular nitrogen (Eqn. 3), and the reaction products was determined. With 0.3_M KSCN a dediazoniation-rate increase of 16.5% was achieved, and the amounts of rearranged and exchanged product were reduced to 88% and 70%, respectively, of the values found in pure TFE. The dediazoniation products formed are ArF ( 3 ), ArOCH₂CF₃ ( 4 ), ArSCN ( 5 ), ArNCS ( 6 ) and traces of 5, 7-dimethylindazole ( 7 ). All the data are in agreement with, and support the previously proposed mechanism (Equ. 1) of heterolytic dediazoniation of arenediazonium salts.     

Thermodynamics of reactions involving zinc and cadmium ions in alkaline solutions

The standard Gibbs energies of assumed reactions in aqueous solutions involving zinc and cadmium ions were calculated.     

Dependence of the solubility of atmospheric oxygen in weakly alkaline aqueous solutions on surfactant concentration

The solubility of atmospheric oxygen in solutions of surfactants of different natures at 293 K and pH 8 is determined by gas chromatography. It is found that additives of nonionic surfactants decrease the oxygen content in the solution in the premicellar region and increase its solubility in the micellar region. It is shown that, for anionic surfactants, a decrease in the solubility of O₂ is observed over the entire concentration range.     

Dediazoniations of Arene Diazonium Ions in Homogenous Solution. Part IV: Change-over from a heterolytic to a homolytic mechanism in 2,2,2-trifluoroethanol pyridine mixtures

There are only two dediazoniation products of benzenediazonium tetrafluoroborate in 2,2,2-trifluoroethanol (TFE), namely phenyl 2,2,2-trifluoroethyl ether ( 1 ) and fluorobenzene ( 2 ). The reaction kinetics are strictly first-order with respect to the diazonium salt. The addition of increasing amounts of pyridine to the system results in a gradual decrease in the yields of 1 and 2 and an increase in the yields of the homolytically formed products, benzene ( 3 ), biphenyl ( 4 ), isomeric phenylpyridines ( 5 ) and diazo tar ( 6 ). The reaction kinetics show that the rate of dediazoniation of the benzene diazonium salt increases with increasing amounts of pyridine. The reaction with added pyridine is no longer first-order with respect to the diazonium ion. The product analyses and the kinetic data are consistent with the view that in pure TFE this diazonium salt decomposes completely by a heterolytic mechanism. The addition of pyridine brings about a competitive homolytic mechanism which becomes increasingly dominant as the concentration of pyridine increases.     

Dediazoniation of Arenediazonium Ions. Part XXII. Reactions of 2, 6-Dialkyl-Substituted Benzenediazonium Ions in Super Acids,Acetonitrile and Acetone

Reactions of 2,4,6-trimethylbenzenediazonium ( 1 ), 2,6-diethylbenzenediazonium ( 2 ) and 2,6-diisopropylbenzenediazonium ( 3 ) tetrafluoroborates were studied in magic acid, SbF₅/SO₂ClF, acetonitrile and acetone by ¹H-NMR and by analysis of the dediazoniation products. The N_α-N_β rearrangement of β-N¹⁵-labelled tetrafluoroborates 1–3 was followed by ¹⁵N-NMR of the corresponding arylazonaphthols, as well as by MS analysis of the anilines obtained by reduction of the azo compounds. Diazonium salts 2 and 3 were synthesized for the first time and the steric effect of substituents at C(2) and C(6) on the reactions under study is discussed. All the results obtained can be rationalized by heterolytic dediazoniation of diazonium salts 1 – 3 and product formation from the corresponding aryl cations.     

Physico‐chemical study of sucrose and calcium ions interactions in alkaline aqueous solutions

Interactions between sucrose and calcium hydoxide have been the subject of our work. They were studied in dilute solutions (sucrose concentration <40% w/w, with a molar stoichiometric ratio Ca(OH)₂/sucrose <2) by several physico‐chemical methods, titration of free and bound calcium ions, ¹³C NMR spectroscopy, static light scattering and low shear viscosimetry. From these analyses we propose a polymer‐like complex formation with calcium ions bound to at least two sucrose molecules. Moreover, this polymer‐like aggregate can be described as a linear short chain at low Ca(OH)₂ and sucrose concentrations and as dense fractal particles at higher concentrations.     

Spectroscopic study of Mg(II) ion influence on the autoxidation of gallic acid in weakly alkaline aqueous solutions

Gallic acid autoxidation in weakly alkaline aqueous solutions was studied by UV-Vis spectrophotometry and ESR spectroscopy under various conditions. Lowering the pH value from 10 to 8.5 probably changes the mechanism of the autoxidation reaction as evidenced by the different time variations of UV-Vis spectra of solutions. The presence of Mg(II) ions greatly influences the autoxidation reaction at pH 8.5. Although the UV-Vis spectral changes with time follow the similar pattern during the gallic acid autoxidation at pH 10 and at pH 8.5 in the presence of Mg(II) ions, some small differences indicate that Mg(II) ions not only affect the electron density of absorbing species but also influence the overall mechanism of the autoxidation reaction. ESR spectra of free radials formed during the initial stage of gallic acid autoxidation at pH 8.5 in the presence of Mg(II) ions were recorded. Computer simulation of ESR spectra allows partial characterization of these free radicals.     

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes: Part VI. Dimethylglyoxime electroreduction from weakly acidic aqueous solutions in the presence of I− and SCN−

An investigation of the _d-effect in the absence of ionic specific adsorption shows that the overall charge of the reactants in dimethylglyoxime (DMG) electroreduction on mercury from aqueous solutions of pH?2 equals+1, in agreement with a protonation reaction either preceding (DMG + H⁺DMGH^++e → products) or else simultaneous with (DMG + H⁺ + e → products) the charge transfer. In the presence of the specifically adsorbed supporting anions I^? and SCN^? the logarithm Φ of the rate constant for DMG electroreduction corrected for diffuse-layer effects decreases linearly with an increase in the absolute value |q_i| of the charge density at the inner Helmholtz plane due to the adsorbed anions. This behaviour is explained semiquantitatively by assuming that protonation takes place simultaneously with charge transfer, with the DMG molecule in the adsorbed state and the H₃O⁺ ion in the non-adsorbed state. In this way the H₃O⁺ ion, due to the screening effect of the surrounding diffuse-layer ions, escapes the accelerating attractive effect exerted by the adsorbed I^? or SCN^? ions; on the other hand these latter anions exert a retarding repulsive effect upon the electron transfer from the metal to the adsorbed DMG molecule.     

Particle-macromolecule interactions in aqueous solutions Part 1: Solution and flow behaviour of polyvinylalcohol in an aqueous medium

Summary Shear and concentration dependent flow studies on solutions of a narrow molecular weight fraction of 90.3 % hydrolysed polyvinylalcohol are reported, together with precise density measurements on these same solutions. From the derived flowing volume and partial molar volume values it is concluded that two association processes occur; the first appears similar to micellization of surfactants, the second is association of micellar spheres.

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Zusammenfassung Es wurde die Scher- und Konzentrationsabhängigkeit des Fließverhaltens von Polyvinylalkohollösungen (90,3 % hydrolysiert) mit niedriger Polydispersität untersucht. An den gleichen Lösungen wurden auch Präzisionsdichtemessungen ausgeführt. Aus dem berechneten Fließvolumen und dem partiellen Molvolumen kann gefolgert werden, daß zwei Assoziationsprozesse vorliegen. Der erste davon gleicht der Mizellbildung von Tensiden, der zweite entspricht der Assoziation kugelförmiger Mizellen.