Benzamide hydrolysis in strong acids — The last word

Recently it has become apparent that the mechanism of amide hydrolysis in relatively dilute strong acid media is the same as the one observed for ester and benzimidate hydrolysis, two water molecules reacting with the O-protonated amide in the rate-determining step. This is not the whole story, however, at least for benzamide, N-methylbenzamide, and N,N-dimethylbenzamide, since the observed rate constants for these substrates deviate upwards from the observed excess acidity correlation lines at acidities higher than about 60% H2SO4, meaning that another, faster, reaction with a different mechanism is taking over at higher acidities. It has never been clear what this latter mechanism was until the work reported in this paper. An exhaustive excess acidity analysis of all the available measured reaction rate constants for the three substrates in three different acidic media, aqueous H2SO4, aqueous HClO4, and aqueous HCl, shows that this second mechanism involves a second rate-determining proton transfer to the O-protonated benzamide, followed by (or possibly concerted with) irreversible loss of +NH4 to give an acylium ion. Subsequent reaction of this with water (or bisulfate, etc.) eventually gives the observed carboxylic acid product. This latter reaction mechanism has never been previously considered for amide hydrolysis, but it may not be uncommon; at least one other reaction with a similar mechanism is known, and another possible case is suggested.Key words: amides, benzamides, hydrolysis, excess acidity, mechanism, acid media.

Structural and Textural Properties of Perchlorated and Persulphated Mixed (Hydr)Oxides of Zirconium and Titanium

Perchlorated and persulphated mixed hydroxides of zirconium and titanium were prepared by coprecipitation and impregnation in aqueous HClO4 or (NH4)2S2O8 solutions of 0.05, 0.10, 0.20 and 0.40 M concentrations. An alternate sequence of impregnation followed by calcination or vice versa was conducted and the samples obtained studied using XRD, FT-IR, pyridine titration and low-temperature (–196°C) nitrogen adsorption methods. XRD indicated that the presence of titanium stabilized the tetragonal modification of zirconia and almost completely prevented the usual tetragonal → monoclinic transformation upon calcination at 650°C. Both S2O82– and ClO4− anions at their lowest concentration level (0.05 M) partially retarded the crystallization which occurred upon calcination at 650°C in their absence. However, the two anions showed different effects. Whereas the perchlorate anion prevented the formation of a crystalline titania phase (anatase) to a greater extent than that of crystalline zirconia, the persulphate anion showed the opposite effect. Complete inhibition was observed with both anions at a concentration of 0.4 M. This effect was attributed to adsorption of the anions on the hydroxy species of zirconium and titanium formed initially, as demonstrated by IR spectroscopy which showed that the anions were of lower symmetry, viz. C2v, due to their bonding to the hydroxy species. Calcination at 650°C caused the material formed initially to lose virtually all its initial high surface area because of crystallization. The prevention of crystallization by added anions was reflected in the retention of a relatively high surface area even after calcination at 650°C. The recorded difference in the interactions of the anions with the hydroxy species formed initially was also reflected in the texture of the anion-modified solids. The protecting influence of the ClO4− anion increased with its increasing concentration in the system, whereas the corresponding effect with the S2O82– anion increased up to 0.10 M concentration and then decreased at higher concentrations.

A spectroscopic study of the hydrolysis, colloid formation and solubility of Np(IV)

The hydrolysis, colloid formation and solubility of Np(IV) are investigated in aqueous HClO

Synthesis of dimethylplatinum(IV) compounds, [{PtMe2X2}n], [{PtMe2XY}n], and, in solution, fac-[PtMe2X(H2O)3]+, where X and Y are anionic ligands

Oxidative addition of X2 (X = Cl, Br, I) to cis-[PtMe2L2] (L = pyridine, py, or L2 = N,N,N′,N′-tetramethylethylenediamine (tmen) gave [PtMe2X2L2]. For X = Br, I, treatment with aqueous HClO4 gave insoluble [{PtMe2X2}n], but for X = Cl, [PtMe2Cl2(H2O)2] remained in solution, with [{PtMe2Cl2}n] depositing only from concentrated solution. [PtMe2L2] (L = py, 1/2(tmen)) with water gave [PtMe2(OH)2L2], which, on treatment with HClO4 gave cis-[PtMe2(H2O)4](ClO4)2 in solution. Water also reacted with [PtMe2(nbd)] (nbd = norbornadiene) to give [{PtMe2](OH)2}n]•mH2O. Alcohols ROH (R = Me, Et) with cis-[PtMe2py2] gave [PtMe2(OR)(OH)py2], which reacted with aqueous HClO4 solution to give fac-[PtMe2(OR)(H2O)3]ClO4 in solution. Addition of chloride to this solution caused precipitation of [{PtMe2(OR)Cl}n]. Reaction of [{PtMe2XY}n] with AgNO3 in water gave fac-[PtMe2X(H2O)3](NO3) in solution (X = Y = Cl, Br, I or Y = Cl, X = OR); for X = I added acid was necessary to prevent precipitation of [{PtMe2I(OH)}n]. Reaction of a solution of fac-[PtMe2Br(H2O)3](NO3) with AgNO2 gave fac-[PtMe2(NO2)(H2O)3](NO3) in solution, but an analogous reaction with AgSCN gave a complex in solution formulated as fac-[PtMe2(SCN)(H2O)3](NO3) only in low yield. Key words: platinum, methyl, pyridine, aqua, alkoxide, oxidative addition, NMR.

Thermodynamic Study of Ce4+ /Ce3+ Redox Reaction in Aqueous Solutions at Elevated Temperatures: 1. Reduction Potential and Hydrolysis Equilibria of Ce4+ in HCIO4 Solutions

The redox potential (E) of the couple Ce4+/Ce3+ has been determined up to 368 K by means of cyclic voltammetric measurement in aqueous HClO4 solutions with cHClO4 decreasing from 7.45 to 0.023 mol kg-1 . A constant potential of (1.741 V)298 K, resp. (1.836 V)368K, indicating the existence of pure unhydrolysed Ce4+ was obtained at cHClO4 ≥ 6.05 m. At lower HClO4 concentration, the potential as a function of the HClO4 molality, as well as of the pH shows 4 further distinct steps. At 298 K, for instance, the potential became nearly constant at pH values of 0.103, 0.735,1.115, after which it drastically decreased, respectively at 1.679, just before the precipitation of Ce(OH)4 occurred. The curves indicate obviously the stepwise formation of the Ce(IV) mono-, di-, tri- and tetrahydroxo complexes. The slope of the curves E vs. pH increased gradually with increasing temperature. ΔS and ΔH of the redox reaction were determined as functions of T at the different HClO4 concentrations. ΔSis positive at cHClO4 > 1.85 m and turns to be negative at lower concentrations. ΔHis negative at all HClO4 concentrations studied. The cumulative formation constants ßi, of the Ce(IV) hydroxo complexes and the corresponding hydrolysis constants (Kh)i were calculated. An unusual decrease of ßi with increasing temperature has been discussed

The pressure dependence of rates of homolytic fission of metal–ligand bonds in aqueous solution

The volume of activation for the exclusively homolytic decomposition of protonated 4-pyridylmethylchromium(III) ion in aqueous HClO4 at 63.4 °C is +19 cm3 mol−1, with negligible dependence on pressure up to 350 MPa at least. The origins of the strongly positive volumes of activation that characterize homolysis of complex cations in aqueous solution are examined.

Extrapolation from concentrated to dilute aqueous acids

The location and the manner of union of the H0 and HR acidity functions with −log [Formula: see text] have been determined for aqueous HClO4, HCl, and H2SO4 (H0 only) through particularly careful and extensive indicator measurements in dilute and moderately concentrated solutions of these acids. These data were also used to evaluate and compare a number of different ways of extrapolating measurements made in concentrated acids down to dilute solution; over the limited range of acidity investigated, the traditional acidity function method, as commonly applied in the absence of an appropriate acidity function (i.e. in the form of mH0), was found to be the least satisfactory of these extrapolative methods, and the Cox–Yates technique was found to be the best.

Protonated cyclopropanes. X. Studies on the deamination of n-butylamine-1-14C and the decomposition of 3-(n-butyl-1-14C)-1-phenyltriazene

The HNO2 deamination of n-butylamine-1-14C (1-NH2-1-14C) in aqueous HClO4 gave a n-butyl-14C alcohol (1-OH-14C) with about 0.7% scrambling of the label from C-1 to C-2, and a sec-butyl-14C alcohol (2-OH-14C) which showed extensive scrambling between C-1 and C-4, as well as small amounts of rearrangement of the label into C-2 and C-3. Analysis of the data suggested that about 2.5% of the 2-OH-14C product could be derived from equilibrating protonated methylcyclopropane intermediates. The HClO4 catalyzed decomposition of 3-(n-butyl-1-14C)-1-phenyltriazene (1-NNNHPh-1-14C) gave the unrearranged 1-OH-1-14C and a 2-OH-14C which showed only about 17–18% scrambling from C-1 toC-4. It is suggested that the different results observed in the deamination and the triazene decomposition may be due to a more ready initial formation of an ion pair in the triazene decomposition.

Mechanistic Information from the Effect of Pressure on the Kinetics of Hydrolysis of Iodopentaaquochromium(III) Ion

The rate of hydrolysis of iodopentaaquochromium(III) ion has been measured as a function of pressure (0.1 to 250 MPa) and hydrogen ion concentration (0.1 to 1.0 mol kg−1) at 298.2 K and ionic strength 1.0 mol kg−1 (aqueous HClO4–LiClO4). The volumes of activation for the acid independent and inversely acid dependent hydrolysis pathways are −5.4 ± 0.5 and −1.6 ± 0.3 cm3 mol−1 respectively, and are not detectably pressure-dependent. Consideration of these values, together with the molar volume change of −3.3 ± 0.3 cm3 mol−1 determined dilatometrically for the completed hydrolysis reaction, indicates that the mechanisms of the two pathways are associative interchange (Ia) and dissociative conjugate base (Dcb) respectively.