55. Structure and reactivity of the oxyanions of transition metals. Part VIII. Acidities and spectra of protonated oxyanions

Bailey, Neil A.; Carrington, Alan; Lott, K. A. K.; Symons, M. C. R.

doi:10.1039/jr9600000290

Cited by 122 publications

(61 citation statements)

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“…The ultraviolet-visible spectrum of the distillate was similar to that reported by Syinons et al (4) for HMii01. When water was added t o this anhydrous trifluoroacetic acid solution the familiar purple color of permanganate was obtained.…”

Section: Methodssupporting

confidence: 73%

The Chromic Acid Oxidation of 2-Propanol in Trifluoroacetic Acid

Lee¹,

Johnson²

1965

Can. J. Chem.

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The mechanism of the chromic acid oxidation of 2-propanol in trifluoroacetic acid solutions has been studied. The general features of this reaction strongly resemble those observed in aqueous solutions of other strong acids. A "normal" primary kinetic isotope effect is observed, similar thermodynamic parameters are obtained, and the rate behaves in similar way towards changes in acidity.An extensive study of the effect of added salts on this reaction has substantiated that the identity of added anions has a profound effect on the rate of reaction and that any complete reaction mechanism must take this into consideration.I t has been established that chromic acid is a very effective reagent for the oxidation of alcohols and that under similar conditions of acidity and temperature the rate of oxidation is inuch faster in acetic acid solutions than in aqueous solutions (I). T h e kinetics and mechanism of this reaction have been investigated quite thoroughly and it appears that the enhanced rate in acetic acid solutions is due in part to incorporation of an acetate ligand into the chromic acid ester, which acts as an intermediate in this reaction (1,2). I t seemed probable to us that the details of the mechanism of this reaction could be further exposed by undertaking a kinetic study of the chromic acid oxidation of 2-propanol in trifluoroacetic acid solutions. The following paper reports the results of this investigation. EXPERIMENTAL Kinetic nlethodsAll kinetic measurements were made spectrophotometrically using a Unicam model S P 500 spectrophotometer with a thermostated cell compartment. Solutions were made up by adding a weighed amount of anhydrous trifluoroacetic acid (Halocarbon Products) to a dilute aqueous solution of IC2Cr2O7 (Fisher Reagent). This solution was then brought to 25.0 f 0.2" in a thermostated bath and a n excesspf 2-propano1 was added volumetrically either from a pipet or a microsyringe fitted with a Chaney adaptor. The optical density of these solutions a t 3 500 A was then determined a t intervals and a plot of log [CrVI] against time was prepared. A typical rate plot is presented in Fig. 1.

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Section: Methodssupporting

confidence: 73%

The Chromic Acid Oxidation of 2-Propanol in Trifluoroacetic Acid

Lee¹,

Johnson²

1965

Can. J. Chem.

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“…These values are very near those found in perchloric and sulfuric acids (7,8,9). It has been suggested (10) that this spectrum, which is similar to the spectra of ions of the type ROCr03-(25) and Cr03F- (26), indicates that the species in sulfuric acid solution is 03Mn-0S03H.…”

Section: Visible Spectroscopysupporting

confidence: 59%

“…The conversion of permanganate ion to the green species (referred to herein as 'ionization') was measured by following the change in optical density at 546 nm (A, , , for permanganate ion) as the acidity was changed, and values of the logarithm of the ratio [green species]/[MnO4-] calculated (Table 4). (It should be noted that literature values for the pK, of permanganic acid (8,11) are based on observations of this equilibrium, and are therefore in error. )…”

Section: Visible Spectroscopymentioning

confidence: 99%

“…Solutions of permanganate containing more than 90% trifluoroacetic acid are also green, and we hoped the present study would aid in identifying the species responsible. Three species have been proposed to account for the green colour; permanganyl cation (Mn03+) (a proposal based on cryoscopic and conductiolnetric measurements in sulfuric acid) (9), its complex with an acid anion A-(AMn03) (based on conductiometric measurements in sulfuric acid) (lo), and permanganic acid (HMn04) (8,11). The latter possibility now appears unlikely since solid permanganic acid has recently been prepared and found to be a deep violet colour (12).…”

mentioning

confidence: 99%

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Aqueous trifluoroacetic acid as a medium for organic reactions. I. Acidity functions and the identity of the manganese(VII) species found in powerfully acidic media

Spitzer¹,

Toone²,

Stewart³

1976

Can. J. Chem.

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. Can. J. Chem. 54,440 (1976). The HR acidity function has been determined for the trifluoroacetic acid -water system, and extended beyond 100% acid by the addition of trifluoroacetic anhydride, to 99.5 wt.7, anhydride. The existing Ho acidity function is corrected and extended to 1007, trifluoroacetic acid, confirming Eaborn's observation that Ho attains a minimum value at approximately 977, acid.Comparison of the acidity functions shows that although trifluoroacetic acid is only weakly protonating, it is a strongly protonating-dehydrating medium. Changes in the uv-visible spectrum of potassium permanganate produced by changing acid concentration are used to calculate the degree of further ionization of the permanganate ion; the correlation of this with the HR acidity function further suggests the presence of the permanganyl cation, MnO3+, or its trifluoroacetate complex or ester, MnO3.02CCF3, in these solutions. A cryoscopic investigation into the mode of ionization of permanganate in trifluoroacetic acid reveals that extensive ion-pair formation occurs in this solvent. Although this effect prevents an unequivocal determination of the number of solute particles being made the results are consistent with the presence of a permanganyl species in solution.

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“…The inhibition effect occurs due to the competition existing between the substrate and Mn II to react with Cr IV . In aqueous acid media, chromium(VI) was suggested [25,26] to exist mainly as acid chromate, H 2 CrO 4 , as represented by the first step in Figure 5. The enhancement of the oxidation rate upon increasing hydrogen ion concentration confirmed a protolytic process of the chromate ion, i.e.…”

Section: Discussionmentioning

confidence: 99%

Chromic Acid Oxidation of Methylaminopyrazole Formamidine in Sulfuric Acid Medium: A Kinetic and Mechanistic Approach

Fawzy¹

2016

AJPC

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Abstract:The kinetics of chromic acid oxidation of one of aminopyrazole formamidine derivatives, namely N, N-dimethyl-N'-(5-methyl-1H-pyrazol-3-yl) formamidine (MAPF)in sulfuric acid solutions has been investigated at constant ionic strength and temperature. The progress of the reaction was followed spectrophotometrically. The reaction showed a first order dependence on [chromic acid] and fractional-first order dependences with respect to [MAPF] . The final oxidation products of MAPF are identified by spectral and elemental analysis as methylaminopyrazole, dimethylamine and carbon dioxide. A spectral evidence for the formation of chromium(III) product was obtained. A reaction mechanism adequately describing the observed kinetic behavior is proposed, and the reaction constants involved in the different steps of the mechanism have been evaluated. The activation parameters with respect to the rate-determining step of the reaction, along with thermodynamic quantities of the equilibrium constants, are presented and discussed.

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55. Structure and reactivity of the oxyanions of transition metals. Part VIII. Acidities and spectra of protonated oxyanions

Cited by 122 publications

References 0 publications

The Chromic Acid Oxidation of 2-Propanol in Trifluoroacetic Acid

The Chromic Acid Oxidation of 2-Propanol in Trifluoroacetic Acid

Aqueous trifluoroacetic acid as a medium for organic reactions. I. Acidity functions and the identity of the manganese(VII) species found in powerfully acidic media

Chromic Acid Oxidation of Methylaminopyrazole Formamidine in Sulfuric Acid Medium: A Kinetic and Mechanistic Approach

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