Carbonium Ions. VIII. Diarylolefin-Diarylalkyl Cation Equilibria 1

This paper is dedicated to Professor Arthur N . Bourns TADASHI OKUYAMA, MASAYOSHI TOYODA, and TAKAYUKI FUENO. Can. J. Chem. 64, 11 16 (1986).Hydrolyses of Zethylidene-( l b ) , 2-isopropylidene-( l c ) , and 2-benzylidene-l,3-dithiolane ( I d ) were kinetically investigated in aqueous solution. All the individual rate constants involved in this three-step reaction were evaluated. Initial carbon protonation is only partially reversible (k2/k-1 = 1.33,0.68, and 1.02 for l b , l c , and Id, respectively) at higher pH, while the protonation becomes completely reversible below pH 2 where the thud step is rate determining. Complete H-D isotopeexchange at the P-carbon of l b and Id was observed in deuterium media before appreciable hydrolysis took place. It was demonstrated that reversion from the tetrahedral intermediate 3 to 1 occurs extensively during the reaction in the latter acidity range. Relative stabilities and reactivities of the olefinic substrates 1 are discussed. que, au cours de la rCaction, la transformation de l'intenntdiaire tCtraidrique 3 en 1 ne se produit que dans le dernier intervalle de pH. On discute des stabilitCs et des rtactivitts relatives des substrats olCfiniques 1.[Traduit par la revue]

Section: Isotope Exchangesupporting

confidence: 56%

Acid-catalyzed hydrolysis of 2-methylene-1,3-dithiolane. 2. Remarkable effects of β substitution on reversibility of the carbon protonation

Okuyama¹,

Toyoda²,

Fueno³

1986

“…-log a, can be defined. This is comparable to the other acidity functions and has been used, for instance, in alkene protonation studies (194 reduced by replacing the water by a dipolar aprotic solvent such as DMSO or sulfolane, at fixed hydroxide concentrations. This affects only the last two terms in eq.…”

Section: Harnnlett Acidity F I L~z C T I O~~~mentioning

confidence: 96%

Acidity functions: an update

Cox¹,

Yates²

1983

151

. 61, 2225 (1983).The subject of acidity functions is reviewed, with emphasis on rnodcrn developrncnts. H o and othcr scales determined in acid media, using the Hammett cancellation assumption (zero'th order approximation) are discussed bricfly. The extension to strongly basic systems is also documcnted. Literature references to acidity function scalcs in aqueous acid, aqueous acidlorganic solvcnt, and aqueous base media are tabulated, togcther with those in aqueous baselorganic solvent mixtures at both variable and fixed base concentration. Over 400 acidity functions are referenced, including several electrochemical scales. Various approaches to developing universal functions from the data are discussed. Proton activity scales, based on the Yates-McClelland method of pKBH+ determination, and on electrochemical HoF scales, are prominent among these. Carpentier's method is mentioned, and the first-order approximation, assumed linearity among log activity coefficient ratios, is used to introduce the Bunnett-Olsen and excess acidity (X-function) methods. A brief critical discussion of which method to use is provided. Future developments anticipate the second-order approximation, with a quadratic dependence of log ionization ratios upon X-functions. The review cites 337 individual references. [Traduit par le journal]

“…describe the protonation of diarylalkenes in sulfuric acid (20). The HR, acidity function for aqueous trifluoroacetic acid was calculated by subtracting the logarithm of the activity of water (21) (converted to molarity units) from the measured HR values.…”

Section: Acidity Functions In Trifluoroacelic Acid (A) Hr Acidity Funmentioning

confidence: 99%

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. 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.