The Upper Limit on Rates of Proton Transfer

The Bronsted plot for hydrolysis of diphenyldiazomethane by carboxylic acids is smooth but curved, approaching an asymptotic limit below 10 mo1-1 dm3 s-1• Regardless of the details of the theory used, such a limit requires that the transition state is approached via a state (or virtual state) substantially (- 70 kJ mol-1) above the starting state in free energy, but still having an intact bond from H+ to its original conjugate base. Such a state has been called a reaction complex,6 and is thought to be analogous to the intimate ion pair of solvolysis reactions

The Methanol-Trimethoxyborane Azeotrope as a Solvent for Acid-Catalyzed Reactions. Methyl Esterification

Carboxylic acids UIIldergo a rapid and complete acid-catalyzed methyl esterification in the methanol-trimethoxyborane azeotrope. The reaction is first order in cariboxylic acid and first order in strong acid catalyst. Rate constants are very similar to those in methanol alone. These observations suggest that the mechanism is the same as that in solvent, methanol-AAc 2. The reaction can be made the basis of a convenient preparation of methyl esters. It has been used to prepare methyl benzoate, methyl lactate, and the methyl ester of phenylalanine. Acid-catalyzed acetal and ketal formation in the azeotrope is fast but incomplete. Methyl etherification of tertiary alcohols seems too limited, structurally, to be generally useful. No acid-catalyzed addition of methanol to acetonitrile or acrylonitrile could be observed

The Methanol-Trimethoxyborane Azeotrope as a Solvent

The methanol-trimethoxyborane azeotrope is an ea.sily prepa. red solvent of strong H-bond donor and weak H-bond acceptor properties. It has a low viscosity and low boiling point, and is completely transparent in the visible and uv at wave lengths above 220 nm. It has a dieLectric constant of 9 and ion pair formation constants between 103 and 104 for simple electrolytes. The glass electrode pH-meter can be used in this solvent, with the ordinary calomel reference electrode. Indicator acidity measurements are somewhat complicated by the forma•tion of H-bonded ion pairs. The solvent is useful for measuning the relative acidity of strong acids; CH3S020H and CF3S020H are readily differentiated. It should be useful for other physico-chemical measurements and also in preparative chemistry

Direct Chemical Reduction of Triphenylcarbinol to Triphenylmethane

BH3cN- has been found to be a moderately efficient trapping agent for triphenylmethyl cations, roughly comparable to chloride ion. Since BH3CN- is moderately resistant to acid hydrolysis, it can trap triphenylmethyl cations generated by treatment of triphenylcarbinol with HCl in 25% water - 750/o tetrahydrofuran. A quantitative yield of triphenylmetharie, based on unrecovered tripheny~carbinol, can be obtained, but the process is inefficient in its use of BH3CN- because of the competing hydrolysis

The Effect of Ordered Water on a Short, Strong (Speakman-Hadži) Hydrogen Bond

We have determined the structures of the sodium, tetrabutylam- monium (TBA) and bis(triphenylphosphoranylidene)ammonium (PNP) salts of the bis(4-nitrophenoxide) anion by X-ray crystallography. The sodium salt is a dihydrate, with the water oxygens coordinated to the sodium cations, and one hydogen from each water hydrogen bonded to one of the bridging oxygens of the anion. The TBA and PNP salts are anhydrous. Nevertheless the oxygen- oxygen distance is shortest in the sodium salt; 246.5 pm in the sodium salt, 247.5 pm in the TBA salt, and 249 pm in the PNP salt; suggesting that the hydrogen bond is not weakened by the water, and may be strongest in the hydrated salt. (All three compounds show Hadži type ii IR spectra, and are called Speakman-Hadži compounds in this paper.) The 2H chemical shifts of the bridging hydrogen in the three solids are 16.8 ppm for the sodium salt, 16.8 ppm for the TBA salt, and 16.5 ppm for the PNP salt. Again there is no evidence that the water weakens the hydrogen bond. These results can be understood by noting that the additional hydrogen bonds to the bridging oxygens decrease their proton affinity, but the mutual repulsion of the oxygens is also decreased

Acidity at a Liquid-Liquid Interface

By using porous polypropylene as a support, we have measured the contact angle of buffered aqueous droplets on surfaces which were over half bis(2-ethylhexyl)hydrogen phosphate, HA. From the variation of the contact angle with the pH of the aqueous solution the degree of dissociation, a, of HA has been evaluated as a function of pH. By defining the standard states of the interfacial HA and its conjugate base at the 50% ionized aqueous- organic interface, a pKR is obtained for the interfacial acid, equal to the pH of the aqueous solution which is in equilibrium with the 50% ionized interfacial acid. Following Katchalsky and Spitnik14 the ratio of the activity coefficient of the conjugate base yA- to the activity coefficient of the acid, yHA is equated to [a/(l - afn l> ] for a values near 0.5. This permits the evaluation of the empirical parameter, n, and leads to a relation between pH and a. pH = pKa-n log[(1 -a)/a] This relation is found to hold from a > 0.1 to a < 0.9, although it cannot be valid for either a = 0.0 or a = 1.0

Speciation of A 1-Alkyl-4-Cyanopyridinium Iodide Using the Charge Transfer Band

The intensity of the charge transfer band of 4-cyano-l-(3,7-dimethyloc- tyDpyridinium iodide (Py+I~) has been used to estimate the contact ion pair (CIP) concentration in a number of solvents. In several nonhydroxylic solvents with dielectric constant between 4 and 8 the molar integrated intensity of the low frequency charge transfer band is very similar, and these values have been averaged to estimate the intrinsic molar intensity of the CIP. A nonlinear concentration dependence of this intensity in 2-propanol shows the presence of a substantial concentration of free ions in that solvent, and an ion association constant, Kp = 3.9 x 103, was found. Tributylphos- phate gives similar evidence of dissociation, but only at lower concentrations. A value of 3.7 x 106 was estimated for Kp. In the other solvents studied, dissociation is hard to distinguish from other effects which reduce the molar intensity at low concentrations. Both tributyl phosphate and 2-propanol also give molar intensities below the average of less polar solvents at high Py+I_ concentration. This is regarded as evidence for the presence of solvent separated ion-pairs (SSIP). Solvent separated ion-pairs are not abundant, however, in the solvents studied. The CIP:SSIP concentration ratio seems to be 4-6 in tributylphosphate and 2-propanol, and is still higher in the other solvents. With increasing concentration the charge transfer band shifts to higher frequency. The magnitude of the shift depends on the solvent, and is most notable in chlorobenzene. A theory is developed which attributes this shift to the increase in the dielectric constant of the medium brought about by the ion-pairs. The effect could be attributed to higher aggregation, but the degree of aggregation would have to increase continuously with concentration, even at concentrations —10"4 M. The idea of dielectric modification can also be folded together with aggregation, with the former predominant at low concentration and the latter at higher concentration (~10-3 M)