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

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 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

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

Effect of divalent metal ion binding on the Raman spectrum of ATP in aqueous solution

The apparent dissociation constants for the secondary H+ of HATP-3, CaHATP-1, MgHATP-1 and ZnHATP-1 were determined by studying the pH dependent frequency of a particular line in the Raman spectrum of ATP. We obtained the following values for the pKa: 6.50 for ATP alone and 5.48, 4.43, 4.21 for the Zn, Mg and Ca complexes respectively. The Raman technique yields the above results more directly than methods previously employed. The order of binding strengths for the three cations to the triphosphate moiety inferred from the present results is reversed from earlier published data.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/32821/1/0000195.pd