Benzene, acid-base equilibria

In this solvent the reaction is catalyzed by small amounts of trimethyl-amine and especially pyridine (cf. 9). The same effect occurs in the reaction of iV -methylaniline with 2-iV -methylanilino-4,6-dichloro-s-triazine. In benzene solution, the amine hydrochloride is so insoluble that the reaction could be followed by recovery. of the salt. However, this precluded study mider Bitter and Zollinger s conditions of catalysis by strong mineral acids in the sense of Banks (acid-base pre-equilibrium in solution). Instead, a new catalytic effect was revealed when the influence of organic acids was tested. This was assumed to depend on the bifunctional character of these catalysts, which act as both a proton donor and an acceptor in the transition state. In striking agreement with this conclusion, a-pyridone is very reactive and o-nitrophenol is not. Furthermore, since neither y-pyridone nor -nitrophenol are active, the structure of the catalyst must meet the conformational requirements for a cyclic transition state. Probably a concerted process involving structure 10 in the rate-determining step... 

It is thus clear that there are many complications in interpreting acid-base equilibria in aprotic solvents. Nevertheless, with a suitable choice of system there are many cases in which a simple equilibrium of the type B-hHA BH A can be established and studied accurately. For example, Figure 3 shows the results of spectrophotometric measurements of the equilibrium between triethylamine and dinitrophenols in benzene solution. The good agreement with straight lines of unit slope establishes the nature of the equilibrium, and values of the constant Kbha = [BH A ]/[B] [HA] are given by the intercepts of these lines. The same... 

Fig. 3. Concentrations of ion pairs in equilibrium with triethylamine and six isomeric dinitrophenols in benzene solution. A, B, and S represent acid, base, and salt respectively. [Reproduced by permission from M. M. Davis, J. Am, Chem. Soc., 84, 3623 (1962).]...

Tetramethyl-D-glucose reaches equilibrium in benzene rapidly through the concerted action of cresol and pyridine as acid-base catalysts (Table 4.3). Bifunctional reagents, like 2-pyridone and benzoic acid, are especially efficient acid-base catalysts in both polar and nonpolar solvents ... 

The reaction is reversible and therefore the products should be removed from the reaction zone to improve conversion. The process was catalyzed by a commercially available poly(styrene-divinyl benzene) support, which played the dual role of catalyst and selective sorbent. The affinity of this resin was the highest for water, followed by ethanol, acetic acid, and finally ethyl acetate. The mathematical analysis was based on an equilibrium dispersive model where mass transfer resistances were neglected. Although many experiments were performed at different fed compositions, we will focus here on the one exhibiting the most complex behavior see Fig. 5. 

Many papers concerning salt effect on vapor-liquid equilibrium have been published. The systems formed by alcohol-water mixtures saturated with various salts have been the most widely studied, with those based on the ethyl alcohol-water binary being of special interest (1-6,8,10,11). However, other alcohol mixtures have also been studied methanol (10,16,17,20,21,22), 1-propanol (10,12,23,24), 2-propanol (12,23,25,26), butanol (27), phenol (28), and ethylene glycol (29,30). Other binary solvents studied have included acetic acid-water (22), propionic acid-water (31), nitric acid-water (32), acetone-methanol (33), ethanol-benzene (27), pyridine-water (25), and dioxane-water (26). 

Values up to pKa 33 (diphenylmethane) are those reported for equilibrium methods, and were measured either directly using the H acidity function or by comparing acidity with 9-phenyl-fluorene. Above pA 33, we assume a = 0.3 for toluene, cumene, and tripticene, and base other values on pKa = 43 for benzene and an assumed a of 0.9. The scale is based on the Langford and Burwell value of 18.5 for pKa of 9-phenylfluorene. 

In practice, extrapolations of p fR in water have usually used the older acidity function based method, for example, for trityl,61,62 benzhydryl,63 or cyclopropenyl (6) cations.66,67 These older data include studies of protonation of aromatic molecules, such as pKSi = —1.70 for the azulenium ion 3,59 and Kresge s extensive measurements of the protonation of hydroxy- and methoxy-substituted benzenes.68 Some of these data have been replotted as p fR or pKa against XQ with only minor changes in values.25,52 However, for more unstable carbocations such as 2,4,6-trimethylbenzyl, there is a long extrapolation from concentrated acid solutions to water and the discrepancy is greater use of an acidity function in this case gives pA 2° = —17.5,61 compared with —16.3 (and m = 1.8) based on X0. Indeed because of limitations to the acidity of concentrated solutions of perchloric or sulfuric acid pICs of more weakly nucleophilic carbocations are not accessible from equilibrium measurements in these media. 

The ammonolysis of acyl chlorides is commonly employed whenever the preparation of an amide is desired for the identification of a carboxylic acid. The acyl chloride is prepared and treated with aqueous ammonia, or, if the amide is very soluble in water, with ammonia in benzene. In the latter case the ammonium chloride is filtered off, and the amide obtained by evaporation of the benzene with powdered ammonium carbonate. The preparation of amides from carboxylic acids is based upon the formation of an equilibrium mixture when the ammonium salts of the acid are heated ... 

When the hydrocarbon base was hexamethylbenzene (I), which is estimated as 1010 times more basic than benzene (27), the strength of acids such as trifluoromethanesulfonic acid (triflic acid) could be examined (28, 29). The position of the protonation equilibrium (I II, equation 2) was also determined by 13C-NMR spectroscopy. 

Benzodiazepines are very much weaker bases than 2,3-dihydro-diazepines. Furthermore the monocations require much less strong acid than do dihydrodiazepinium cations to convert them into the corresponding dications. The pK value for the base (1) to monocation (4) equilibria has been determined as 4.5 (2,4-dimethyl, potentiometric)443 and 5.76 (2,4-dimethyl, spectroscopic).44 The presence of 2(4)-phenyl groups lowers the basicity still further.44 The pK value for the monocation to dication equilibrium is —1. The benzodiazepinium cation is thus obviously a much less stabilized system than is the dihydrodiazepinium cation.46 The low basicity of benzodiazepines is associated with this fact, with the interaction between the amine groups and the benzene ring, and also with the... 

In this series of runs, the product acid compositions varied from 9 to 15 percent nitric acid for a feed acid composition of 15 percent nitric acid and from 11 to 21 percent nitric acid with a feed composition of 25 percent. The benzene concentrations in the organic product varied from 60 to 100 percent when pure benzene was used as the feed hydrocarbon and from 25 to 60 percent when the feed hydrocarbon contained 60 percent benzene. No nitrobenzene was detected in the organic product for three of the 20 runs indicating for these particular runs that the rates of nitration were essentially zero. Measurements of the equilibrium solubility of benzene in the acid phase were almost identical regardless of whether nitrobenzene or nitrotoluene was used as the diluent in the hydrocarbon phase. Four additional (and duplicate) runs were also made to estimate the probable levels of experimental errors. This estimate was pooled with an estimate based on earlier runs. 

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