Equilibrium acidities

The ionization of a series of 4-substituted pyridines has been studied, and both equilibrium acidities (pK ) and enthalpies of ionization have been recorded at 25°C ... 

When the acidities of hydrocarbons are discussed in terms of the relative stabilities of neutral and anionic forms, particularly with respect to the extent of electron delocalization in the anion, the appropriate data are equilibrium acidity measurements. We have just seen... 

An extensive series of hydrocarbons has been studied in cyclohexylamine, with the use of cesium cyclohexylamide as base. For many of the compounds studied, spectroscopic measurements were used to determine the relative extent of deprotonation of two hydrocarbons and thus establish relative acidity. For other hydrocarbons, the acidity was derived by kinetic measurements. It was shown that the rate of tritium exchange for a series of related hydrocarbons is linearly related to the equilibrium acidities of these hydrocarbons in the solvent system. This method was used to extend the scale to hydrocarbons such as toluene for which the exchange rate, but not equilibrium data, can be obtained. Representative values of some hydrocarbons withpAT values ranging from 16 to above 40 are given in Table 7.2. 

The following table gives exchange rates in methanolic sodium methoxide for a number of hydrocarbons and equilibrium acidities for some. Determine whether there is a correlation between kinetic and thermodynamic acidity in this series of compounds. If so, predict the thermodynamic acidity of the hydrocarbons for which no values are listed. 

This process is referred to as internal return, i.e., the base returns the proton to the carbanion faster than exchange of the protonated base with other solvent molecules occurs. If internal return is important under a given set of conditions, how would the correlation between kinetics of exchange and equilibrium acidity be affected How could the occurrence of internal return be detected experimentally ... 

Solvent isotope effects are usually in the range / h20+ = 2-3. These values reflect the greater equilibrium acidity of deuterated acids (Section 4.5) and indicate that the initial protonation is a fast preequilibrium. 

An isomeric form of the anion that is initially formed is converted to the aU-cis system rapidly at room temperature.Data on the equilibrium acidity of the parent hydrocarbon are not available, so the stability of the anion cannot be judged quantitatively. The NMR spectrum of the anion, however, is indicative of aromatic character. 

Table 14. Equilibrium Acidities of Esters, Diketones, and Nitrometbanes [64]...

Benzyl-type carbanions and their metallo compounds, derived from aromatic or hetero-aromatic precursors, bearing carbon- or hetero-substituents, are readily available with variable substitution patterns due to their mesomeric stabilization (see Section 1.3.2.2)2. Even dicarbanions are accessible without difficulty3,4. The equilibrium acidities of many aromatic hydrocarbons have been determined5-7. The acidities of a-hetero-substituted toluenes8 are similar to those of the corresponding allylic compounds and can usually be generated by the same methods. 

Kresge et a/.498 have drawn attention to the fact that detritiation of [3H]-2,4,6-trihydroxy- and [3H]-2,4,6-trimethoxy-benzenes by concentrated aqueous perchloric acid gives correlations of log rate coefficient with — H0 with slopes of 0.80 and 1.14 respectively. Protonation to give the carbon conjugate acids is, however, governed by h0lA0 and h0l 9S, respectively, which suggests that the difference in kinetic acidity dependence is a property of the substrate and should not be interpreted as a major difference in mechanism. The kinetic difference can be eliminated by an appropriate comparison of kinetic and equilibrium acidity dependencies. In equation (230)... 

In this chapter it is clearly impossible to do more than sample the extensive literature on the carbon acidity of sulfinyl and sulfonyl compounds, as it illuminates the electronic effects of these groups, particularly in connection with linear free-energy relationships. There are three main areas to cover first, as already indicated, equilibrium acidities (pKa values) second, the kinetics of ionization, usually studied through hydrogen isotopic exchange and finally, the kinetics of other reactions proceeding via carbanionic intermediates. 

Finally, in many cases the acidity equilibria cannot be measured but the rate of proton transfer or transmetallation can be measured to give an ionic or ion pair kinetic acidity. Studies using the rates of proton transfer have included the use of isotopes such as tritium and deuterium5,6. The rate is then used to calculate the Brpnsted slope, a, by plotting the logarithm of the proton transfer rate against the pK, as determined by the equilibrium acidity, for a series of compounds. From this plot, the approximate pKa of an unknown compound can be determined by comparison of the same type of compounds. 

One of the earliest measurements of the gas-phase equilibrium acidity of propene involved measuring the rates of reaction of propene with hydroxide ion in both directions33. The resulting equilibrium constant gave A//acid = 391 1 kcalmol-1. In the case of ethylene, the acidity and independently measured electron affinity of vinyl radical were used to determine the bond dissociation energy, a quantity difficult to obtain accurately by other means8. 

TABLE 10. Equilibrium acidities of selected allylic compounds in dimethyl sulfoxide at 25°Ca... 

In order to investigate the effect of chain length of alkenes upon acidity and aggregation, Thiele and Streitwieser probed the equilibrium acidity of a series of polyenes using UV VIS-spectroscopy in THF at 25 °C Ph(CH=CH) CH2Ph (n = 1, DP3 n = 2, DP5 n = 3, DP7 n = 4, DP9)70. The equilibrium acidity was determined using the transmetallation reaction of equation 3 with Cs+ as the counterion. The results were consistent with... 

Kinetic and equilibrium acid-base properties of diaminonaphthalenes0,1 ... 

Application of ion exchange refining to the process for the manufacture of dextrose may be understood from the following description of such a process. Triple-washed starch is diluted with ion-free water to the desired concentration and is acidified with a definite quantity of mineral acid such as hydrochloric or sulfuric. It is desirable that the acidified starch slurry be held for at least thirty minutes in order to permit an effective equilibrium acidity to be reached. The starch slurry may contain a quantity of soft water salts which consume acid, and since this consumption is variable, the acidity is checked and adjusted to the desired level following the holding period. The conversion is then carried out at elevated temperature and under pressure for a period sufficient to yield maximum dextrose. The conversion may be carried out batchwise in an autoclave, or continuously. 

The results of ab initio calculations are compatible with the conclusion that -SH stabilizes an adjacent carbanion more than —OH363 Furthermore, the above discussion represents a viable alternative to the d-orbital model for explaining the enhanced kinetic and equilibrium acidities of molecules containing sulfur groups. 

Table I. Rate Constants of the Adsorption/Desorption and Equilibrium Acidity Constants...

F. G. Bordwell. Equilibrium Acidities in Dimethyl Sulfoxide Solution. Acc. Chem. Res. 1988,21, 456-A63. 

F. G. Bordwell, J.-P. Cheng, J. A. Harrelson Jr. Homolytic Bond Dissociation Energies in Solution from Equilibrium Acidity and Electrochemical Data. J. Am. Chem. Soc. 1988,110, 1229-1231. 

Quantitative and semiquantitative equilibrium acidity scales have been established for weak carbon acids in cyclohexylamine (CHA)113, dimethoxyethane (DME)114, dimethyl-sulphoxide (DMSO)115 and THF116. 

The equilibrium acidities of aniline and 26 of its derivatives have recently been measured127 in DMSO solution and they cover a range of 15 pKa units. The pKa values of anilines in DMSO are 10 units higher than the ion-pair pKa values obtained in liquid ammonia. This effect has been attributed to the greater capacity of ammonia to solvate the proton. When the pKa values determined in DMSO were compared with those derived in H2O-DMSO or EtOH-DMSO, it was evident that the latter were appreciably lower. This behaviour is different from that found for the carbon acids. The H2O-DMSO mixture can solvate nitranions better than pure DMSO, in line with the fact that nitranions are better HBA than carbanions128 (see Table 10). 

Bordwell129 developed a method of estimating relative bond dissociation energies (BDE) for families of acids, HA, by combining equilibrium acidity constants, pXeA, with the oxidation potential of their conjugated bases, A, both measured in DMSO ... 

Kinetic and equilibrium acidities of several families of nitroalkanes have been discussed extensively in the chapter by Lewis172, where the effects of changing substituents and the nature of the base, together with the role of the solvent on rates of ionization and equilibria, have been considered. 

Since the equilibrium acidities of nitro-activated carbon acids have been considered172 in the preceding volume of this series, we show in Table 16 some significant values determined in DMSO from the enlarged compilation of DMSO equilibrium acidities proposed by Bordwell66. 

TABLE 21. Equilibrium acidity of nitro-substituted diphenylmethanes in 30% H2O-70% DMSO at 25 °C... 

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