Carbanions carbon acidity measurements

The most widely studied physical property of carbanions is their basicity, which of course is a direct measure of the acidity of the parent carbon acid. Carbon acidity measurements date back to the early part of the twentieth century and a myriad of techniques have been employed for the measurements. Although early measurements were only able to provide semiquantitative data, more recent ones have resulted in accurate acidity measurements across a vast range of effective acid dissociation constants, Ka values. This section will begin with a brief description of definitions and methodologies followed by representative data as well as applications of those data. 

In the discussion of the relative acidity of carboxylic acids in Chapter 1, the thermodynamic acidity, expressed as the acid dissociation constant, was taken as the measure of acidity. It is straightforward to determine dissociation constants of such adds in aqueous solution by measurement of the titration curve with a pH-sensitive electrode (pH meter). Determination of the acidity of carbon acids is more difficult. Because most are very weak acids, very strong bases are required to cause deprotonation. Water and alcohols are far more acidic than most hydrocarbons and are unsuitable solvents for generation of hydrocarbon anions. Any strong base will deprotonate the solvent rather than the hydrocarbon. For synthetic purposes, aprotic solvents such as ether, tetrahydrofuran (THF), and dimethoxyethane (DME) are used, but for equilibrium measurements solvents that promote dissociation of ion pairs and ion clusters are preferred. Weakly acidic solvents such as DMSO and cyclohexylamine are used in the preparation of strongly basic carbanions. The high polarity and cation-solvating ability of DMSO facilitate dissociation... 

A study of the rates of water-catalyzed detritiation of six disulfonyl-activated carbon acids contains results of interest in connection with electronic effects192. Thus the carbanion stabilizing abilities of several groups as measured kinetically lie in the order SOzPh > SOzEt > S02Me. 

The bottom portion of Table 8.1 consists of very weak acids pK. above 17). In most of these acids, the proton is lost from a carbon atom, and such acids are known as carbon acids. The pa s values for such weak acids are often difficult to measure and are known only approximately. The methods used to determine the relative positions of these acids are discussed in Chapter 5. The acidity of carbon acids is proportional to the stability of the carbanions that are their conjugate bases (see p. 227). 

This chapter will begin with a brief overview of the development of carbanion chemistry followed by a section devoted to the structure and stability of carbanions. Methods of measuring carbon acidity and systematic trends in carbanion stability will be key elements in this chapter. Next, processes in which carbanions appear as transient, reactive intermediates will be presented and typical carbanion mechanisms will be outlined. Finally, some new developments in the field will be described. Although the synthetic utility of carbanions will be alluded to many times in this chapter, specific uses of carbanion-like reagents in synthesis will not be explored. This topic is exceptionally broad and well beyond the scope of this chapter. 

The next issue that arises from the weak acidity of carbon acids involves the degree of self-dissociation. In Eq. 3, the equihbrium constant is determined by measuring the concentration of the four species in the equation, but this requires that the carbon acid self-dissociates to an extent that a measurable quantity of the carbanion is formed. Again, because carbon acids are generally weak, this requirement often is not met and therefore another type of equilibrium measurement... 

For reviews of methods used to measure the acidity of carbon acids, see Jones Q. Rev., Chem. Soc. 1971, 25, 365-378 Fischer Rcwicki Prog. Org. Chem. 1968, 7, 116-161 Reutov Beletskaya Butin. Ref. 11. Chapter I an earlier version of this chapter appeared in Russ. Chem. Rev. 1974, 43. 17-31) Ref. 6. For reviews on acidities of carbon acids, see Gau Assadourian Veracini Prog. Phys. Org. Chem. 1987, 16, 237-285 in Bunccl Durst Comprehensive Carbanion Chemistry, pt. A Elsevier New York, 1980, the reviews by Pcllcritc Brauman. pp. 55-96 (gas phase acidities) and Streitwicscr Juaristi Nebenzahl, pp. 323-381. 

A carbanion can be regarded as the conjugate base of the corresponding carbon acid and thus the thermodynamic acidities of hydrocarbons and substituted hydrocarbons provide a very convenient quantitative measure of the extent of carbanion formation in the generalized reaction... 

The acidities of carbon acids have been extensively discussed before, especially by Cram5 who has constructed a scale of pK values based on the work of Mc-Ewan, Streitwieser, Applequist, and Dessy and known as the MSAD scale. Table 3 shows a selection of these pK values, and also values given by Reutov and co-workers6 derived from polarographic measurements. It is clear from the values shown in Table 3 that substituents such as a-phenyl, a-benzyl, and a-carbethoxy increase the extent of carbanion formation from carbon acids very considerably,... 

Bordwell, F. G. Matthews, W. S. Vanier, N. R. Acidities of carbon acids. IV. Kinetic vs equilibrium acidities as measures of carbanion stabilities. The relative effects of phenylthio, diphe-nylphosphino, and phenyl groups./. Am. Chem. Soc. 1975, 97, 442-443. 

When HX is a carbon acid the value of the rate coefficient, ) for a thermodynamically favourable proton transfer rarely approaches the diffusion limit. Table 1 shows the results obtained for a few selected carbon acids which are fairly representative of the different classes of carbon acids which will be discussed in detail in Sect. 4. For compounds 1—10, the value of k i is calculated from the measured value of k, and the measured acid dissociation constant and, for 13, k, is the measured rate coefficient and k1 is calculated from the dissociation constant. For 11 and 12, both rate coefficients contribute to the observed rate of reaction since an approach to equilibrium is observed. Individual values are obtained using the measured equilibrium constant. In Table 1, for compounds 1—10 the reverse reaction is between hydronium ion and a carbanion whereas for 11, 12 and 13 protonation of unsaturated carbon to give a carbonium ion is involved. For compounds 1—12 the reverse reaction is thermodynamically favourable and for 13 the forward reaction is the favourable direction. The rate coefficients for these thermodynamically favourable proton transfers vary over a wide range for the different acids. In the ionization of ketones and esters, for which a large number of measurements have been made [38], the observed values of fe, fall mostly within the range 10s—101 0 1 mole-1 sec-1. The rate coefficients observed for recombination of the anions derived from nitroparaffins with hydronium ion are several orders of magnitude below the diffusion limit [38], as are the rates of protonation and deprotonation of substituted azulenes [14]. For disulphones [65], however, the recombination rates of the carbanions with hydronium ion are close to 1010 1 mole-1 sec-1. Thermodynamically favourable deprotonation by water of substituted benzenonium ions with pK values in the range —5 to —9 are slow reactions [27(c)], with rate coefficients between 15 and 150 1 mole-1 sec-1 (see Sect. 4.7). 

Acidity constants for ionization of weak carbon acids in water caimot be determined by direct measurement when the strongly basic carbanion is too unstable to exist in detectable concentrations in this acidic solvent. Substituting dimethyl-sulfoxide (DMSO) for water causes a large decrease in the solvent acidity because, in contrast with water, the aprotic cosolvent DMSO does not provide hydrogenbonding stabilization of hydroxide ion, the conjugate base of water. This allows the determination of the pfC s of a wide range of weak carbon acids in mixed DMSO/water solvents by direct measurement of the relative concentrations of the carbon acid and the carbanion at chemical equilibrium [3, 4]. The pfC s determined for weak carbon acids in this mixed solvent can be used to estimate pfC s in water. 

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