Phase Carbon Acidity Measurements

Carbon acidities have been measured in several solvents, but the greatest amount of 

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A very useful thermodynamic cycle links three important physical properties homolytic bond dissociation energies (BDE), electron affinities (EA), and acidities. It has been used in the gas phase and solution to determine, sometimes with high accuracy, carbon acidities (Scheme 3.6). " For example, the BDE of methane has been established as 104.9 0.1 kcahmol " " and the EA of the methyl radical, 1.8 0.7 kcal/mol, has been determined with high accuracy by photoelectron spectroscopy (PES) on the methyl anion (i.e., electron binding energy measurements). Of course, the ionization potential of the hydrogen atom is well established, 313.6 kcal/ mol, and as a result, a gas-phase acidity (A//acid) of 416.7 0.7 kcal/mol has been... 

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. 

Another class of acids of interest in organic chemistry is the group of carbon acids. Here we may discern three kinds of effects on acidity. The first of these is illustrated by the acidity of methane (pKa a 48) compared with that of cyclohexane (pKa a 52) (Table 3.1). It would appear that the trend is in the direction of decreasing acid strength with substitution of hydrogen by alkyl. Note that the tendency here is in the direction opposite to the effect in alcohols if we take Brauman s gas-phase results to be the more accurate indication of intrinsic acid strength. The hydrocarbon data are from solution measurements subject to considerable uncertainty, and the differences are small. It seems risky to interpret the results in terms of intrinsic molecular properties. 

Marcus5 8 taught us that the most appropriate and useful kinetic measure of chemical reactivity is the intrinsic barrier (AG ) rather than the actual barrier (AG ), or the intrinsic rate constant (kQ) rather than the actual rate constant (k) of a reaction. These terms refer to the barrier (rate constant) in the absence of a thermodynamic driving force (AG° = 0) and can either be determined by interpolation or extrapolation of kinetic data or by applying the Marcus equation.5 8 For example, for solution phase proton transfers from a carbon acid activated by a ji-acceptor (Y) to a buffer base, Equation (1), k0 may be determined from Br A ns ted-type plots of logki or... 

Solid phase chemical analyses included determination of total organic C content, and the distribution of S between iron monosulfides (acid-volatile sulfur or AVS) and pyrite (the difference between total reducible sulfur and AVS). Total organic carbon was measured coulometrically following combustion at 1050°C (7). Acid-volatile sulfur and total reducible sulfur analyses followed the procedure of Canfield et al. (8). A microbiological assay of the abundance of sulfate reducing bacteria was performed according to (9). 

The value of the surface pressure in the two-phase region where it is constant is called the surface-vapour pressure it measures the tendency of the molecules to escape from the liquid film into the vapour film, and is smaller the greater the lateral adhesion between the molecules. The adhesion increases steadily the longer the hydrocarbon chains in tride-cylic acid it first becomes great enough to form a separate liquid phase lauric acid, with twelve carbons, is at, or just above, its critical point. ... 

Using a selected ion flow tube (SiFT) technique, DePuy and coworkers studied such rates of deuterium-hydrogen exchange for a series of neutral carbon acids . Table 1 contains some selected rates of exchange with DO from DePuy s work these rates are approximate measures of relative acidity in the gas phase. 

The absorption infrared spectra of ABA (n = 1-9) and nAOBA (n = 3-12) have been measured in the spectral range from 100 to 4000 cm ( 3 cm-1) and in the temperature interval from 100 to 550 K ( 1 K) [57,58,60]. The assignment of the spectral infrared bands to the vibrations of atoms of the dimer ring (Fig. 2) has been made based on calculations of frequencies and shapes of normal coordinates performed for dimers of homologies of carbon acids [292]. Vibrations of the parareplaced benzoic acid and its alkyl radical have been interpreted on the basis of calculations carried out in Refs. 294 and 295, respectively. The maxima recorded at 2900 cm-1 (Voh band), 1692 cm-1 (vc=o band), and 940 cm-1 (p0H band) for all phases of the ABA and AOBA are direct evidence for the connection of molecules ABA and AOBA via hydrogen bonds. 

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