Acidity scale

Fig. IV-18. (a) Electron micrograph of a collapsing film of 2-hydroxytetracosanoic acid. Scale bar 1. [From H. E. Ries, Jr., Nature, 281, 287 (1979).] (b) Possible collapse mechanism. [Reprinted with permission from H. E. Ries, Jr. and H. Swift, Langmuir, 3, 853 (1987) (Ref. 223). Copyright 1987, American Chemical Society.]...

A new dimension to acid-base systems has been developed with the use of zeolites. As illustrated in Fig. XVIII-21, the alumino-silicate faujasite has an open structure of interconnected cavities. By exchanging for alkali metal (or NH4 and then driving off ammonia), acid zeolites can be obtained whose acidity is comparable to that of sulfuric acid and having excellent catalytic properties (see Section XVIII-9D). Using spectral shifts, zeolites can be put on a relative acidity scale [195]. An important added feature is that the size of the channels and cavities, which can be controlled, gives selectivity in that only... 

Where do hydrocarbons lie on the acidity scale As the data in Table 8.1 show, both methane (pKa 60) and ethylene (plC, = 44) are very weak acids and thus do not react with any of the common bases. Acetylene, however, has piCa = 25 and can be deprotonated by the conjugate base of any acid whose pKa is greater than 25. Amide ion (NH2-), for example, the conjugate base of ammonia (pKa - 35), is often used to aeprotonate terminal aikynes. 

The subscript R in pA R indicates that the acidity constant KR is based on the ifR acidity scale. 

DMSO can be used as a dipolar non-hydroxylic solvent for the measurement of pKa values for various phenols bearing strongly electron-withdrawing substituents. These acidity scales in DMSO have been correlated with those in H20 or in the gas phase92. 

The parameter Ho reflects the ability of the solvent system to donate protons, but it can be applied only to acidic solutions of high dielectric constant, mostly mixtures of water with acids such as nitric, sulfuric, perchloric, and so on. It is apparent that the Ho treatment is valid only when/i //hi+ is independent of the nature of the base (the indicator). Since this is so only when the bases are structurally similar, the treatment is limited. Even when similar bases are compared, many deviations are found. Other acidity scales have been set up, among them // for bases with a... 

A new acidity scale has been developed based on calorimetric measurement of A-methylimidazole and A-methylpyrrole in bulk solvents. A revised version of this method was shown to give better results in some cases. Another scale of solvent acidities was developed based on the hydrogen-bond donor acidities in aqueous DMSO. ... 

Table 8.1 is a thermodynamic acidity scale and applies only to positions of equilibria. For the distinction between thermodynamic and kinetic acidity, see p. 228. 

Entry a-Chloro Acid Scale (mole) Temperature (°) Yield" (%)... 

Hardly surprisingly the tendency of alkanes to lose proton and form carbanions is not marked, as they possess no structural features that either promote acidity in their H atoms, or are calculated to stabilise the carbanion with respect to the undissociated alkane (cf. carboxylic acids, p. 55). Thus CH4 has been estimated to have a pKa value of 43, compared with 4.76 for MeC02H. The usual methods for determining pKa do not, of course, work so far down the acidity scale as this, and these estimates have been obtained from measurements on the iodide/organo-metallic equilibria ... 

The acidity of 30 has been measured with reference to Streitwieser s acidity scale (87) a pKA value of 15.8 for 30 as compared to 22.8 for fluorene and 18.5 for 9-phenylfluorene demonstrates a significant increase in acidity (87). 

Excess acidity scales 6 Determination of basicities 17 Medium effects 22 Slope values other media 23 Other matters 25... 

Cram26 had developed an acidity scale based on the ion pair acidity and used this and other measures (such as the acidity function technique) in compiling his so-called MSAD acidity scale, named after W. K. McEwen, A. Streitwieser, D. E. Applequist and R. E. Dessy. The scale used 9-phenylfluorene (pK = 18.5) as its standard and is considered at least approximately to refer to the dilute aqueous solution as the standard state. On this scale ethylene is assigned a pK value 0.5 units lower than benzene however, in another early compilation27 ethylene is 1 pK unit higher than benzene. In an updated MSAD scale, ethylene was found to be 1 pK unit less acidic than benzene6,28,29. 

Allyl anion is too strongly basic to be studied as the free anion in solution. Bordwell developed an acidity scale based on equation 1 in dimethyl sulfoxide (DMSO) at 25 °C3 and applied the method to a number of more acidic substituted allylic systems. A summary of some results is shown in Table 10. DMSO is sufficiently polar that there is little ion... 

Wilson, D.A. and Crump, D.K., Alkylene Phosphonic Acid Scale Inhibitor Compositions, U.S. Patent 4,540,508, 1985. 

Tunon, I., Silla, E. and Pascual-Ahuir, J. L. Theoretical study of the inversion of the alcohol acidity scale in aqueous solution. Toward an interpretation of the acid-base behavior of organic compunds in... 

Once the preferred conformation of each term of the series has been settled, the basicity (and acidity) scales provided by CNDO calculations on the assumption of standard geometrical models are identical to those obtained from i.c.r. and u.p.s.. Quantum calculations even allowed us to predict the basicity of some Lewis bases which could not be experimentally determined because of side-reactions occurring during the measurements. 

In addition it is possible to explain the inversion about the acidity scale of aliphatic alcohols observed on passing from the gas phase to aqueous solutions. Jam s formalism (53) for solvation energy was successfully used on this occasion. 

The exact p R+ value for 20+ could also not be determined by the earlier method. Therefore, dimethyl sulfoxide (DMSO)-water-tetramethylammonium hydroxide (0.011 M) system was used for the measurement. The H scale (Hammett acidity scale) for the system ranges from 12 in water up to 26.2 in... 

Many gas-phase acidity scales have been proposed, according to the nature of the interacting systems35-39. 

By keeping either BH+ or HBA constant, hydrogen-bond basicity or acidity scales can be built41-43. 

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. 

For NO2 the recommended values of am and ap (benzoic acid scale) in the IUPAC document79 are 0.73 and 0.78, respectively, compared with the traditional values of 0.71 and 0.78, respectively (Section III.B). When these values are used for correlations of processes taking place in other than highly aqueous media, the possibility of specific solvent effects should be borne in mind. The fact that the 0 values of NO2 are very much at the upper end of the scale for commonly used substituents means that they exert a strong influence in regression analysis and there is danger of their biassing a correlation unduly. 

Several alternative attempts were made to quantify Lewis-type interactions235,236. Following the HSAB principle, the applicability of a one-parameter Lewis acidity scale will inevitably be restricted to a narrow range of structurally related Lewis bases addition of parameters results in more general relationships237-239. The quantitative prediction of the gas-phase stabilities of Lewis acid-Lewis base complexes is still difficult. Hence the interpretation, not to mention the prediction, of solvent effects on Lewis acid-Lewis base interactions is often speculative. 

Fig. 3. FMB-spectra of the CHs-protons of pentamethylbenzene as a fonotion of acidity scale 8 c/s 298°K. (a) CFsCOOH, (b) CFsCOOH+12 mole % HsO.BF, (c) CFsCOOH+ 20 mole % HsO.BFs, (d) CFsCOOH+ 30 mole % HsO.BFs.

In higher acidity, the rate constant may correlate better with (the Hammet-Deyrup acidity scale) than with the stoichiometric concentration of H+. Since nearly all the studies involve hydrolysis reactions, the depletion of the reagent water may be an important consideration also. 

The reader should note, however, that some acidity values have changed since first reported, owing both to changes in the absolute acidities used to anchor the relative acidity scale, and to the correction of a significant error in the 1979 ICR acidity scale. The current data compilation reflects these changes. ... 

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