Acids, gas phase

A very fine example was provided by the extensive use of Professor Pitzer s electrolyte activity coefficient theory within several acid gas phase equilibrium models. 

Experimental and theoretical studies of the basicity and acidity of benzene-substituted indoles were performed169. In aqueous solution, the pK values are above —4 for the bases and above 15 for the acids. Gas-phase ionization enthalpies have been calculated using the AMI semiempirical method. 

As, Se, Sb, Br, I and Hg) were observed on the base-treated filters using the combined techniques of PGAA, INAA and IC. The base-treated filters proved to be very efficient collectors of the acidic gas-phase species, but apparently allow some elemental and organic species to pass through as shown by studies with activated charcoal-impregnated filters. 

To demonstrate this point, consider an example. Figure 6.5 shows the saturated water content for an acid gas mixture (the solid line representing the water content). Note the similarity between this curve and the curves shown in figure 4.5. The trapezoidal region represents the acid gas phase transition. For this acid gas mixture the dew point is at 7.56 MPa and the bubble point is 8.60 MPa. 

Tian, Z. Pawlow, A. Poutsma, J. C. Kass, S. R. Are carboxyl groups the most acidic sites in amino acids Gas-phase acidity, H/D exchange experiments, and computations on cysteine and its conjugate base, J. Am. Chem. Soc. 2007,129, 5403-5407. 

Figure 3-18 shows the spectra obtained recently by Millikan and Pitzer, of gaseous and solid formic acid (1410, 1411). They find band widths for monomer (gas phase), dimer (gas phase), high polymer (glass, 20-77°K), and solid (20-77°K) of, respectively 50 cm", 500 cm, 600 cm", and 400 cm". Figure 3-19 contrzists the Pg band structures for the various deuterated formic acids (gas phase) (1411). The comparisons made by BratoX, Hadii, and Sheppard (270) include those spectra shown in Fig. 3-20. The spectral similarity in the p, region for a variety of H bonded carboxylic acids in GCU solution is apparent. Here the band widths are about 400 cm" (979).

Carboxylic acid gas phase reactions tend to be rather surface sensitive, hence one must be cautious in accepting the reported Arrhenius parameters. The high activation energy bond fissions are particularly noteworthy in this regard. Kinetic results for the gas phase unimolecular reactions of the carboxylic acids are summarized in Table 38. 

The acidities of thio- and dithiocarboxylic acids have been discussed in an earlier review [3]. For example, the values of PhCOOH, PhCOSH and PhCSSH are 4.20,2.48 and 1.92, respectively. The acidities of some chalcogeno-carboxylic acids have been estimated through theoretical studies. The gas-phase acidities (AH) of formic acid and its sulfur congeners are 342.1 for HC(0)0H, 332.2 for HC(0)SH, 328.8 for HC(S)OH and 325.8 kcal/mol for HC(S)SH. Therefore, the acidity appears to increase roughly in proportion to the number of sulfur atoms present [23]. For selenocarboxylic acids, gas-phase acidities are 340.4 for HC(0)0H, 327.6 for HC(0)SeH, and 321.9 kcal/mol for HC(Se)OH, indicating that selenocarboxylic acids are more acidic than their parent carboxylic acids [24]. It was also predicted that selenocarboxylic acids maybe more acidic than their corresponding thioic acids [14]. 

Under anhydrous conditions, traditional mineral acid and Friedel-Crafts systems (liquid phase), as well as supported phosphoric acid (gas phase), can be used to produce dimers and trimers through to relatively high molecular weight viscous liquid polymers from C3 and C4 olefins. These same catalyst systems are also used in the alkylation of aromatic hydrocarbons. 

Hydrolysis itself accelerates racemization. Shorter acid exposure at higher temperatures, such as 160°C for 1 hr, decreases racemization by about 50% compared to conventional hydrolysis. Liquid-phase methanesulfonic acid, gas-phase microwave, conventional, and gas-phase micro-wave hydrolysis produce progressively higher rates of racemization. Additional phenol, however, significantly reduces racemization during microwave hydrolysis. " ... 

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