Ionization of carboxylic acids

From the previous discussion, in which it was pointed out that both a and P must be functions of Y, we see that pR, which is a function of a and p, must also be a function of Y. It will therefore be convenient to compare the composition of the electrical effect transmitted by various groups G with Y and the reaction conditions held constant. Again, the most useful reaction for this purpose is the ionization of carboxylic acids in water at 25°. Not only are there more sets of data available for this reaction than for any other, but the sets studied for this reaction generally include more substituents than do sets for other reactions or under other reaction conditions. 

Measurements of the dissociation constants of m-nitrobenzoic acid and p-nitrobenzoic acid began with Ostwald in 1889114. However, the first reasonably precise values were obtained by Dippy and coworkers in the nineteen-thirties, as part of an extensive study of the ionization of carboxylic acids by conductimetric methods115. The pKa values in water at 25 °C of m-nitrobenzoic acid and p-nitrobenzoic acid were found to be 3.493 and 3.425, respectively, compared with 4.203 for benzoic acid itself. On the basis of these values Hammett28,116 proposed 0 values of 0.710 for m-NOy and 0.778 for p-NOy (see Section II.A). These a values have commonly been used thereafter, often rounded to 0.71 and 0.78, respectively. 

Quantitative application of electrostatic theory to the effect of substituents on the ionizations of carboxylic acids, such as chloroethanoic acid, is rendered... 

Figure 12 shows the spectra of slurries of humic acid and Cu +-humate in H 0. The main difference between the spectrum of sodium humate (Fig. i 1C pH 8.0) and that of humic acid (Fig. I2A) is the occurrence of the band centered at —1700 cm in the latter. Upon neutralization, this band decreases in intensity considerably and new bands appear at —1570 and -1395 cm consistent with the ionization of carboxylic acid groups. The fact that the —1700 cm band has practically disappeared at pH 8.0 indicates that this band is due almost exclusively to carboxyl groups with little contribution from aldehyde and ketone groups. This is consistent with what has been reported by other workers (Wagner and Stevenson, 1965 Theng et al.. 1967) using the KBr pellet method on dried samples. 

The first example of a surface eroding polymer was a partially esterified copolymer of methyl vinyl ether and maleic anhydride, published in 1978 [1], These polymers solubilize by ionization of carboxylic acid groups as shown in Scheme 1 and because at low pH the carboxyl groups are unionized and hence the polymer is insoluble, these materials are useful as enteric coatings to protect oral dosage forms from dissolution in the stomach [4-6]. 

Solution Carboxylic acids are acidic in aqueous solutions. The ionization of carboxylic acids proceeds according to the following reaction ... 

At least two of the recognized mechanisms for the formation of electrical double layers (Hunter, elal. 1981 Russel etal., 1989) are relevant to LB film depositions (1) ionization of carboxylic acid group and amphoteric acid groups on solid surfaces, and (2) differences between the affinities of two phases for ions or ionizable species. The latter mechanism includes the uneven distribution of anions and cations between two immiscible phases, the differential adsorption of ions from an electrolyte solution to a solid surface, and the differential solution of one ion over the other from a crystal lattice. Since the solid-liquid and the film-liquid interfaces are flat, large surfaces and since both have a large, solid-like concentration, the analysis that follows applies to both interfaces. For an interface conformed by a thin film of an amphiphilic compound with the hydrophilic end of the molecule in contact with the water subphase, the equilibrium of charges is based on pH and subphase concentration. The effect of pH is highlighted by the definition of the of the carboxylic acid ... 

A large number of LFERs were discussed above many more can be found in the literature or other physical organic chemistry texts. This is especially true for Hammett-related sigma constants. For example, here is an abbreviated list of some you may encounter in the literature a", a", and carboxylic acids other than benzoic acid for induc-... 

Experimental values for equilibrium constants give only values for the product ap. Each is therefore uncertain by a numerical factor. We can multiply all the p s by a common factor A if we divide all the as by the same factor. In order to resolve this ambiguity, we have to assume the value of some specific p or a. Hammett did this in the case of his benzene derivatives by assuming that p = 1 for ionization of carboxylic acids in water. If then the pK values for benzoic acid and for a substituted benzoic acid are (pX )o... 

Figure 14 Dependence of the advancing contact angle (0a) of buffered aqueous solutions of different values of pH on the SAMs comprising a mixture of carboxylic acids and amides, generated by reactions of the interchain anhydride and alkylamines (n-C H2 +iNH2, n = 0, 1, 4, 6, 11, and 18). The curves are labeled by the respective alkylamines on the right side of the plot. Two dashed lines on the top and the bottom of the plot are reference data for 0a of CH3(CH2)i5S/Au and H02C(CH2)i5S/Au, respectively, as indicated. Arrows indicate the onset points of ionization of carboxylic acid groups.

The first ionization constants were determined by Ostwald very soon after Arrhenius discovered the phenomenon of ionization. Ostwald applied the law of mass action to the ionization of carboxylic acids at various dilutions. Using Kohlrausch s method to obtain the equivalent conductance (Ac), and Arrhenius s derivation of the degree of ionization (a) from equation (5 1), he substituted a in the mass law formula (as in equation (5 2)) and obtained constants which varied only a little with dilution. 

SAMPLE PROBLEM 14.2 Ionization of Carboxylic Acids in Water... 

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