Nonprotonic acids

On the other hand, dipolar solvent molecules may also compete with the aquo ligands in the coordination sphere of the metal cation, e.g., in the nonprotonated acid form (HX). Adduct formation in the absence of proton exchange or ion association (e.g., in nonaqueous solvents) is denoted as an Lewis acid-base interaction. The adduct formation may be described and evaluated by using the Hammett function introduced previously (Equations 8.54 and 8.55) according to - °... 

The reduction of molybdate salts in acidic solutions leads to the formation of the molybdenum blues (9). Reductants include dithionite, staimous ion, hydrazine, and ascorbate. The molybdenum blues are mixed-valence compounds where the blue color presumably arises from the intervalence Mo(V) — Mo(VI) electronic transition. These can be viewed as intermediate members of the class of mixed oxy hydroxides the end members of which are Mo(VI)02 and Mo(V)0(OH)2 [27845-91-6]. MoO and Mo(VI) solutions have been used as effective detectors of reductants because formation of the blue color can be monitored spectrophotometrically. The nonprotonic oxides of average oxidation state between V and VI are the molybdenum bronzes, known for their metallic luster and used in the formulation of bronze paints (see Paint). 

In acidic aqueous solution the displacement of 1,1-cyclobutanedicar-boxylate (cbdca2-) from [Pt(cbdca)(NH3)2] (Figure 1) resembles the successive displacements of two monodentate carboxylates [18]. Rate constants of 8 x 10 5 s-1 and 8.0 x 10 4 s-1 (298.2 K) were found for the ring-opening step of nonprotonated and protonated species, respectively. 

Experiments were performed at 5°C in order to arrest the cis-trans isomerization of the protonated Schiff base. Spectra with one equivalent of acid and different mixing times showed one NOE cross-peak between H15 of the retinal molecule and the proton on the counterion, as shown for a mixing time of 0.4 s in Figure 10. The strong chemical shift dependence of the H15 resonance on the concentration of the acid dictated the use of less than one equivalent of the protonating formic acid, and therefore an incomplete protonation (>80%) of the retinal, in order to avoid an overlap between the formate and the H15 peaks in the spectrum. This should not affect the observed result since an average chemical shift, between those of HI 5 of the retinal in its nonprotonated and protonated... 

For an acidic and basic drug, the solubility over the GI pH range varies depending on the intrinsic solubility (SD) of the compound (i.e., solubility of unionized or nonprotonated species), pfCa, and the solubility of the salt form.71 72... 

In silicate melts and other nonprotonated solvents, the Bronsted-Lowry equation is not applicable and is conveniently replaced by the Lux-Flood acid-base definition (Lux, 1939 Flood and Forland, 1947), according to which free oxygen 0 replaces A basic oxide is one capable of furnishing oxygen ions, and an acidic oxide is one that associates oxygen ions ... 

A kinetic study has been carried out in order to elucidate the mechanism by which the cr-complex becomes dehydrogenated to the alkyl heteroaromatic derivative for the alkylation of quinoline by decanoyl peroxide in acetic acid. The decomposition rates in the presence of increasing amounts of quinoline were determined. At low quinoline concentrations the kinetic course is shown in Fig. 1. The first-order rate constants were calculated from the initial slopes of the graphs and refer to reaction with a quinoline molecule still possessing free 2- and 4-positions. At high quinoline concentration a great increase of reaction rate occurs and both the kinetic course and the composition of the products are simplified. The decomposition rate is first order in peroxide and the nonyl radicals are almost completely trapped by quinoline. The proportion of the nonyl radicals which dimerize to octadecane falls rapidly with increase in quinoline concentration. The decomposition rate in nonprotonated quinoline is much lower than that observed in quinoline in acetic acid. 

This step is not favored by the protonation of the d-complex for polar reasons. A fast equilibrium between the protonated tr-complex and the nonprotonated quinoline in acetic acid cannot be excluded, however [Eq. (20)]. 

The fact that such selectivity was not found with homolytic alkylation of nonprotonated heteroaromatics (Table I) or with homocyclic aromatics indicates that polar factors play a major role in the reactivity of alkyl radicals with protonated bases. These effects were determined by the study of the relative reaction rates in the alkylation of 4-substituted pyridines in acidic medium. The results obtained with methyl, n-propyl, w-butyl, sec-butyl, i-butyl, and benzyl radicals are summarized in Table III. 

Photochemical a-hydroxyalkylation by alcohols is strictly connected with photochemical alkylation. With quinoline an acidic medium leads to alkylation hydroxy alkylation takes place with nonprotonated bases. The suggested mechanism is substantially identical for both processes. A IT -> 7T excited state would cause the initial hydrogen abstraction (Scheme 11). 

If the mechanism in acid and without acid are the same, one might have expected 4-alkylation under both conditions, and the failure to observe any 4-alkylation when acid is not present is as yet unexplained. Possibly with nonprotonated bases the hydroxyalkylation occurs according to Scheme 11, in which dimerization of two radicals within the solvent cage would lead to attack only at position 2, while in acid the attack could take place, at least in part, according to Scheme 12 but with protonated base, leading to both the isomers (2 and 4), as in the hydroxyalkylation by oxidation of alcohols. The much higher affinity of alkyl radicals toward protonated heteroaromatic bases in comparison with nonprotonated bases would support this interpretation. 

G.N. Lewis extended and generalized the acid-base concept to nonprotonic systems.5,6 He defined an acid as a substance that can accept electrons and defined a base as a substance that can donate electrons. Lewis acids are electron-deficient molecules or ions such as BF3 or carbocations, whereas Lewis bases are molecules that contain readily available nonbonded electron pairs (as in ethers, amines, etc.) [Eq. (1.6)]. 

The spectrum of ammonia chemisorbed on a silica-alumina cracking catalyst was studied to determine whether the acidity of these catalysts is due to a Lewis (nonprotonic) or a Bronsted type of acid (28, 29). This work was based on the premise that ammonia chemisorbed on Lewis sites would retain a NH3 configuration while ammonia chemisorbed on a Bronsted site would form NHt. The NH3 configuration was expected to have bands near 3.0 and 6.1 p and the NHt near 3.2 and 7.0 p. 

Tab. 6.2 Energy Gain through Resonance in Nonprotonated and Protonated Carboxylic Acid Derivatives...

In most studies the reaction mixtures were quenched by dilution with water. When dealing with heteroaromatic substrates care must be taken to ensure that only one species (either free base or conjugate acid) of both reactant and product is present in the resulting acid solution. In some cases, on diluting with water the pH of the resultant solution would fall close to the pAa values for the substrate and/or its nitro derivative. In these cases, to avoid measuring absorptions arising from both the proton-ated and nonprotonated forms, the reaction mixtures are quenched with sodium hydroxide solution, so that the resulting pH is far from the p/L, value in the reference cell a solution of sodium hydroxide of comparable concentration should be used. 

---