Acidity constants phenols

Table 17.1 Acidity Constants of Some Alcohols and Phenols...

Szmant and Suld88 measured pK values for various substituted benzoic acids and phenols in 48% aqueous ethanol by potentiometric titration. Substituent constants... 

It is not only p-substituted phenols that have been connected by methylene groups an extended calixarene based on naphthalene units [57] has also been synthesized and its acidity constants have been measured (see Table 25 Poh and Lim, 1989, 1990). In contrast to the calix[4]arenes, the pKa of the expanded calixarene [57] did not differ much from the of the analogue [58]. This may have two explanations (i) the conformation of the expanded calixarene is different from phenol-based calix[4]arenes and a special stabilization of the anion does not occur, (ii) this stabilization occurs but it is also present in the analogue. A stabilization by the neighbouring... 

The amounts oi adsorption of the polymer on latex and silica particles were measured as follows. Three milliliters of the polymer solution containing a known concentration was introduced into an adsorption tube(lO ml volume) which contained 2 ml of latex (C = l+.O wt %) and silica(C = 2.0 wt %) suspensions. After being rotated(l0 rpm) end-over-end for 1 hr in a water bath at a constant temperature, the colloid particles were separated from the solution by centrifugation(25000 G, 30 min.) under a controlled temperature. The polymer concentration that remained in the supernatant was measured colorimetrically, using sulfuric acid and phenol for the cellulose derivatives(12), and potassium iodide, iodine and boric acid for PVA(13). From these measurements, the number of milligrams of adsorbed polymer per square meter of the adsorbent surface was calculated using a calibration curve. 

The resultant (modified) equilibrium constant is called the acidity constant of phenol, and has the new symbol Ka, which has a value is 10-10 for phenol. Ka is also called the acid constant, the acid dissociation constant or just the dissociation constant. The value of Ka for phenol is clearly tiny, and quantifies just how small the extent is to which it dissociates to form a solvated proton. 

The major activity in gas-phase studies now depends on the use of modem techniques such as ion cyclotron resonance (ICR). Thus, as already mentioned (Section ELD). Fujio, Mclver and Taft131 measured the gas-phase acidities, relative to phenol, of 38 meta- or para-substituted phenols by the ICR equilibrium constant method, and their results for +R substituents led them to suggest that such substituents in aqueous solution exerted solvation-assisted resonance effects. It was later163 shown by comparison of gas-phase acidities of phenols with acidities of phenols in solution in DMSO that solvation-assisted resonance effects could also occur even when the solvent did not have hydrogen-bond donor properties. Indeed for p-NC>2 and certain other substituents these effects appeared to be larger than in aqueous solution. 

There are a number of limitations on the Brpnsted relationship. First of aU, the relation holds only for similar types of acids (or bases). For example, carboxylic acids may have a different a values compared to sulfonic acids or phenols. Because charge, and likewise solvation, can greatly influence the reaction rate, deviations of net charge from one catalyst to another can also influence Brpnsted plots. Another limitation on this relationship relates to temperature. Reaction rates and the corresponding dissociation constants for the acids must all be measured at the same temperature (and, most rigorously, in the same solvent). For some systems, this may prove infeasible. A third limitation is that the reaction must indeed be subject to general acid (or base) catalysis. For certain catalysts, deviations from a linear relationship may indicate other modes of action beyond general acid/... 

Fig. 8. Constant acid-constant base frequency shift-enthalpy relations. Solid lines are constant acid lines. The phenol line also contains values for p-t-butylphenol, 4-, phenol, 0, p-chlorophenol, , m-trifluormethylphenyl, A. The acid butanol is H...

A simple case where the general a constants in Table 8.5 do not succeed in correlating acidity constants is when the acid or base function is in direct resonance with the substituent. This may occur in cases such as substituted phenols, anilines, and pyridines. For example, owing to resonance (see Fig. 8.4), a para nitro group decreases the pKa of phenol much more than would be predicted from the o para constant obtained from the dissociation of p-nitrobenzoic acid. In such resonance cases (another example would be the anilines), a special set of o values (denoted as oJpara) has been derived (Table 8.5) to try to account for both inductive and resonance... 

The air-water volume ratio (Va / Vw) in a cloud is about 10s (Seinfeld, 1986). Consider now a given cloud volume that contains a certain total amount of (a) 2-4-dinitro-6-methyl phenol (DNOC), and (b) 4-chloroaniline (4-CA). Calculate the fraction of total DNOC and 4-CA, respectively, present in the water phase at equilibrium at 10°C for pH 2, 4, and 6. Neglect the effect of temperature on the acidity constant. 

The mobility and bioavailability of CPs, and thus their bioremediation, are affected by their chemical and physical properties. The solubility of CPs in water decreases as the number of chlorine substituents increases. In addition, the increase in the number of chlorosubstituents in the phenol ring increases the lipophilicity of CPs and thus their tendency to bioaccumulate. The water solubilities, pKa (acidity constant) and pK0W (octanol-water partition constant) values for environmentally important CPs are summarized in Figure 8.1. 

The acid constant of phenol, 1.7 X 10 10, is much larger than that of the aliphatic alcohols. This we attribute to resonance with the structures Fj G, and H,... 

A nitro group substituted in phenol should increase the acid constant by virtue of the inductive effect of the electronegative group (with N+ attached to the ring) moreover, in the ortho and para positions there would occur an additional resonance effect, due to the contribution of structures such as the following ... 

These place a positive charge on the oxygen atom of the unionized molecule, and so cause it to repel the proton. On analysis of the experimental values for Ka at 25°C it is found that the inductive effect of a nitro group increases Ka by a factor of about 45, and the resonance effect in the ortho and para positions gives another factor of about 22. The acid constant of a nitrophenoi can be found approximately by multiplying that for phenol, l.t X 10 10, by the factor 45 for every meta nitro group and 1000 for every ortho or para nitro group in the molecule. The comparison of the values calculated in this way with those found by experiment is shown in Table 8-1. ... 

The amount of glucuronide arising from administration of a precursor (at dose level A) which is converted to the acid or phenol by a process having a first-order velocity constant fa is given by... 

Since phenol is benzene with a hydroxyl group, the reactivity of phenol and phenolic compounds is in many ways dictated by the chemical properties of the benzene ring. The first property to consider is acidity. A compound is considered an acid when it can release a proton (H ) while in solution. The acid constant Ka of a compound defines to what extent the proton is released. Strong acids will completely dissociate, whereas weak acids (HA) are at equilibrium with their dissociated state ... 

Based on their chemical structure, the organic chemicals were divided into a number of categories alkanes, alkenes, amines, aromatic hydrocarbons, benzenes, carboxylic acids, halides, phenols, and sulfonic acid. Linear regression analysis has been applied using the method of least-squares fit. Each correlation required at least three datapoints, and the parameters chosen were important to ensure comparable experimental conditions. Most vital parameters in normalizing oxidation rate constants for QSAR analysis are the overall liquid volume used in the treatment system, the source of UV light, reactor type, specific data on substrate concentration, temperature, and pH of the solution during the experiment. 

Dixon WT, Murphy D (1976) Determination of the acidity constants of some phenol radical cations by means of electron spin resonance. J Chem Soc Faraday Trans 272 1221-1230 Dizdaroglu M, Scherz H, von Sonntag C (1972) y-Radiolyse von meso-Erythrit in wassriger Losung. Z Naturforsch 27b 29-41... 

---