Acidic function protonic concept

The reaction family concept was exploited by representing the reactions by various reaction families incorporating the metal function (dehydrogenation/hydrogenation) and the acid function (protonation/deprotonation, H/Me-shift, PCP isomerizations, and P-scission). The optimized Cm model provided excellent parity between the predicted and experimental yields for a wide range of operating conditions. This shows that the fundamental nature (feedstock and catalyst acidity independent) of the rate parameters in the model. 

A good example of the use of the functional-group concept is for acid-base properties. Alcohols, ROH, are structurally related to water, HOH, in that both possess a hydroxyl function. We may then expect the chemistry of alcohols to be similar to that of water. In fact, both are weak acids because the OH group has a reactive proton that it can donate to a sufficiently strongly basic substance, written as B here ... 

The concepts of hydrogen ion concentration and pH discussed previously are meaningful only for dilute aqueous solutions of acids. A widely used means of gauging acidity in other media and at high concentrations is the Hammett acidity function H0, which is defined in terms of the behavior of one or more indicator bases B, for which there is the protonation equilibrium... 

This chapter examines the fundamental structural properties of organic molecules that directly or indirectly affect pKa values of organic acids, making it possible to estimate theoretical upper limits for concentrations of common acidic functional groups, to assess methods of quantitative analysis of acidic functional groups in humic substances, and to examine several models that have been proposed for the description of proton binding by humic substances. Some of the concepts that appear obvious in the context of this chapter have been included for the simple reason that the literature on acidity of humic substances reveals that these points have frequently been either misunderstood or neglected. 

In 1923, Broensted was the first to develop an acid-base concept that was no longer related to substances, but rather to the function of particles. Acids are proton donors and are capable, with suitable reaction partners, to donate protons to base particles or proton acceptors, i.e. protolysis or proton transfer reaction. For example, HC1 molecules, as acid particles, transfer protons when colliding with H20 molecules (see Fig. 7.3). In this sense, the proton donors of pure sulfuric acid are H2S04 molecules, the acid particles of the sulfuric acid solution are the hydronium ions (or also the hydrogen sulfate ions in concentrated solutions). In weak acids, the protolysis equilibrium is to be considered, equilibria and their constants are well defined. 

This leaves the second order rate coefficient for toluene nitration in aqueous solution (k) as the only missing parameter. The most reliable data are probably those of Coombes et al(25). Unfortunately, they used nitric acid concentrations much below those found in heterogeneous nitrations and there is some uncertainty as to how they should best be extrapolated. Thus Cox and Strachan(9 ) have attributed deviations in their results to the probability of k values in their nitrating mixtures being higher than those measured by Coombes et al. Schiefferle(26) found it necessary to modify the acidity function concept to account for the contribution of the nitric acid dissociation to the proton concentration and suggested that k should be related to the total acidity rather than just the sulphuric acid concentration. A model based on this concept gave a better fit to his experimental results. 

Concepts of acidity and basicity are, in practice, defined and evaluated by their utility. Since overly formd definitions can be restrictive the concepts of acidity evolve towaids more comprehensive definitions. For example the Lewis definition includes the Broensted definition simply regarding the proton as an electron acceptor. Because the interaction of Broensted acids and bases in solutions involves a common process, protic transfer, scales of acidity can be established, for example the Hammett [1] acidity function. For Lewis acid-base interaction there is no common process to provide a unique basis for comparisons of acid strength. Experimentally, the strength of a Lewis acid depends upon the particular Lewis base. The classification of acids and bases as hard or soft in the principle of hard and soft acids and bases (HSAB principle) clarifies the interactions of Lewis acids and bases [2a]. Strong interactions occur between hard acid and hard base, or between soft acid and soft base, hi the hard-hard interaction there is a considerable electrostatic contribution to bonding and in the soft-soft interaction there is a major covalent contribution to bonding. The use of density functional analysis has clarified the concepts of hardness and softness and an empirical ranking of Lewis acids, based on local hardness is, proposed [2c]. 

In these examples we control the extent of protonation of A by physically adjusting the pH. The important point is that the pH is adjusted relative to the pfCa of the conjugate acid of the base A to change the protonation state. The pH therefore tells us the proton donor ability of the solution toward species A . In other words, it tells us the power of the solution to donate a proton to a particular base. In the next section we introduce a similar concept, acidity functions, which give the proton donor ability of extremely concentrated add solutions. 

Acidic/Basic Lewis acidity/basicity determines the solvent s ability to donate or accept a pair of electrons to form a coordinate bond with solute and/or between solvent molecules. A scale for this acid/base property was proposed by Gutman (DN and AN donor and acceptor number, respectively) based on calorimetric determination. The complete proton transfer reaction with formation of protonated ions is determined by proton affinity, gas phase acidity, acid or base dissociation constants. Both concepts differ in terms of net chemical reaction. Acidity functions are not unique properties of the solvent system alone, but depend on the solute (or family of closely related solutes) with respect to which the thermodynamic tendency is measured. ... 

These and similar solvents are distinguished by the fact that they themselves cannot eliminate any protons. On the other hand one is still dealing with water-like solvents in the case of alcohols, since the selfdissociation of the alcohols resembles that of water in order of magnitude (see Gurney, 1953). The ability of free electron pairs to function prompted Lewis (1923) to a more general definition of the acid-base concept. 

Amphoteric compounds are compounds that may function as either acid or base, depending upon conditions. We have already met this concept in Section 4.5.4, where simple alcohols and amines have two pATa values according to whether the compound loses or gains a proton. Of course, with alcohols and amines, acidity and basicity involve the same functional group. Other amphoteric compounds may contain separate acidic and basic groups. Particularly... 

Given these definitions, it is crucial to emphasize again that no compound is inherently an acid or a base. A compound only functions as an acid (in the Bronsted sense) if it donates a proton to something that is, there must be a base present (a proton acceptor) to have an acid-base reaction. Thus a compound acts as an acid only in the presence of a base, and a substance can only act as a base in the presence of an acid. A good example of this concept is the fact that HC1 in the vapor phase is an undissociated, covalent molecule. This... 

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