Basic function protonic concept

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 ... 

These definitions describe the Lowry-Bronsted concept of acidity and basicity. The Lewis concept of acids and b s is more general, and may be useful for reactions in which protons are not involved. A Lewis acid is any substance that can accept electrons, and a Lewis base is any substance that can donate electrons. Small, high-charge metal ions such as Al + and Fe " are strong Lewis acids because they tend to complex with functional groups on molecules (Lewis bases) by accepting electrons from them ... 

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... 

In addition we will consider the possibility to obtain reliable theoretical information on the preferred attach sites for proton and metal cations and on the potential energy surfaces (PES) that cannot be determined experimentally even with the most modern and sophisticated mass-spectrometric instruments [22,23j. Furthermore, we will propose the way to rationalize some of chemical properties by using the concepts of hardness, softness and other reactivity indices (Fukui functions) for which an exact definition exists only in the framework of DFT 1111. These last fascinating tools can contribute to increase funhemiore the DFT use going in the "core" of molecules to predict and explain basic chemical concepts. 

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]. 

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. ... 

The polyprotic acid, para-hydroxybenzoic acid, has two functional groups, a carboxylic acid and a phenolic group. Each group has an ionizable proton. Given just two basic pieces of titration data, we used concepts from this and the preceding chapter to determine the pK values for both ionizable groups as well as the pH at the two equivalence points. As a check, note that the pK values of these groups are comparable to the values for their parent compounds, acetic add and... 

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