Bronsted-Lowry acidity

Boron trioxide is not particularly soluble in water but it slowly dissolves to form both dioxo(HB02)(meta) and trioxo(H3B03) (ortho) boric acids. It is a dimorphous oxide and exists as either a glassy or a crystalline solid. Boron trioxide is an acidic oxide and combines with metal oxides and hydroxides to form borates, some of which have characteristic colours—a fact utilised in analysis as the "borax bead test , cf alumina p. 150. Boric acid. H3BO3. properly called trioxoboric acid, may be prepared by adding excess hydrochloric or sulphuric acid to a hot saturated solution of borax, sodium heptaoxotetraborate, Na2B407, when the only moderately soluble boric acid separates as white flaky crystals on cooling. Boric acid is a very weak monobasic acid it is, in fact, a Lewis acid since its acidity is due to an initial acceptance of a lone pair of electrons from water rather than direct proton donation as in the case of Lowry-Bronsted acids, i.e. 

It will be seen from these examples that the process of self-ionization in a protonic solvent involves the transfer of a proton from one solvent molecule to another. Thus, the solvent is acting simultaneously as a Lowry-Bronsted acid and as a base. 

Table 3.3 pvalues for some Lowry -Bronsted acids... 

Neutralization reactions between Lowry-Bronsted acids and bases are frequently employed in chemical analysis. Methods based on them are sometimes termed acidimetric or alkalimetric. 

The pH of a Lowry-Bronsted acid DEcre-ases as its concentration INcreases. 

Thus, Lewis s definition is a much broader definition that includes coordination compound formation as acid-base reactions, besides Arrhenius and Lowry-Bronsted acids and bases. Examples ... 

IV. Bridge and Endohydrogens and Relative Lowry-Bronsted Acidity... 

The structures, relative stabilities, and relative Lowry-Bronsted acidities of carboranes and boranes as well as related anions, Lewis base adducts, and heteroelement analogs are rationalized primarily on the basis of rudimentary coordination numbers. The principal factors, in decreasing order of importance, are (a) the various deltahedra and deltahedral fragments, (b) the placement of bridge and endohydrogens, (c) the placement of carbon and other heteroelements, and d) the resulting coordination number of boron. 

The relative Lowry-Bronsted acidities of the various groups within the carboranes and related species are tentatively listed in Table VI. Further experiments to determine the equilibrium positions involving species such as... 

Lowry-Bronsted acidities of, 18 132-136 reactions of, 18 48 structural patterns, 18 3-7 structural rules, 18 85-97 structures of, 18 68, 69 transition element group (TEG)-substituted, 18 79... 

Lowry-Bronsted acidity of bridge hydrogens, 18 135 of carboranes, 18 132-136 Low-temperature condensation, of high-temperature species, 14 121-171 activation enthalpy in, 14 128-129 atomic species in, 14 123-125 condensation process in, 14 129-130 experimental methods in, 14 130-141 formation of high temperature species, 14 131-139... 

Compounds with a low HOMO and LUMO (Figure 5.5b) tend to be stable to selfreaction but are chemically reactive as Lewis acids and electrophiles. The lower the LUMO, the more reactive. Carbocations, with LUMO near a, are the most powerful acids and electrophiles, followed by boranes and some metal cations. Where the LUMO is the a of an H—X bond, the compound will be a Lowry-Bronsted acid (proton donor). A Lowry-Bronsted acid is a special case of a Lewis acid. Where the LUMO is the cr of a C—X bond, the compound will tend to be subject to nucleophilic substitution. Alkyl halides and other carbon compounds with good leaving groups are examples of this group. Where the LUMO is the n of a C=X bond, the compound will tend to be subject to nucleophilic addition. Carbonyls, imines, and nitriles exemplify this group. 

Among organic compounds, we can expect appreciable Lowry-Bronsted acidity from those containing O -H, N -H, and S—H groups. 

Rewrite the following equations to show the Lowry-Bronsted acids and bases actually involved. Label each as stronger or weaker, as in Sec. 1.22. 

What is the Lowry-Bronsted acid in (a) HCl dissolved in water (b) HCl (unionized) dissolved in benzene (c) Which solution is the more strongly acidic ... 

Step (1) is the difficult step, and its rate largely or entirely controls the overall rate of addition. This step involves attack by an acidic, electron-seeking reagent— that is, an electrophilic reagent—and hence the reaction is called electrophilic addition. The electrophile need not necessarily be a Lowry-Bronsted acid transferring a proton, as shown here, but, as we shall see, can be almost any kind of electron-deficient molecule (Lewis acid). 

Table 3.3 pA, values for some Lowry-Bronsted acids... 

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