Bronsted—Lowry acid

These other concepts of acids and bases are not so easily applied quantitatively as the Lowry-Bronsted concept. Nevertheless they have proved to be very useful as ways of classifying chemical substances and—more importantly— these ideas have been a stimulus to many advances in inorganic chemistry. 

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. 

A wide variety of solutes behave as weak acids that is, they react reversibly with water to form H30+ ions. Using HB to represent a weak acid, its Bronsted-Lowry reaction with water is... 

Since Arrhenius, definitions have extended the scope of what we mean by acids and bases. These theories include the proton transfer definition of Bronsted-Lowry (Bronsted, 1923 Lowry, 1923a,b), the solvent system concept (Day Selbin, 1969), the Lux-Flood theory for oxide melts, the electron pair donor and acceptor definition of Lewis (1923, 1938) and the broad theory of Usanovich (1939). These theories are described in more detail below. 

Any text on acids and bases would not be deemed complete if mention were not made of the extended definition of acids and bases that is embodied in the Lowry-Bronsted theory. The theory basically proposed a more general definition of acids and bases to overpower the limitations of the theory arising from the Arrhenius concept. 

Certain acids can give up two or three protons these are called diprotic (for example, H2S04) or triprotic (for example, H3P04) acids, respectively. According to the Lowry-Bronsted concept, each acid is regarded as giving up only one proton. Diprotic or triprotic acids are perceived as giving out protons in different steps. 

The Lowry-Bronsted theory suffers from the following drawbacks (i) it explains the acidic or the basic nature of a substance only if a solvent such as water is present. In other words, molecules such as HC1, H2S04, etc. are not acids in the absence of water (ii) this concept fails to explain how acidic oxides (such as C02, S02, etc.) are neutralized by basic oxides (such as CaO, BaO) since there is no involvement of protons in these reactions, e.g. ... 

It was G. N. Lewis who extended the definitions of acids and bases still further, the underlying concept being derived from the electronic theory of valence. It provided a much broader definition of acids and bases than that provided by the Lowry-Bronsted concept, as it furnished explanations not in terms of ionic reactions but in terms of bond formation. According to this theory, an acid is any species that is capable of accepting a pair of electrons to establish a coordinate bond, whilst a base is any species capable of donating a pair of electrons to form such a coordinate bond. A Lewis acid is an electron pair acceptor, while a Lewis base is an electron pair donor. These definitions of acids and bases fit the Lowry-Bronsted and Arrhenius theories, and cover many other substances which could not be classified as acids or bases in terms of proton transfer. 

The compounds, BeH2, BH3 and CH4 are generally considered as neither Bronsted-Lowry acids nor Bronsted-Lowry bases. ... 

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. 

On the other hand an acid orprotogenic solvent will be a poor proton acceptor, accentuating basic properties. This effect is exemplified by the solution of nitric acid in anhydrous hydrofluoric acid, which shows how nitric acid (normally regarded as a strong acid) can behave as a Lowry-Bronsted 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 Lowry-Bronsted theory says an acid is a proton donor. 

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 ... 

The concepts of the Lowry-Bronsted theory may explain the various reactions that take place during many non-aqueous titrations. Thus, an acid is a proton donor and a base is a proton acceptor. Therefore, when an acid HA undergoes dissociation it gives rise to a proton and the conjugate base A of the acid ... 

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... 

Amines cire both Bronsted-Lowry bases (they accept hydrogen ions from acids) and Lewis bases (they furnish an electron pair to Lewis acids). As Bronsted-Lowry bases they have values. Aliphatic amines have values of approximately 1(H, and aromatic amines have values near 10 °. (These values compare to a value of =10" for ammonia.)... 

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. 

A point of this reaction is the release of H+ ions—the Bronsted-Lowry approach considers this the appearance of a proton from the acid. A Bronsted-Lowry acid is a proton donor. Note that the hydrogen ion is a proton, hydrogen s nucleus without the electron found in the atom. Then, a Bronsted-Lowry acid must contain a hydrogen. Of course, if the solvent were not to be water, this statement may not work because the cation released could be other than the hydrogen ion, but there might be other ions performing the same service (liquid ammonia autoionizes, Problem 17.3). 

A Bronsted-Lowry acid is a substance that transfers a proton to another substance. Since a hydrogen ion is a proton, all Arrhenius acids are Bronsted-Lowry acids. However, the slight difference in definitions allows us to consider additional substances as acids. We can also consider reactions that do not occur in aqueous solutions ... 

In the chemical reaction shown below, identify the Bronsted-Lowry acid, the Bronsted-Lowry base, the conjugate acid, and the conjugate base ... 

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