Br0nsted a and

Table 27 Br0nsted a and (3 values for the deprotonation of substituted benzyl carbene...

The limitations of the Arrhenius theory of acids and bases are overcome by a more general theory, called the Bronsted-Lowry theory. This theory was proposed independently, in 1923, by Johannes Br0nsted, a Danish chemist, and Thomas Lowry, an English chemist. It recognizes an acid-base reaction as a chemical equilibrium, having both a forward reaction and a reverse reaction that involve the transfer of a proton. The Bronsted-Lowry theory defines acids and bases as follows ... 

This idea was realized very successfully by Shibasaki and Sasai in their heterobimetallic chiral catalysts [17], Two representative well-defined catalysts. LSB 9 (Lanthanum/Sodium/BINOL complex) and ALB 10 (Aluminum/Lithium/BINOL complex), are shown in Figure 8D.2, whose structures were confirmed by X-ray crystallography. In these catalysts, the alkali metal (Na, Li, or K)-naphthoxide works as a Br0nsted base and lanthanum or aluminum works as a Lewis acid. 

Hexaborane(lO) behaves as a Br0nsted acid analogous to pentaborane(9) 14,15,16) (see Section VII. A.a.) and decaborane(14) 208>. Strong bases react quantitatively with hexaborane(lO) to form the nonahydrohexa-borate(—1) ion which is moderately stable at room temperature in an inert atmosphere 18>179>. 

The goal of this volume is to provide (1) an introduction to the basic principles of electrochemistry (Chapter 1), potentiometry (Chapter 2), voltammetry (Chapter 3), and electrochemical titrations (Chapter 4) (2) a practical, up-to-date summary of indicator electrodes (Chapter 5), electrochemical cells and instrumentation (Chapter 6), and solvents and electrolytes (Chapter 7) and (3) illustrative examples of molecular characterization (via electrochemical measurements) of hydronium ion, Br0nsted acids, and H2 (Chapter 8) dioxygen species (02, OJ/HOO-, HOOH) and H20/H0 (Chapter 9) metals, metal compounds, and metal complexes (Chapter 10) nonmetals (Chapter 11) carbon compounds (Chapter 12) and organometallic compounds and metallopor-phyrins (Chapter 13). The later chapters contain specific characterizations of representative molecules within a class, which we hope will reduce the barriers of unfamiliarity and encourage the reader to make use of electrochemistry for related chemical systems. 

Mechanistic details have been disclosed which imply that the cyclization follows a pathway by rapid protonation of one of the carbonyl groups followed by attack of the forming enol at the other carbonyl group <1995JOC301>. The reaction can be carried out under milder conditions and with improved yields using catalysts other than Br0nsted acids and the reaction has been greatly extended in substrate scope. 

The peaks at 53 and 62ppm correspond to physisorbed TMPO while the peaks at 62 and 53 ppm can be assigned to TMPO adsorbed on Br0nsted (B) and/or Lewis (L) acid sites. Upon hydration the intensity of both these signals diminished, indicahng that they in fact correspond to Lewis acid sites. Had they been Br0nsted sites the interaction between TMPO and the catalyst would have been too strong to be influenced by hydration. Upon addihon of a metal promoter additional resonances are observed at 87, 76, 68 and 65 ppm. Hydrahon experiments confirm that these new sites are Bronsted acid sites. Furthermore, quantification of the distinct acid sites identified on the catalyst has been carried out, based on the assumption that each TMPO molecule can only be adsorbed on one acid site. 

A nonbonding electron pair can serve as a Lewis base and attack an electron-deficient carbon or serve as a Br0nsted base and pull off a proton. The decision of whether a lone pair serves as a base or a nucleophile will not concern us in this section, only how to recognize the generic class. As a Br0nsted base, the electron flow starts from the lone pair of the base. Section 3.2 showed how to rank Br0nsted bases. A synthetically useful proton transfer has a Teq greater than 10 . See Section 3.6 for the calculation of the proton transfer /feq. See Section 3.2.4 for common examples. 

The free-energy change, AG, of such a reaction in the gas phase is simply PA(RO ) — PA(B). When B is OH and ROH is phenol, then AGg is PA(PhO ) — PA(OH ) = —41 kcalmoR, so in this reaction the phenol molecule acts as the Br0nsted acid and the OH anion as the Br0nsted base. Clearly, relative proton-affinity values determine the relative acidity scale of molecules in the gas phase. 

Table 4.1. Val ues of interest for the reaction-progress variable X, the compression variable i, the Br0nsted coefficient a, and their significance.

Write all the species (except water) that are present in a phosphoric acid solution. Indicate which species can act as a Br0nsted acid, which as a Br0nsted base, and which as both a Br0nsted acid and a Br0nsted base. 

The amide ion is a strong Br0nsted base and does not exist in water. 

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