Description
In 1923, G.N. Lewis proposed an electronic definition of acids and bases founded on electron pair sharing. Compared with the protonic definition of Br¨onsted based on proton exchange, the new Lewis definition broadened considerably the field of acid/base reactions. It incorporates heterolysis, coordination, solvation, complexation, hydrogen-bond formation, halogen-bond formation and electrophilic and nucleophilic reactions into acid/base chemistry. It is therefore not surprising that discussions of Lewis acidity and Lewis basicity appear in almost every textbook on general, organic and inorganic chemistry.
A major criticism, however, is often made of the Lewis definition. Contrary to the Br¨onsted definition, it is not possible to establish any universal order of acid or base strength. In the Br¨onsted definition, the proton is used as the reference and the quantitative study of proton exchange reactions between bases, by means of electrochemical or spectrometric methods, enabled the strength of several thousand Br¨onsted acids and bases to be measured unambiguously. Thermodynamic databases for proton exchange in the gas phase (NIST webbook, http://webbook.nist.gov.) or in water (e.g. D.D. Perrin, Dissociation Constants of Organic Bases in Aqueous Solution, Butterworths, London, 1965, and supplement, 1972) are therefore extremely useful in analytical, organic and inorganic chemistry and in biochemistry.
In contrast, in the Lewis definition there is no single reference that is naturally operational. Since there is no obvious reason to choose one reference rather than another, there are potentially as many acidity or basicity scales as possible references. Concerning basicity, however, which is the subject of this book, the statistical treatment of various scales of Lewis basicity, as well as theoretical studies, show that a limited number of factors influence the strength of Lewis bases. Consequently, the judicious choice of a few reference Lewis acids should allow Lewis basicity scales to be constructed and used as a general guide to basicity. Although none of these scales can be considered as universal, each will have a domain of validity that is sufficiently wide to be useful in many branches of chemistry and biochemistry.
It is with this objective that this book presents thermodynamic and spectroscopic data on the strength of Lewis bases coming from both the literature and our laboratories.We do not aim to provide exhaustive scales but rather a selective guide. From a mass of data, sometimes contradictory and often lacking consistency, we have chosen typical reference Lewis acids. These are SbCl5, BF3, 4-FC6H4OH, CH3OH, 4-NO2C6H4OH, 4-NO2C6H4NH2, I2, ICl, ICN, Li+, Na+, K+, Al+, Mn+, CpNi+, Cu+ and CH3NH+ 3 . This choice is justified in the first chapter. For each of these acids, we have selected only the data determined by the most accurate techniques and/or the most reliable methods. In cases of doubt, some measurements from the literature have been repeated in our laboratories. Additional measurements have also been carried out in order to fill significant gaps. Finally, data have been made homogeneous either by means of the usual thermodynamic relationships, in order to refer to the same standard state and to the same temperature, or by means of extrathermodynamic relationships, specifically established, in order to refer them to the same solvent. In all, more than 2400 equilibrium constants of acid/base reactions, and thus of Gibbs energies, about 1500 complexation enthalpies and, for spectroscopic scales, nearly 2000 infrared and ultraviolet shifts of absorption bands upon complexation are gathered together in this book.
