The Protonic Concept

A more general definition was suggested independently by Bronsted and Lowry . Proton-transfer reactions are considered as responsible for boijh the self-ionization of the amphoteric solvent molecules and for most acid-base reactions in their solutions. Acids and bases are defined as proton donors and proton acceptors respectively and acid-base reactions are regarded as being due to proton transfer reactions (protolysis). The most significant difference from the Arrhenius definition is that the proton itself is neither acid nor base. Even the solvent molecules can act either as acids or bases, a phenomenon which is responsible for the autoprotolysis of the pure liquid solvents. 

The activity of the hydrated protons is a quantitative measure of the acidity of the solution and it can be measured by electrochemical methods with a high degree of accuracy. Another advantage is the fact that ionization of a compound, neutralization and hydrolysis (or more general solvolysis) are all considered as protolytic reactions. When is the ionic product of water or of the protonic solvent used and Ka the dissociation constant of the acid, 

This concept is clearly not restricted to aqueous systems, but can also be applied to protolytic reactions in other systems, such as in solutions of liquid 

The asymmetry in the definition has been critisized. The acidic function is limited to a compound or ion that contains hydrogen in an ionizable form, whilst any substance or ion capable of combining with a proton is exerting a basic function. 

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The protonic concept of acid and bases is applicable to many of these high lemperatures solvent systems such as the fused ammonium salts which possess the oniunt ion or solvated proton, and the fused anionic acids which arc salts possessing a metallic ion and a hydrogen containing anion. One of the most useful of the anionic acids is KHF which is used to dissolve ore minerals containing silica, tilania and other refractory oxides. 

Mechanism of acid-base catalysis It is accepted that acid-base catalysis involves a reversible acid-base reaction between the substrate and catalyst. This is in agreement with the protonic concept of acids and bases, since acid catalysis depends on the tendency of the acid to lose a proton, while base catalysis depends upon the tendency of the base to gain a proton. The mechanism of reaction involving H and OH- ion catalysis may be expressed as follows, by taking the example of hydrolysis of esters. 

A rigidized molecule obtained when the two a-carbons of the trimethine chain are linked by a dimethylene bridge cannot be planar. According to the resonance concept, its stability increases as a bathochromic effect of 41 nm is observed (122). The of the bistyryl dye obtained by the substitution of the -proton in the chain of a styryl dye by a dialkylamino group is nearly the same as for the parent styryl dye (123). 

It has been revealed that the formation of protonic acid sites from molecular hydrogen is observable for the catalysts other than Pt/S042--Zr02, and the protonic acid sites thus formed act as catalytically active sites for acid-catalyzed reaction. We propose the concept "molecular hydrogen-originated protonic acid site" as a widely applicable active sites for solid acid catalysts. 

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. 

This theory was advanced by G. N. Lewis (1916, 1923, 1938) as a more general concept. In his classic monograph of 1923 he considered and rejected both the protonic and solvent system theories as too restrictive. An acid-base reaction in the Lewis sense means the completion of the stable electronic configuration of the acceptor atom of the acid by an electron pair from the base. Thus ... 

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. 

First, we shall discuss reaction (5.7.1), which is more involved than simple electron transfer. While the frequency of polarization vibration of the media where electron transfer occurs lies in the range 3 x 1010 to 3 x 1011 Hz, the frequency of the vibrations of proton-containing groups in proton donors (e.g. in the oxonium ion or in the molecules of weak acids) is of the order of 3 x 1012 to 3 x 1013 Hz. Then for the transfer proper of the proton from the proton donor to the electrode the classical approximation cannot be employed without modification. This step has indeed a quantum mechanical character, but, in simple cases, proton transfer can be described in terms of concepts of reorganization of the medium and thus of the exponential relationship in Eq. (5.3.14). The quantum character of proton transfer occurring through the tunnel mechanism is expressed in terms of the... 

If one wishes to obtain a fluorine NMR spectrum, one must of course first have access to a spectrometer with a probe that will allow observation of fluorine nuclei. Fortunately, most modern high field NMR spectrometers that are available in industrial and academic research laboratories today have this capability. Probably the most common NMR spectrometers in use today for taking routine NMR spectra are 300 MHz instruments, which measure proton spectra at 300 MHz, carbon spectra at 75.5 MHz and fluorine spectra at 282 MHz. Before obtaining and attempting to interpret fluorine NMR spectra, it would be advisable to become familiar with some of the fundamental concepts related to fluorine chemical shifts and spin-spin coupling constants that are presented in this book. There is also a very nice introduction to fluorine NMR by W. S. and M. L. Brey in the Encyclopedia of Nuclear Magnetic Resonance.1... 

Ans. (a) and (b) 17 (all the protons arc in the nucleus, so it is not necessary to specify the nucleus). Here are two questions which sound different, but really arc the same. Again, you must read the questions carefully. You must understand the concepts and the terms and you must not try to memorize. 

The same system was used by Frechet s group of to achieve a multicomponent one-pot cascade reaction with mutually interfering acid and proline-derived pyrrolidine catalysts [31]. The concept is illustrated in Figure 5.1. The protonation of imidazo-lidone (3) by the immobilized PSTA (5) gives the desired iminium catalyst (6), while... 

The relative ease with which proton transfer is accomplished is responsible for the importance of the generalized acid-base concept in solution chemistry. The Br0nsted concept of acidity is most useful in this respect. Br0nsted defined an acid as a species that tends to give up a proton and a base as a species that tends to accept a proton. In this sense any proton transfer process having the general form... 

The active site is viewed as an acid-base, cation-anion pair, hence, the basicity of the catalyst depends not only on the proton affinity of the oxide ion but also on the carbanion affinity of the cation. Thus, the acidity of the cation may determine the basicity of the catalyst. Specific interactions, i.e., effects of ion structure on the strength of the interaction, are likely to be evident when the carbanions differ radically in structure when this is likely the concept of catalyst basicity should be used with caution. 

According to the Arrhenius theory of acids and bases, the acidic species in water is the solvated proton (which we write as H30+). This shows that the acidic species is the cation characteristic of the solvent. In water, the basic species is the anion characteristic of the solvent, OH-. By extending the Arrhenius definitions of acid and base to liquid ammonia, it becomes apparent from Eq. (10.3) that the acidic species is NH4+ and the basic species is Nl I,. It is apparent that any substance that leads to an increase in the concentration of NH4+ is an acid in liquid ammonia. A substance that leads to an increase in concentration of NH2- is a base in liquid ammonia. For other solvents, autoionization (if it occurs) leads to different ions, but in each case presumed ionization leads to a cation and an anion. Generalization of the nature of the acidic and basic species leads to the idea that in a solvent, the cation characteristic of the solvent is the acidic species and the anion characteristic of the solvent is the basic species. This is known as the solvent concept. Neutralization can be considered as the reaction of the cation and anion from the solvent. For example, the cation and anion react to produce unionized solvent ... 

If we use the Bronsted concept of an acid as a proton donor and a base as a proton acceptor, consideration of acid-base catalysis may be extended to solvents other than water (e g., NH CH3COOH, and SO,). An acid, on donating its proton, becomes its conjugate base, and a base, on accepting a proton, becomes its conjugate acid ... 

Equilibria involving acids and bases are discussed from within the Lowry-Br0nsted theory, which defines an acid as a proton donor and a base as a proton acceptor (or abstracter ). The additional concept of pH is then introduced. Strong and weak acids are discussed in terms of the acidity constant Ka, and then conjugate acids and bases are identified. 

EL pyridine-phenylene copolymers 564 [666] and 565 [667] have been synthesized and studied by Bryce and coworkers. Although a rather low el(<0.1%) was reported for the devices, an interesting phenomenon was found for polymers 565. When the PLED (ITO/PEDOT/565/Ca/Al) was fabricated using acidic solutions, a strong red shift in the EL band compared to that obtained with the neutral solution (from 510 to 575 nm) was observed. The authors explained this concept by planarization of the protonated polymer chain as a result of intramolecular hydrogen bonding N H- O. Variation of pyridine linkage in copolymers 565, 566, and 567 affects the PL and EL emissions (AEL = 444, 432,... 

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