Proton transfer definition

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. 

A1C13, or S02 in an inert solvent cause colour changes in indicators similar to those produced by hydrochloric acid, and these changes are reversed by bases so that titrations can be carried out. Compounds of the type of BF3 are usually described as Lewis acids or electron acceptors. The Lewis bases (e.g. ammonia, pyridine) are virtually identical with the Bransted-Lowry bases. The great disadvantage of the Lewis definition of acids is that, unlike proton-transfer reactions, it is incapable of general quantitative treatment. 

The products of proton transfer in aqueous solution may also react with water. For example, the CIST ion produced when HCN loses a proton to water can accept a proton from a water molecule and form HCN again. Therefore, according to the Bronsted definition, CN is a base it is called the conjugate base of the acid HCN. In general, a conjugate base of an acid is the species left when the acid donates a proton ... 

The Bronsted definitions of acids and bases are more general than the Arrhenius definitions they also apply to species in nonaqueous solvents and even to gas-phase reactions. For example, when pure acetic acid is added to liquid ammonia, proton transfer takes place and the following equilibrium is reached ... 

Bronsted-Lowry definition A definition of acids and bases in terms of the ability of molecules and ions to participate in proton transfer. 

In Chapter H, we introduce a second definition of acids and bases, the Lewis definition, which focuses attention on electron movement rather than proton movement Until then, acid-base always means proton transfer."... 

Any reaction in which a proton is transferred from one substance to another is an acid-base reaction. More specifically, the proton-transfer view is known as the Bronsted-Lowiy definition of acids and bases. In an acid-base reaction, an acid donates a proton, and a base accepts that proton. Any species that can give up a proton to another substance is an acid, and any substance that can accept a proton from another substance is a base. The production of two water molecules from a hydroxide anion (a base) and a hydronium ion (an acid) is just one example of an acid-base reaction acids and bases are abundant in chemistry. 

In both cases the nitrogen atom uses its pair of nonbonding electrons to make a new covalent bond. This similarity led G. N. Lewis to classify ammonia as a base in its reaction with B (CH3)3 as well as in its reaction with H3 O . Whereas the Br< )nsted definition focuses on proton transfer, the Lewis definition of acids and bases focuses on electron pairs. 

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. 

Although the concepts of specific acid and specific base catalysis were useful in the analysis of some early kinetic data, it soon became apparent that any species that could effect a proton transfer with the substrate could exert a catalytic influence on the reaction rate. Consequently, it became desirable to employ the more general Br0nsted-Lowry definition of acids and bases and to write the reaction rate constant as... 

As Skinner has pointed out [7], there is no evidence for the existence of BFyH20 in the gas phase at ordinary temperatures, and the solid monohydrate of BF3 owes its stability to the lattice energy thus D(BF3 - OH2) must be very small. The calculation of AH2 shows that even if BFyH20 could exist in solution as isolated molecules at low temperatures, reaction (3) would not take place. We conclude therefore that proton transfer to the complex anion cannot occur in this system and that there is probably no true termination except by impurities. The only termination reactions which have been definitely established in cationic polymerisations have been described before [2, 8], and cannot at present be discussed profitably in terms of their energetics. It should be noted, however, that in systems such as styrene-S C/4 the smaller proton affinity of the dead (unsaturated or cyclised) polymer, coupled, with the greater size of the anion and smaller size of the cation may make AHX much less positive so that reaction (2) may then be possible because AG° 0. This would mean that the equilibrium between initiation and termination is in an intermediate position. 

The most unexpected features of our results are the slow increase of conductivity after the polymerisations of isobutylene to a definite, stable maximum and the finding of approximately one tritium atom, i.e., one C-Al bond, per polymer molecule. These observations are puzzling because at first thought it appears that only those polymer molecules which had been started by initiation (and not those started by proton transfer)... 

In 1923 the American chemist G.N. Lewis provided a broad definition of acids and bases, which covered acid-base reactions not involving the traditional proton transfer an acid is an electron-pair acceptor (Lewis acid) and a base is an electron-pair donor (Lewis base). The concept was extended to metal-ligand interactions with the ligand acting as donor, or Lewis base, and the metal ion as acceptor, or Lewis acid. 

These similarities indicate that the mechanism for (188) is apparently exactly the same as that for (135) except that the attack of water occurs on a sulfonyl group in (188), instead of on a sulfinyl group as in (135), and that a proton transfer is also part of the rate-determining step of the spontaneous hydrolysis of cr-disulfones. It may be recalled that in the case of the spontaneous hydrolysis of sulfinyl sulfones we determined that the purpose of the proton transfer was either to assist the attack of a water molecule on the substrate (136) or to assist the departure of the ArSOz group (137), but could not make a definite decision between the two alternatives from the information available. Thus the mechanism for the spontaneous hydrolysis of cr-disulfones is either as in (189) (where attack of water on a sulfonyl group is aided by the removal of a... 

The study of chemical reactions requires the definition of simple concepts associated with the properties ofthe system. Topological approaches of bonding, based on the analysis of the gradient field of well-defined local functions, evaluated from any quantum mechanical method are close to chemists intuition and experience and provide method-independent techniques [4-7]. In this work, we have used the concepts developed in the Bonding Evolution Theory [8] (BET, see Appendix B), applied to the Electron Localization Function (ELF, see Appendix A) [9]. This method has been applied successfully to proton transfer mechanism [10,11] as well as isomerization reaction [12]. The latter approach focuses on the evolution of chemical properties by assuming an isomorphism between chemical structures and the molecular graph defined in Appendix C. 

According to the Br0nsted-Lowry definitions, any species that contains hydrogen can potentially act as an acid, and any compound that contains a lone pair of electrons can act as a base. Therefore, neutral molecules can also act as bases if they contain an oxygen, nitrogen or sulphur atom. Both an acid and a base must be present in a proton transfer reaction, because an acid cannot donate a proton unless a base is present to accept it. Thus, proton-transfer reactions are often called acid-base reactions. 

A Bronstcd-I.owry aeid is defined as a proton donor, a Bronsted-Loory base as a proton acceptor. The definitions apply generally toprotic systems those in which proton transfers can occur. A general equation expressing proton transfer in aqueous solution is ... 

The Brensted definition also includes the possibility that an ion is an acid. For instance, a hydrogen carbonate ion, HC03, one of the species present in natural waters, can act as a weak proton donor and in water, it takes part in a proton transfer equilibrium (Fig. 10.3) ... 

More recently, Apeloig and Nakash have studied diastereoselectivity in the reaction of (E)-5 with p-methoxyphenol53. In both benzene and THF, the stereochemistry of the products was independent of the phenol concentration. The syn/anti ratios of the addition products were 90 10 in benzene and 20 80 in THF. They have suggested that intramolecular proton transfer after rotation of the Si—Si bond of the phenol-coordinated intermediate is responsible for the formation of the anti-addition rather than intermolecular proton transfer. This must be a special case due to much slower (by a factor of 109-1012) rates of addition of phenol to (E)-5. Since phenolic oxygen is definitely less basic than alkyl alcoholic oxygen, coordination of oxygen in the zwitterionic intermediate in the reaction of (E)-5 with phenol must be loose and hence the intermediates should have much chance of rotation around the Si—Si bond. 

It is important to note, though, that for these reactions the situation is quite different from that in Equations (33a), (34a), and (36). In pericyclic reactions aromaticity is mainly a special characteristic of the transition state whereas the reactants and products are not aromatic or less so than the transition state. This is quite different from the proton-transfer reactions discussed in this chapter where the aromaticity of the transition state is directly related to that of the reactants/products. An analogy with steric effects on reaction barriers may illustrate the point. In a reaction of the type of Equation (38), steric effects at the transition state will definitely increase the intrinsic... 

A quantitative method to determine the strength of Lewis acids and to establish similar scales as discussed in the case of Brpnsted acids would be very useful. However, establishing such a scale is extremely difficult and challenging. Whereas the Brpnsted acid-base interaction always involves proton transfer, which allows a meaningful quantitative comparison, no such common relationship exists in the Lewis acid-base interaction. The result is that the definition of strength has no real meaning with Lewis acids. 

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