Proton transfer Subject

After these findings, evidence was presented that pyrazoles which crystalline as cyclic dimers 9a/9b and tetramers 11a are subject to double and quadruple proton transfers respectively (92JA9657 95JOC1965). An im-... 

Rates of addition to carbonyls (or expulsion to regenerate a carbonyl) can be estimated by appropriate forms of Marcus Theory. " These reactions are often subject to general acid/base catalysis, so that it is commonly necessary to use Multidimensional Marcus Theory (MMT) - to allow for the variable importance of different proton transfer modes. This approach treats a concerted reaction as the result of several orthogonal processes, each of which has its own reaction coordinate and its own intrinsic barrier independent of the other coordinates. If an intrinsic barrier for the simple addition process is available then this is a satisfactory procedure. Intrinsic barriers are generally insensitive to the reactivity of the species, although for very reactive carbonyl compounds one finds that the intrinsic barrier becomes variable. ... 

Up to this point, we have dealt with the subject of acid-base chemistry in terms of proton transfer. If we seek to learn what it is that makes NH3 a base that can accept a proton, we find that it is because there is an unshared pair of electrons on the nitrogen atom where the proton can attach. Conversely, it is the fact that the hydrogen ion seeks a center of negative charge that makes it leave an acid such as HC1 and attach to the ammonia molecule. In other words, it is the presence of an unshared pair of electrons on the base that results in proton transfer. Sometimes known as the electronic theory of acids and bases, this shows that the essential characteristics of acids and bases do not always depend on the transfer of a proton. This approach to acid-base chemistry was first developed by G. N. Lewis in the 1920s. 

The reaction of ammonia and hydrogen chloride in the gas phase has been the subject of several studies in the last 30 years [56-65], The interest in this system is mainly that it represents a simple model for proton transfer reactions, which are important for many chemical and biological processes. Moreover, in the field of atmospheric sciences, this reaction has been considered as a prototype system for investigation of particle formation from volatile species [66,67], Finally, it is the reaction chosen as a benchmark on the ability, of quantum chemical computer simulations, to realistically simulate a chemical process, its reaction path and, eventually, its kinetics. 

DR. EYRING Well, I think we are all conscious of the fact that Bell writes extensively on the subject of tunneling in connection with proton transfer. In fact, there is a recent book that was published within the last year that is on that particular topic [Bell, R. P. "The Tunnel Effect in Chemistry" Chapman and Hall London, 1980]. 

Directions for Future Work. The measurement of rates of proton transfer from a single acid to more bases differing only in thermodynamic base strength should allow the construction of BrjSnsted plots of kinetic versus thermodynamic acidity. The bases we have used at this early stage of development of the subject have involved different proton acceptor atoms and cannot be so used (although comparison of the Et N transfer rates of... 

It is a well established fact that the preferred site of protonation of aniline in solution is the nitrogen atom, due to the higher stabilization upon solvatation of the N-protonated species with respect to the ring protonated ion (/). In the gas phase however, the situation is less clear, and the preferred site of protonation has been the subject of many experimental and theoretical studies. Based on proton transfer equilibria at 600 K and upon correlation of the proton affinities... 

In 1903, Lapworth described his findings of the action of potassium cyanide on benzaldehyde [28], He postulated that cyanide adds to benzaldehyde to form V, followed by proton transfer of the a-labile hyd rogen, forming intermediate VI which is now referred to as an acyl anion equivalent. Addition to another molecule of benzaldehyde occurs to form VII (Scheme 1). The unstable cyanohydrin of benzoin VII then collapses to form benzoin and potassium cyanide. Additionally, Lapworth tested the reversibility of the addition of cyanide to benzaldehyde by first forming hydroxybenzyl cyanide (protonated variant of V) and subjecting it to benzaldehyde and base, in which benzoin was recovered. 

Polybrominated Biphenyls. The photolytic degradation of PBBs in solution has been the subject of several studies. Available data in the literature indicate that brominated biphenyls photodegrade by reduction in solvents capable of proton transfer with the formation of lower brominated biphenyls. For example, the irradiation of FireMaster BP-6 and 2,2, 4,4, 5,5 -hexabromobiphenyl in methanol at wavelengths >286 nm produced mainly penta- and tetrabromobiphenyl (Ruzo and Zabik 1975). FireMaster BP-6 photolyzed7 times faster than its chlorinated counterpart, 2,2, 4,4, 5,5 -hexachloro-biphenyl (Ruzo and Zabik 1975). Although an earlier study tentatively identified dimethoxy tetrabromobiphenyl as a photolysis product of FireMaster BP-6 (Ruzo and Zabik 1975), later work did not detect this compound (Ruzo et al. 1976). Earlier studies indicated that the debromination usually occurs with the stepwise preferential loss of bromine from the ortho and para positions of the biphenyl ring (i.e., 2, 2, 6, and 6 positions) (De Kok et al. 1977 Ruzo and Zabik 1975 Ruzo et al. 1976 Trotter 1977). Thus, the photolysis of 2,2, 4,4, 5,5 -hexachlorobiphenyl, the major component of FireMaster BP-6, would be expected to produce 23, 4,4, 5-pentabromobiphenyl and subsequently... 

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