Proton generation

The position of the second CC double bond in the structural fragment E follows finally from the correlation of the C signals at 5c = 37.8 and 49.8 with the //signals at 3h = 4.47 and 4.65. Note that trans protons generate larger cross-sectional areas than cis protons as a result of larger scalar couplings. 

Acylation of alcohols is often performed in the presence of an organic base such as pyridine. The base serves two purposes. It neutralizes the protons generated in the reaction and prevents the development of high acid concentrations. Pyridine also becomes directly involved in the reaction as a nucleophilic catalyst (see Section 8.5). 

The H NMR spectra of acetic acid and acetamide are quite different. The OH proton generates a single sharp peak at room temperature, while the NHi protons generate a broad, double-humped peak that turns into two sharp peaks at lower temperatures. This suggests that the NH, protons occupy different chemical environments, while the OH proton occupies a single environment. 

The reactivity of the closely related system TpMe2PtMeH2 toward electrophiles in arene solvents has also been reported recently (68). The boron-based Lewis acid B(C6F5)3 induced elimination of methane and formation of an aryl(dihydrido) platinum(IV) complex via arene C-H activation (Scheme 17, A -> C). The active acid may be either B(C6F5)3 or alternatively a proton generated from B(C6F5)3 and trace water. It was proposed that the acid coordinates to a pyrazole nitrogen (shown in Scheme 17, B) forming an intermediate five-coordinate platinum(IV) complex, which readily eliminates methane. 

The evolution of hydrogen at the cathode leads to the buildup of alkalinity the evolution of oxygen at the anode causes an increase in acidity because the protons generated by Eq. (2) immediately undergo hydration to yield H30+ entities. This is indeed experimentally observed. The chlorine evolution reaction (Eq. 2a) is central and concomitant to the evolution of oxygen. 

FIGURE 3.8 Deprotection of carboxyl groups by acid-catalyzed hydrolysis (A) of amides and (B) of esters. Protonation generates a relatively stable carbenium ion that usually requires heat to fragment it. 

Removal of one of the methylene protons generates a carbanionic center, but the corresponding single Lewis formula is a poor description of the electronic structure. More mesomeric forms of 2A may be written to give a more adequate fomu-lation. Alternatively, a circle may be drawn to symbolize the 3c2e n bond in 2A, resulting from overlapping p orbitals perpendicular to the plane of atoms involved. 

Many parameters can be monitored, for example, free-ion concentrations, membrane potentials, activities of specific enzymes, rate of proton generation, transport of signaling molecules, and gene expression. 

Protons, generated indirectly by deprotonation of an intermediate during anodic oxidation of an organic substrate, are obviously produced in stoichiometric amounts. Reactions induced by these protons are typically acid-catalyzed conversions of the initial oxidation product or proton-induced reactions/deactivation of unconverted substrate. 

Direct formation of protons by anodic oxidation of dihydrogen, H2, on activated Pt anodes allows the formation of well-defined quantities of acid [4, 5]. However, this method of proton generation has not been used in typical EGA-induced reactions. 

Rowlett and Silverman used a Brpnsted plot to examine the interaction of external buffers with human carbonic anhydrase II. The buffers act as proton acceptors in the removal of the proton generated by the enzyme-catalyzed reaction. The Brpnsted plot displays a plateau at a value of about 10 for the catalytic rate... 

Fig. 7.1. Three regimes of interaction in the hydrogen molecular ion. (a) At large distances, R>16 a.u., the. system can be considered as a neutral hydrogen atom plus a proton. The polarization of the hydrogen atom due to the field of the proton generates a van der Waals force, (b) At intermediate distances, 16>/ >4 a.u. the electron can tunnel to the vicinity of another proton, and vice versa. A resonance force is generated, which is either attractive or repulsive, (c) At short distances, R<4 a.u., proton-proton repulsion becomes important. (Reproduced from Chen, 1991c, with permission.)...

It is evident from the structure of the oxazol-5(4//)-one, also sometimes called an azlactone or oxazolinone, why it is so susceptible to epimerization. Removal of the a-proton generates a five-membered ring with six n-electrons, an aromatic system according to the Hiickel 4n + 2 rule (Scheme 5). 

A sulfur ylide is formed with base which then abstracts the a proton, generating dimethylsulfide and the aldehyde or ketone. 

Countercations influence the rate and selectivity of this reaction. The activity order, as for cations, was found to be Ag > Cu, H > Fe > Al > Pd > La > Zn (190). The distributions of product hydrocarbons were found to be similar to those observed for H3PW12O40 (Table XIX), suggesting similar reaction mechanisms. Ag and Cu salts of H3PW12O40 are much more active than the acid form catalyst. Protons generated by the reaction of Ag+ with H2 are presumed to give the more active catalyst (797). 

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