Protonation, reaction

In the already discussed diaza-substituted redox systems, which are represented by the general formula 71, the reduced level 71 red is not only an azine but also contains two amidrazone units. It is therefore understandable that 71 red is easily stepwise protonated. (About the less important protonation of 7isEM cf. l.c. and 

In a solution containing more than 99.9% of violet to blue 77sem ( sem lO - 10 cf. 6.1.) a yellow to red colour is immediately produced by excess acid. With base the colour of 71 sem is so inunediately restored, a phenomenon which resembles that of a normal acid-base indicator. UV-spectra, however, discloK an acid-driven disproportionation which transforms 77sem quantitatively into 77qx and and 77 red H or 77 red 2H respectively. If the free azine 77 red is reformed by base it comproportionates with 77ox. buildup up the original concentration of 

Redox potentials of species sensitive to protons are pH-dependent according to the following Eq. (7) i. 

The intersections of the pH-dependent with the pH-independent parts of the curves in Fig. 12 mark the pK-value of the corresponding azine. With the very weakly basic 35 no protonation occurs even at pH 2.9. 

As already mentioned, dipyridnium salts I, together with 5- 7 cannot reversibly be reduced down in RED in water. Due to its high basicity, RED is probably proton-ated, whereby e.g. 74 may be formed, which as a dienamine could be consumed by consecutive reactions. 

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It has been detected spectroscopically in great abundance, especially in the hotter stars, and it is an important component in both the proton-proton reaction and the carbon cycle, which account for the energy of the sun and stars. 

A.ccekrator-Producedlsotopes. Particle accelerators cause nuclear reactions by bombarding target materials, which are often enriched in a particular stable isotope, with rapidly moving protons, deuterons, tritons, or electrons. Proton reactions are most commonly used for production purposes. 

Many organic reactions involve acid concentrations considerably higher than can be accurately measured on the pH scale, which applies to relatively dilute aqueous solutions. It is not difficult to prepare solutions in which the formal proton concentration is 10 M or more, but these formal concentrations are not a suitable measure of the activity of protons in such solutions. For this reason, it has been necessaiy to develop acidity functions to measure the proton-donating strength of concentrated acidic solutions. The activity of the hydrogen ion (solvated proton) can be related to the extent of protonation of a series of bases by the equilibrium expression for the protonation reaction. 

The close agreement of the three methods supports the contention that protonation at low temperatures first occurs at nitrogen and is followed by a proton shift to give the iminium salt (M). The rate of this rearrangement is dependent on temperature, the nature of the amine, and the nature of the carbonyl compound from which the enamine was made. Even with this complication the availability of iminium salts is not impaired since the protonation reaction is usually carried out at higher temperatures than —70°. Structurally complicated enamines such as trichlorovinyl amine can be readily protonated (17,18). 

Identify which protonation reaction alkene A protonated alkene A, alkene B protonated alkene B)... 

While a different explanation for the diastereoselection in these protonation reactions has been proposed, the stereochemical sense of protonation can be rationalized as arising from protonation of the chelated intermediate from the least hindered diastereotopic face of the nitronate anion (i.e. anti to the /i-methyl group)20-21. 

Increased acidity of this hydrogen atom may lead to self-protonation reactions with electrogenerated bases (in aprotic solvents or in unbuffered solutions). 

The molecular mechanism of the enantioselective protonation reaction by antibody 14D9 was revealed by a crystal structure analysis [19[. A catalytic carboxyl group AspH 101 was found at the bottom of the catalytic pocket and found to be necessary for catalysis by mutagenesis to Asn or Ala. The mechanism or protonation involves an overall syn addition of water to the enol ether in a chiral binding pocket ensuring complete enantioselectivity (Figure 3.4). 

These transfer reactions are interrelated by two successive protonation reactions in both O... 

The protonation reactions for ionizable molecules have been defined in Section 3.1. When a solute molecule, HA (or B), is in equilibrium with its precipitated form, HA(s) (or B(s)), the process is denoted by the equilibrium expression... 

Fig. 5.38 Reduction of 10-3m phenylglyoxylic acid at the mercury streaming electrode in acetate and phosphate buffers containing 1 m KN03 (1) pH 5.02, (2) pH 5.45, (3) pH 5.85, (4) pH 6.25. The curves 2, 3 and 4 are shifted by 0.2 V, 0.4 V and 0.6 V with respect to curve 1. The first wave is controlled by the surface protonation reaction while the second is a direct reduction of the acid anion. (According to J. Koryta)...

Although the reduction potentials argue for thymine, as the most easily reducable base in protic solvents like water, subsequent protonation reactions need to be considered as well. The coupling of single electron reduction with a subsequent protonation step will strongly affect the ease of single electron reduction. Table 2 contains the pKa-values of some nucleobases in their reduced and neutral states [37]. It is clear that the thymine radical anion, due to its rather neutral pKa-value of about 7 is unlikely to become pro-tonated either by water or by the adenine counter base in the DNA strand. 

In acidic aqueous solution, protonation reactions of tetracyanodioxotech-netate(V) give a complicated equilibration, leading to formation of [TcO(OHXCN)4]- and [TcO(H2OXCN)4] . At pH values less than 1, these monomer species are fairly stable, while at pH 2-5, [Tc203(CN)8]4 is formed rapidly. This complicated feature is seen in a plot of /cobsd against pH (Fig. 4). When thiocyanate ion is added to this system at pH 1, it replaces a water molecule or hydroxy group in the coordination site. 

Soil pH is the most important factor controlling solution speciation of trace elements in soil solution. The hydrolysis process of trace elements is an essential reaction in aqueous solution (Table 3.6). As a function of pH, trace metals undergo a series of protonation reactions to form metal hydroxide complexes. For a divalent metal cation, Me(OH)+, Me(OH)2° and Me(OH)3 are the most common species in arid soil solution with high pH. Increasing pH increases the proportion of metal hydroxide ions. Table 3.6 lists the first hydrolysis reaction constant (Kl). Metals with lower pKl may form the metal hydroxide species (Me(OH)+) at lower pH. pK serves as an indicator for examining the tendency to form metal hydroxide ions. 

Our study of these elementary reactions was extended to protonation reactions of unsaturated species (Fig. 26) 60). For protonation of ketones and aldehydes the... 

Cu is produced via a proton reaction on enriched 64Ni, followed by separation from the 64Ni target ... 

Derivatization of some mono(cyclopentadienyl) complexes to yield new monosubstituted species can often be accomplished by metathetical exchange (Equation (26)) or protonation reaction.295 Protonolysis of (CsPr 4H)Ca[N(SiMe3)2](THF) with several terminal alkynes HC CR in either toluene or hexanes produces the... 

In reaction (11) the metal-hydride addition suggests a protonation reaction whereas, in reaction (12) the addition appears to be a hydride transfer reaction. If the reaction is indeed a hydride transfer reaction then the introduction of p-electron donating substituents, which place more electron density at the carbonyl carbon, (the site of hydride attack) will inhibit hydride addition. The data in Table 2 show that the introduction of p-electron donating substituents reduces the turnover frequency. This is consistent with hydride attack at the benzaldehyde carbonyl carbon, (12). 

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