Proton species

Several different mechanism involving either the free base or the protonated species according to the medium. 

According to a kinetic study which included (56), (56a) and some oxaziridines derived from aliphatic aldehydes, hydrolysis follows exactly first order kinetics in 4M HCIO4. Proton catalysis was observed, and there is a linear correlation with Hammett s Ho function. Since only protonated molecules are hydrolyzed, basicities of oxaziridines ranging from pii A = +0.13 to -1.81 were found from the acidity rate profile. Hydrolysis rates were 1.49X 10 min for (56) and 43.4x 10 min for (56a) (7UCS(B)778). O-Protonation is assumed to occur, followed by polar C—O bond cleavage. The question of the place of protonation is independent of the predominant IV-protonation observed spectroscopically under equilibrium conditions all protonated species are thermodynamically equivalent. 

The reason for the slow hydrolysis compared to that of structurally similar compounds like nitrones or 0,lV-acetals might be the following (b-67MI50800) in the protonated species (77) assistance of the lone pair of electrons at nitrogen is sterically hindered due to the large angle of its orbital to the plane of the ring. 

The slope of the straight lines in Fig. 1 is pH-dependent. This has been explained on the ground of an equilibrium between the free enamine and the nitrogen-protonated species. This acid-base equilibrium is built up very rapidly [Eq. (3)], and causes a decrease in concentration of the reactive... 

The double bond of the nitrogen-protonated species is stable with respect to electrophilic attack under the reaction circumstances, since the free... 

Many protonic species react with liberation of hydrogen ... 

Reduction of the pH of solutions of carbonylate anions yields a variety of protonated species and, from acid solutions, carbonyl hydrides such as the unstable, gaseous H2Fe(CO)4 and the polymeric liquids H2Fe2(CO)g and H2Fe3(CO)n are liberated. The use of ligand-replacement reactions to yield hydrides of higher nuclearity has already been noted. 

In the hydrolysis of acridine antimalarials, the role of the protonated species of the substrate appeared to be important even in aqueous solution buffered at pH 7.3, i.e., under conditions of physiological interest.Moreover, out of the three possible modes of reaction mathematically possible (H3O+-HB, HgO-f BH+, and OH -f BH2+, where B is the basic substrate) the one not involving the protonated substrate can be ruled out on structural grounds. 

The potential surface of protonated pyrrole (see Scheme 5) was analyzed for minimums and saddle points with the CNDO/2-FK and MNDO/2 methods. The a-protonated species 11 was found to be the most stable one in this series (80JPR147). 

Figure 12.9 MALDI-TOF mass spectrum of chicken egg-white lysozyme. The peak at 14,307.7578 daltons (amu) is due to the monoprotonated protein, M+H+, and that at 28,614.2188 daltons is due to an impurity formed by dimerization of the protein. Other peaks are various protonated species, M+H rH ...

The 2-position is largely unreactive toward electrophiles, but nucleophilic substitution occurs there with some facility, especially in acidic medium. The protonated species is about 20 times more reactive than the neutral molecule (70BSF2705). Exhaustive chlorination in the presence of antimony trichloride gave pentachlorobenzothiazole (64GEP1168911). Direct chlorination of the parent heterocycle with aluminium or ferric... 

The mechanism (strong H2S04 solvent) was originally thought to involve the protonated species (VIII) rather than the alternative (IX) since logarithms of rates... 

Sometimes we need to know how the concentrations of the ions present in a solution of a polyprotic acid vary with pH. This information is particularly important in the study of natural waters, such as rivers and lakes (Box 10.1). For example, if we were examining carbonic acid in rainwater, then, at low pH (when hydronium ions are abundant), we would expect the fully protonated species (H2C03) to be dominant at high pH (when hydroxide ions are abundant), we expect the fully deprotonated species (C032 ) to be dominant at intermediate pH, we expect the intermediate species (HC03, in this case) to be dominant (Fig. 10.20). We can verify these expectations quantitatively. 

FIGURE 10 JO The fractional composition of the species in carbonic acid as a function of pH. Note that the more fully protonated species are dominant at lower pH. 

Al(III) is an example of an aquatic ion that forms a series of hydrated and protonated species. These include AlOrf Al(OH)J, Al(OH)3, and other forms in addition to AP. (For simplicity, we omit the H2O molecules that complete the structures of these complexes.) Most of these species are amphoteric (able to act as an acid or a base). Thus the speciation of Al(III) and many other aquatic ions is sensitive to pH. In this case, an aggregate variable springs from the conservation of mass condition. In the case of dissolved aluminum, the total dissolved aluminum is given by... 

The third possibility is that the molecular species is m/z 163 and that m/z 195 and m/z 217 are both adducts. If this is the case, it must be possible to explain the differences of 32 Da and 54 Da easily. Can this be done Commonly occurring adducts in electrospray involve the mobile phase and either sodium (relative molecular mass (RMM) 23) or potassium (RMM 39). The simplest interpretation of this spectrum is that the molecular weight of the analyte is 162, the ion at m/z 163 is the protonated species, that at m/z 195 is (M - - H - - CH30H)+ and that at m/z 217 is (M-h Na-h CH30H)+. Since the HPLC mobile phase contained methanol (molecular weight of 32), this is not an unreasonable conclusion. 

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