Nitrogen, protonated

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... 

It is noteworthy that the kinetics indirectly provided the evaluation of the basicities of these enamines [Eq. (4)]. The pK values for 4-(2-methyl-propenyl)morpholinc, l-(2-methylpropenyl)piperidine, and l-(2-methyl-propenyl)pyrrolidine are 5.47, 8.35, and 8.84, respectively (27). Since the protonation of the j8-carbon atom does not possess the character of a real equilibrium at pH 7 and up [for compound 1 even at pH 1 and up] the basicity must be fully ascribed to the equilibrium between enamine and the corresponding nitrogen-protonated conjugate acid. 

Unlike imino nitrogen, protonated carbon nuclei are abundant throughout the molecules of nucleic acids providing many sites as possible probes into the molecular motions. How-... 

Knowing the equilibrium constants for the protonation of the amino and imidazole nitrogens, and the kinetics of deprotonation of the C(2) carbon of imidazole, it was possible to investigate nitrogen-protonation microequilibria and the C(2)-deprotonation microkinetics of histidine, histamine and other related compounds164. 

Crepaux, D., Lehn, J. M., Dean, R. R. Nuclear Spin-Spin Interactions. X. Signs of Geminal and Vicinal Nitrogen-Proton Coupling Constants. Stereochemistry and Medium Effects on NH Couplings. Mol. Phys. 16, 225 (1969). 

Amino substituents in acyclic derivatives have been discussed by Eggert and Djerassi (396), who emphasize structural and conformational effects, whereas Batchelor has investigated SCS(NH2) and nitrogen protonation shifts in methylated cyclohexylamines (424). The, 3C NMR spectra of amino acids have been compared with those of amines and carboxylic acids (425,426). The transmission mechanisms of amino, ammonium, trimethylammonium, acetamido, and di-acetamido groups have been examined by Faure and co-workers (427), the SCSs of nitro groups by Ejchart (400), and those of azido functions in steroids by Lukacs and co-workers (428). 

Enolization and ketonization kinetics and equilibrium constants have been reported for phenylacetylpyridines (85a), and their enol tautomers (85b), together with estimates of the stability of a third type of tautomer, the zwitterion (85c). The latter provides a nitrogen protonation route for the keto-enol tautomerization. The two alternative acid-catalysed routes for enolization, i.e. O- versus Af-protonation, are assessed in terms of pK differences, and of equilibrium proton-activating factors which measure the C-H acidifying effects of the binding of a proton catalyst at oxygen or at nitrogen. 

Some other ten rr-electron N-heterocycles, but with a nitrogen atom shared by two rings, also present two or more conjugated nitrogen protonation sites. One such system is imidazo[l,2-a] pyridine (1-azaindolizine, [98]), which may be viewed as an... 

The direct nitration of imidazole with acidic reagents is difficult due to facile nitrogen protonation (pA aH 7). Nitration of imidazoles proceeds in the 4- and 5-positions with the amidine 2-position being quite inert. Imidazole can be directly nitrated to 4,5-dinitroimidazole but no further. 2,4,5-Trinitroimidazole (TNI) can be prepared from the successive nitration of 2-nitroimidazole the latter synthesized from the diazotization of 2-aminoimidazole in the presence of excess sodium nitrite and a copper salt. The nitrative cleavage of polyiodoimidazoles also provides a route to polynitroimidazoles. " ... 

Pre-eminent amongst examples is the case of amides, which do not show the typical basicity of amines. Acetamide, for example, has pATa — 1.4, compared with a 10.7 in the case of ethylamine. This reluctance to protonate on nitrogen is caused by delocalization in the neutral amide, in which the nitrogen lone pair is able to overlap into the n system. This type of resonance stabilization would not be possible with nitrogen protonated, since the lone pair is already involved in the protonation process. Indeed, if amides do act as bases, then protonation occurs on oxygen, not on nitrogen. Resonance stabilization is still possible in the D-protonated amide, whereas it is not possible in the A-protonated amide. Note that resonance stabilization makes the D-protonated amide somewhat less acidic than the hydronium ion (pATa — 1.7) the amide oxygen is more basic than water. 

The proton remaining in the 5-position is evidently very unreactive. Metallation attempts with AgNOs lead only to the substitution of a nitrogen proton, as can be shown by the following ethylation ... 

Prolonalion on the ring N(2) nitrogen. Protonation on the amino group. Acidity of the hydroxy group. 

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