Reversible protonation/deprotonation

RATE COEFFICIENTS FOR PROTONATION (ki FORWARD) AND DEPROTONATION (Aij REVERSE) OF AZULENE IN AQUEOUS HCIO4. AT 7.5 °C197... 

As seen from Scheme 1.19, the cation-radicals transform into radicals that are more or less stable and can be protonated reversibly. If the radicals formed are unstable, they perish before protonation. If the initial cation-radicals have no hydrogen atoms, their stability appears to be higher. Deprotonation is typical for cation-radicals that contain proton-active hydrogen atoms and form, after deprotonation, either quite stable or, the reverse, quite unstable radicals. 

Systems. As already observed for methane (vide supra), with decreasing acidity it becomes more and more difficult to protonate reversibly C—H bonds. Nevertheless, when alkanes with more than two carbon atoms are used as starting material, carbenium ions are generated by competitive protolytic and oxidative processes. Depending on the strength of the superacid system, proton exchange can take place by two competitive reactions (i) directly via reversible protonation and (ii) via deprotonation of the carbenium ion and reprotonation of the alkene. 

In the dehydration the ketone is first converted into an enol by protonation and deprotonation. Protonation of the alcohol O is then followed by an El elimination reaction (loss of H2O and then loss of H+) to give the product. As in the formation of the enol ether discussed earlier, the principle of microscopic reversibility suggests that the ketone must be converted into an enol before El elimination occurs. 

The existence of surface hydroxyls is evidenced in the above schemes. These surface groups may be protonated (reverse of reaction(l)) or deprotonated (reaction (2)), according to the solution pH ... 

Scheme 9.42. A representation of the base-catalyzed aldol condensation reaction (using the hydroxide ion [OH ] as a typical base) between two equivalents of ethanal (acetaldehyde [CH3CHO]). The steps, both deprotonation-protonation and carbon-carbon bond forming are reversible (the retroaldol reaction). The carbon-carbon bond forming step is shown as occurring on the re face of the aldehyde and with an approximate Burgi-Dunitz trajectory. Attack at the si face is equally likely and the product is racemic.

Once formed the tetrahedral intermediate can revert to starting materials by merely reversing the reactions that formed it or it can continue onward to products In the sec ond stage of ester hydrolysis the tetrahedral intermediate dissociates to an alcohol and a carboxylic acid In step 4 of Figure 20 4 protonation of the tetrahedral intermediate at Its alkoxy oxygen gives a new oxonium ion which loses a molecule of alcohol m step 5 Along with the alcohol the protonated form of the carboxylic acid arises by dissocia tion of the tetrahedral intermediate Its deprotonation m step 6 completes the process... 

Chemical off—on switching of the chemiluminescence of a 1,2-dioxetane (9-benzyhdene-10-methylacridan-l,2-dioxetane [66762-83-2] (9)) was first described in 1980 (33). No chemiluminescence was observed when excess acetic acid was added to (9) but chemiluminescence was recovered when triethylamine was added. The off—on switching was attributed to reversible protonation of the nitrogen lone pair and modulation of chemically induced electron-exchange luminescence (CIEEL). Base-induced decomposition of a 1,2-dioxetane of 2-phen5l-3-(4 -hydroxyphenyl)-l,4-dioxetane (10) by deprotonation of the phenoHc hydroxy group has also been described (34). 

Reaction of the cyclopentadienyl rhodium and iridium tris(acetone) complexes with indole leads to the species 118 (M = Rh, Ir) [77JCS(D)1654 79JCS(D)1531]. None of these compounds deprotonates easily in acetone, but the iridium complex loses a proton in reaction with bases (Na2C03 in water, r-BuOK in acetone) to form the ri -indolyl complex 119. This reaction is easily reversed in the presence of small amounts of trifluoroacetic acid. 

The reversibility of aromatic diazotization in methanol may indicate that the intermediate corresponding to the diazohydroxide (3.9 in Scheme 3-36), i. e., the (Z)-or (is)-diazomethyl ether (Ar — N2 — OCH3), may be the cause of the reversibility. In contrast to the diazohydroxide this compound cannot be stabilized by deprotonation. It can be protonated and then dissociates into a diazonium ion and a methanol molecule. This reaction is relatively slow (Masoud and Ishak, 1988) and therefore the reverse reaction of the diazomethyl ether to the amine may be competitive. Similarly the reversibility of heteroaromatic amine diazotizations with a ring nitrogen in the a-position may be due to the stabilization of the intermediate (Z)-diazohydroxide, hydrogen-bonded to that ring nitrogen (Butler, 1975). However, this explanation is not yet supported by experimental data. 

Challis and Long497 have used the fast flow technique described above (p. 217) to measure the equilibrium protonation of azulene in a range of aqueous perchloric acid media at 7.5 °C and hence the rates of the forward protonation and reverse deprotonation, the overall exchange rate being the sum of these. Some representat i ve values are given in Table 141. Coupled with data obtained at other temperatures... 

Tetraene 4 (Scheme 1.3), when treated with 40 mol % of triflic acid in methylene chloride at -23 °C for 1 h, gives the adducts 5 and 6 in a 1 1 ratio as the main reaction products. The formation of these adducts has been justified [21] by a stepwise mechanism that requires an initial reversible protonation of 4 to produce the allyl cation 7, which then cyclizes to 8 and 9 in a non-reversible process. Deprotonation of 8 and 9 gives 5 and 6, respectively. 

Electrochemical methods have been used extensively to elucidate the mechanism of reduction of tetrazolium salts. In aprotic media, the first step is a reversible one-electron reduction to the radical 154 as confirmed by ESR spectroscopy.256,266 As shown in Scheme 26, this radical can then disproportionate to the tetrazolium salt and the formazan anion (166) or take up another electron to the formazan dianion (167). The formation of the dianion through a direct reduction or through the intermediate tetrazolyl anion (168) has also been proposed.272-28 1,294 In aqueous solutions, where protonation/deprotonation equilibria contribute to the complexity of the reduction process, the reduction potentials are pH dependent and a one-electron wave is seldom observed. 

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