Picric acid anion protonation

Methanol is one of the easy solvents to work with using the electric-field-jump technique. The preparation of the solvent is not nearly as arduous as is that of some other solvents such as acetonitrile. In methanol we observed that picric acid anion protonates at the diffusion controlled rate whereas dipicryl-amine sterically hinders the proton from recombining with it. 

Actually, highly nucleophilic anions such as Cl" are deleterious for the synthesis of crown ether complexes, if they are not effectively stabilized by hydrogen bonding from protic solvents or other proton donors such as picric acid Anions that have been found useful for crown ether complex preparation are of soft HSAB character, e.g. SCN", CIO4, Br", I", and picrate. 

Although there is a wealth of data in the literature on acid-base behavior in aprotic solvents,60 61 there are few examples of the use of buffers for polarography and voltammetry in aprotic solvents. This has occurred because most investigators have sought to keep all potential proton donors out of the system and thereby stabilize anion radicals. Although the picric acid-picrate system has been used as a buffer in a number of studies in aprotic solvents, its use in voltammetric work is limited because of the ease of reduction of picric acid. 

Evidently the a-H is likely to be involved in the early stages of the decomposition process [16], perhaps moving to an adjoining nitro group to form a nitronic acid tautomer, 4 [35,45,46]. These are known to be reactive and unstable [47], The transfer or loss of a proton to yield a nitronic acid or a nitronate (aci) anion has also been invoked as the initial step in the decompositions of other energetic molecules, e.g. picric acid [35,48] and amine-sensitized nitromethane [49-51]. 

The crystal and molecular structure was determined of the picrates (174) of aniline (la), its A-Me derivatives, 1,2- (lj), 1,3- (lk) and 1,4-phenylenediamine (11). Except for lj, combining with two picric acid molecules, the rest show 1 1 stoichiometry. The main structural feature is H-bonding of various strengths, with O-N distances from 267 pm for A-methylaniline to 288 pm for lj. The acid proton in these compounds sits on the N atom forming ammonium salts, as shown by the Cl-O bond distances near 125 pm for a picrate anion, rather than 130 to 134 pm for picric acid or its tt-complexes with benzene and PAH. The picrates of N-methylaniline and lk also show jr-stacking to some extent, especially in the latter case266. 

To act as an efficient protonophoric uncoupler a weakly acidic compound must have properties that allow it, in both the uncharged protonated and the anionic deprotonated forms, to enter and cross the membrane lipid bilayer. The compound must have a suitable plQ such that on the more acidic, intermembrane side of the membrane a significant proportion is protonated, whilst on the less acidic matrix side a proportion is deprotonated. A compound that is not acidic enough may transfer a single proton across the membrane, but will not release it in the matrix, and hence cannot repeat the cyde. In contrast, too strong an acid will remain deprotonated even in the intermembrane space. For this reason 2,4-dinitrophenol (pJCa 4.04) is a stronger uncoupler than 2-nitrophenol (plQ 7.14) and picric acid (pKa 0.53) does not act as a protonophoric uncoupler in mitochondria, although all three compounds have a similar lipophilidty [88]. 

At the methylidene group carbon atom of 3 a relatively high electron density NMRCCDCls) 5 = 8S.5 ppm) is located which could be caused by the mesomeric electron-releasing effect of the silicon-substituted enamine nitrogen atom. Therefore this methylidene group carbon atom is able to pick up a proton from some acids. Even weak acids react with 3 if a stable Si-0 bond can be formed with the corresponding anion. 1,4-Addition of methanesulfonic acid, picric acid, benzoic acid and hydroquinone to 3 (solution in THE) led to hexacoordinate salen-silicon complexes by precipitation of compounds 4, 5, 6 and 7 respectively (Scheme 1). The Si chemical shifts of the solids obtained (CP/MAS-NMR) are presented in Table 2. 

While one might have expected a reinforcement of the anion binding with the protonation of the receptors XCU, XCTU, XCUE and XCA, the addition of a stoichiometric amount of a strong acid (e.g. picric acid) leads to a complete release of the anion and the further addition of a base restores cleanly the host-guest complex. This acid-base control of the binding properties is highly reminiscent of allosteric processes encountered in natural systems. Finally, the more flexible XtubeU and XtubeTU are unable to bind efficiently anions in chloroform... 

---