Zwitterion macro

Niflumic acid, which has two pKa values, was studied both pH-metrically and spectroscopically using the shake-flask method [224]. The monoprotonated species can exist in two forms (1) zwitterion, XH 1 and (2) ordinary (uncharged) ampholyte, XH°. The ratio between the two forms (tautomeric ratio) was measured spectroscopically to be 17.4. On assuming that a negligible amount of zwitterion XH partitions into octanol, the calculated micro-log/1 for XH° was 5.1, quite a bit higher than the macro-log/1 3.9 determined pH-metrically in 0.15 M NaCl. It is noteworthy that the distribution coefficient D is the same regardless of whether the species are described with microconstants or macroconstants [275]. 

This intermediate reacts with another molecule of BF3OEt2 forming a neutral molecule and Et03 +BF. Komratov et al. [158] discovered macro-zwitterions, apparently analogous to the type shown above, on the polymerization of THF through BF3 in the presence of methyloxirane. Depending on the chain length, temperature and polarity of the medium [157], both ends will either be free or will form an ion pair the latter may be of the contact or solvent-separated type. At 293 K, most active centres exist in the form of cyclic zwitterions [158]. 

Marino EL and Dominquez-Gil A, Determination of the macro- and micro-ionization constants of a dipolar zwitterionic cephalosporin cefadroxil, Int. J. Pharm., 8(1), 25-33 (1981). 

Codoy-Alcantar, C. Nelen, M.I. Eliseev, A.V. Yatsi-mirsky, A.K. Molecular recognition by natural macro- 25. cycles. Part II. Esterolytic activity and chiral discrimination of amino acid derivatives by the zwitterionic form... 

Materials studied include polyesters and polyethers [73], PMMA [74], star-shaped styrene block copolymers [75, 76], PC [77], isoprene-styrene graft copolymers [76], polyethylene oxide-maleic anhydride-vinyl methyl ether copolymers [78], polybutylene terephthalate [79], PE [80], (phenoxymethyl)thizrane [81], polytrimethylene carbonate [82], polybutylene succinate [83], polymethyldiundecenylsilane [84], poly(N-vinyl carbazole) [85], titanium-containing copolymers [86], styrene-macro zwitterion polymers [87], and poly(octadecene-alt-maleic anhydride) [88]. 

The two moles of the "genetic zwitterion" 1 react with each other to produce its dimer 2 which is the smallest species of propagation (Eq 2). Then, the propagation species grows by its reaction with 1 (Eq 3). Zwltterlons 2 and 3 (nil) are called "macro zwitterion", which are dlfferenciated from genetic zwitterion 1. The Intermolecular reaction between two moles of macrozwltterlon and the Intramolecular cycllzatlon of macro zwitterion are also possible, although the contribution of each process depends upon the natures of monomers, reaction conditions and the extent of monomers conversion. 

In addition, free monomers sometimes take part in propagation, i.e., Mn and Mg may react respectively with the cationic and anionic sites of zwitterion (genetic or macro)(Eqs A and 5). 

The Interaction between two zwltterions (genetic zwitterion or macro zwitterion) occurs via the opening of the oxazolinium ring of the cationic site (Eq 6). 

There is one possible way into this problem if it were to be thought worth pursuing. Figure 12.15a exhibits the familiar four-way equilibrium of zwitterion formation, for whose solution three equilibrium constants are required typically, these are the macro-pK values and fCg and a model for one of the micro-pFQs FCj -K4. If the monocation of Figure 12.15a is replaced by a dication as in 84 (Figure 12.15b), both macro-pK values should remain accessible, whereas K5, while not exactly repHcating K2, should provide an acceptable qualitative substitute for it If fCj and J<2 appear well separated, then the question is answered, though a more equivocal picture may well be found in this case and others. Here, the N-methylated lactam 127 has been chosen deUberately to avoid the tautomeric complications that... 

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