Positional isomery

Orts-funktion, /. position ftmction. -isomerie, /. place isomerism, position isomerism, Ortsteln, m, Min.) bog iron ore hardpan, ortsverinderlich, a. movable, portable, Orts-verinderung,/. change of position, -wech-sel, m. change of position or location, -zahl, /. position number, index number. 

Selten-gummi, n. side wall (of a rubber tire), -isomerie, /. chain isomerism, -kante, /. lateral edge, -kette, /. side chain, -ketten-isomerie, /. side-chain isomerism, -kraft, /. component force, component, -lage,/. lateral position, -l ge, /. length of a side, lateral length, -riss, m. side elevation. -lohr, n., -rohre,/. side tube, branch tube, branch pipe, seitens, adv. on behalf (of), on the part (of), seltenstandlg, a. lateral. 

The rapid formation of the (Z)-diazoate is followed by the slower (Z/J )-isomeri-zation of the diazoate (see Scheme 5-14, reaction 5). Some representative examples are given in Table 5-2. Both reactions are first-order with regard to the diazonium ion, and the first reaction is also first-order in [OH-], i.e., second-order overall. So as to make the rate constants k and k5 directly comparable, we calculated half-lives for reactions with [ArNj ]0 = 0.01 m carried out at pH = 9.00 and 25 °C. The isomerization rate of the unsubstituted benzenediazonium ion cannot be measured at room temperature due to the predominance of decomposition (homolytic dediazoniations) even at low temperature. Nevertheless, it can be concluded that the half-lives for (Z/ )-isomerizations are at least five powers of ten greater than those for the formation of the (Z)-diazohydroxide (reaction 1) for unsubstituted and most substituted benzenediazonium ions (see bottom row of Table 5-2). Only for diazonium ions with strong -M type substituents (e.g., N02, CN) in the 2- or 4-position is the ratio r1/2 (5)/t1/2 (1) in the range 6 x 104 to 250 x 104 (Table 5-2). 

Aufgrund dieser Isomerie ist der genaue Substitutionsort bei Benzofurazan-1-oxiden (und ver-wandten Systemen) haufig nicht festzulegen ein als 4-substituiertes (4,5-disubstituiertes etc.) Benzofurazan-1 -oxid kann den Substituenten auch in Position 7 (bzw. 6, 7 etc.) tragen. 

Might the observations of the remaining oxygen-substituted MIPT s provide additional clues There are four possible mono-methoxylated MIPT s all have been synthesized and all have been explored in man. The 4-methoxy-isomer was of modest activity and deserves, and has received, a recipe of its own. The 5-methoxy-isomeris the one described here, and is extremely potent (orally, but less so parenterally). But as one goes to the 6- and 7-positional isomers, the two remaining positions, the psychopharmacological activity seems to be lost. This is... 

Dissociative processes were also implicated in HCl cleavage reactions of ci5-[PtMeR(PEt3)2] (R = methyl, phenyl, or mesityl) and their subsequent isomeri-zations. The cleavage mechanism is believed to involve a fast preequilibrium between substrate and chloride, combined with a parallel protonation of both platinum species, where the proton attacks the Pt—Me bond (Scheme 3). The spontaneous isomerization of the solvated intermediates ci5-[PtR(S)(PEt3)2] was followed either by P NMR or UV/visible spectroscopy. Dissociative loss of the solvent molecule, S, was implicated because of the relative insensitivity of the reaction rate to the steric bulk of R, and because of the positive values of AS. ... 

The same arguments can be applied to other energetically facile interconversions of two potential reactants. For example, many organic molecules undergo rapid proton shifts (tautomerism) and the chemical reactivity of the two isomers may be quite different. It is not valid, however, to deduce the ratio of two tautomers on the basis of subsequent reactions which have activation energies greater than that of the tautomerism. Just as in the case of conformational isomeri, the ratio of products formed in subsequent reactions will not primarily be controlled by the position of the facile equilibrium. 

Labeling experiments earried out with a-deuterated -styryl(phenyl)iodonium tetrafluoroborate 5 resulted in the same 85 15 E/Z isomerie mixture of alkenes 6. However, whereas the Z-isomer 6 retained the label at the original a-position, scrambling of deuterium between both the a and the P positions was detected in the ii-isomer (Scheme 35.2). The analysis of the H-NMR spectrum of the product mixture showed that the deuterium was distributed equally both at the a and P positions of the ii-isomer 6. Unlabeled phenylacetylene was also formed as byproduct in these experiments. 

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