Solvent effects bromide/pyridin

A full quantum-mechanical description of the Menshutkin reaction has been obtained for gas phase and solution by using density functional theory (DFT) and the self-consistent isodensity polarizable continuum model (SCI-PCM). Ammonia and pyridine were the nucleophiles and methyl chloride and methyl bromide, the electrophiles. In the gas phase an initial dipole complex intermediate is followed by a transition state leading to an ion pair. In the solvent-effect calculations, the dipole complex disappears with both cyclohexane and DMSO. The transition state is stabilized compared with the gas phase. The ion-pair product is strongly stabilized and in DMSO it is dissociated into free ions. 

Observed first-order rate constants for reaction of trans-[Rh(tn)2Cl2], tn = 1,3-propanediamine, with bromide are independent of bromide concentration, as indeed they are for all similar systems except one. The rate-determining step is replacement of the first chloride by water, with subsequent bromide anation and replacement of the second chloride quicker. Qualitative kinetic observations, including solvent effects, have been described for replacement of dialkyl sulfides by pyridine or bipyridyl in [Rh(SR2)3Cl3]. ... 

Under these standard reaction conditions, the acetonitrile/pyridine mixture can replace the DMF/pyridine one. This solvent mixture is also quite convenient for running the preparation of arylzinc halides. Yields are good to excellent (60-90%) and even higher than those obtained in DMF. However, with bromophenol, no organozinc species was formed in acetonitrile as observed in DMF. The formation of arylzinc species is also effective in a mixture of solvents of acetonitrile-DMF-pyridine (8/1/1) in the presence of C0CI2 (13%) as the catalyst precursor and zinc bromide (30%). This method has also been applied to the formation of organozinc halides from alkyl and alkenyl halides. So far, only low yields have been obtained using the standard reaction conditions in DMF-pyridine. Results are reported in Table 8. 

Separations of various anions Using a solvent mixture composed of n-butyl alcohol, pyridine, and 1-5m ammonia solution in the proportions of 2 1 2, the following Rf values are obtained for the sodium or potassium salts chloride, 0-24 bromide, 0-36, iodide, 0-47 chlorate, 042, bromate, 0-25, iodate, 0-09 nitrite, 0-25 nitrate, 0-40 arsenite, 019 arsenate, 0 05 phosphate, 0-04 and thiocyanate, 0 56. The positions of the anions may be detected by spraying with ammoniacal silver nitrate potassium iodide and hydrochloric acid are particularly effective for chlorates, bromates, and iodates. The RF values provide the basis for the separation of a number of mixtures of anions, e.g. chloride and iodide, bromide or iodide and nitrate. 

From the effect of solvent on the volumes of activation, it was argued that the zwitterion mechanism, (3) and (4), was in better agreement with the data . It is seen from Table 45 that for malonic acid, the volume of activation decreases as the polarity of the solvent decreases. A similar decrease in AF with decreasing solvent polarity was observed in the reaction of n-butyl bromide with pyridine to give N-... 

The quaternization of pyridine in non-polar solvents has been studied as a means of clarifying the much debated nature of the displacement reaction under such circumstances . Swain and Eddy i deduced evidence for their theory of specific solvation from the reaction of pyridine and methyl bromide in benzene containing various hydroxylic solutes. Swain and Langs-dorf45ii> found the Hammett plot for the reaction of substituted benzyl bromides with pyridine in acetone to be markedly concave, and indeed to fall into two separate lines for meta- and / zm-substituents. The curvature and the division illustrate the effects of substituents upon reactions of intermediate character. Ingold and his co-workers from reactions in sulphur dioxide,... 

---