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The reactions of methyl 4-nitrophenyl, ethyl 4-nitrophenyl, and ethyl 2,4-dinitrophenyl thionocarbonates (MNPTOC, ENPTOC, and EDNPTOC, respectively) with a series of 3- and 4-substituted pyridines are subjected to a kinetic investigation in water, 25.0 degrees C, ionic strength 0.2 M (maintained with KCl). Under amine excess, pseudo-first-order rate coefficients (k(obsd)) are obtained, which are linearly proportional to the free-pyridine concentration. The second-order rate coefficients (k(N)) are obtained as the slopes of these plots. The Brönsted-type plots found for the two mononitro derivatives coincide in one straight line (same slope and intercept) of slope beta = 1.0. The EDNPTOC pyridinolysis shows a curved Brönsted-type plot with slopes beta(1) = 0.1 (high pK(a)), beta(2) = 1.0 (low pK(a)), and pK(a)(o) = 6.8 (pK(a) value at the center of curvature). These plots are consistent with the existence of a zwitterionic tetrahedral intermediate (T(+/-)) on the reaction pathway whereby expulsion of aryloxide anion from T(+/-) is rate determining (k(2) step) at low pK(a) for EDNPTOC (and in the whole pK(a) range for MNPTOC and ENPTOC), and there is a change to rate-limiting formation of T(+/-) (k(1) step) at high pK(a) for EDNPTOC. Comparison of these Brönsted plots among them and with similar ones permits the following conclusions: (i) There is no variation of k(N) by substitution of methoxy by ethoxy as the nonleaving group of the substrate. (ii) The pK(a)(o) value is smaller for the less basic aryloxide nucleofuge due to a larger k(2) value. (iii) The change of C=S by C=O as the electrophilic center of the substrate results in larger values for both k(-)(1) (amine expulsion rate) and k(2), and also a larger k(-)(1)/k(2) ratio for the carbonyl derivative. There is also an increase of k(1) by the same change. The K(1)k(2) (= k(1)k(2)/k(-)(1)) values are larger for the pyridinolysis of methyl 2,4-dinitrophenyl and methyl 4-nitrophenyl carbonates compared to the corresponding thionocarbonates (EDNPTOC and MNPTOC, respectively). (iv) Pyridines are more reactive than isobasic secondary alicyclic amines toward ENPTOC when either the k(1) step or the k(2) step is rate limiting. This is explained by the softer nature of pyridines than alicyclic amines (k(1) step) and the greater nucleofugality (k(-)(1)) of the latter amines than isobasic pyridines, leading to a larger k(2)/k(-)(1) ratio for pyridines (k(2) is little affected by the amine nature), and therefore a larger K(1)k(2) value when the k(2) step is rate determining.

Spray drying is a well-known method of particle production which comprises the transformation of a fluid material into dried particles, taking advantage of a gaseous hot drying medium, with clear advantages for the fabrication of medical devices. In fact, it is quite common the production of microspheres and microcapsules designed for drug delivery systems. This review describes the different stages of the mechanism of the spray-drying process: atomization, droplet-to-particle conversion and particle collection. In particular, this work addresses the diversity of available atomizers, the drying kinetics and the importance of the configuration of the drying chamber, and the efficiency of the collection devices. The final properties of the dried products are influenced by a variety of factors, namely the spray dryer design, the feed characteristics and the processing parameters. The impact of those variables in optimizing both the spray-drying process and the synthesis of dried particles with desirable characteristics is discussed. The scalability of this manufacturing process in obtaining dried particles in submicron-to-micron scale favors a variety of applications within the food, chemical, polymeric, pharmaceutical, biotechnology and medical industries.

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The reactions of methyl 4-nitrophenyl carbonate (MNPC) with a series of secondary alicyclic amines (SAA) and quinuclidines (QUIN), methyl 2,4-dinitrophenyl carbonate (MDNPC) with QUIN and 1-(2-hydroxyethyl)piperazinium ion (HPA), and phenyl 2,4-dinitrophenyl carbonate (PDNPC) with SAA are subjected to a kinetic investigation in aqueous solution, at 25.0 degrees C and an ionic strength of 0.2 M. By following spectrophotometrically the nucleofuge release (330-400 nm) under amine excess, pseudo-first-order rate coefficients (k(obsd)) are obtained. Plots of k(obsd) vs [amine] at constant pH are linear, with the slope (k(N)) being pH independent. The Brönsted-type plot (log k(N) vs amine pK(a)) for the reactions of SAA with MNPC is biphasic with slopes beta(1) = 0.3 (high pK(a) region) and beta(2) = 1.0 (low pK(a) region) and a curvature center at pK(a)(0) = 9.3. This plot is consistent with a stepwise mechanism through a zwitterionic tetrahedral intermediate (T(+/-)) and a change in the rate-determining step with SAA basicity. The Brönsted plot for the quinuclidinolysis of MNPC is linear with slope beta(N) = 0.86, in line with a stepwise process where breakdown of T(+/-) to products is rate limiting. A previous work on the reactions of SAA with MDNPC was revised by including the reaction of HPA. The Brönsted plots for the reactions of QUIN and SAA with MDNPC and SAA with PDNPC are linear with slopes beta = 0.51, 0.48, and 0.39, respectively, consistent with concerted mechanisms. Since quinuclidines are better leaving groups from T(+/-) than isobasic SAA, yielding a less stable T(+/-), it seems doubtful that the quinuclidinolysis of PDNPC is stepwise, as reported.