Complexation between nickel(II), cobalt(III) and hydrazones derived from pyridoxal 5′-phosphate and hydrazides of 2-,3-,4-pyridinecarboxylic acids in aqueous solution

Pseudo-first-order rate constants for the decarboxylation of 3-hydroxy- and 3-amino- picolinic acids in aqueous solution at 150 °C were determined and plotted as a function of acidity. Each rate profile has a maximum at an acidity well above the isoelectric point and this is attributed to decarboxylation of an intermediate protonated at the 2-position, analogous to the intermediates involved in the decarboxylation of salicylic and anthranilic acids. There is also a shoulder on each rate profile at a lower acidity corresponding to the isoelectric point where the Hammick ylide mechanism has a rate maximum in most picolinic acid decarboxylations. It is concluded that 3-hydroxy- and 3-aminopicolinic acids decarboxylate by the ylide mechanism at low acidity and by the protonation mechanism at higher acidities. In agreement with this interpretation, the 13C kinetic isotope effect in 3-hydroxypicolinic acid decarboxylation is 2.0% on the ylide part of the curve, 1.3% where decarboxylation of the protonated intermediate is rate determining, and drops to 0.4% in the intermediate region. Comparison of the rate constants for ylide decarboxylation with those for other 3-substituted picolinic acids shows that 3-hydroxy and 3-amino substituents facilitate decarboxylation, probably by their inductive and field effects on the developing negative charge at the 2-position of the transition state.

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Kinetics and Mechanism of Decarboxylation of some Pyridinecarboxylic Acids in Aqueous Solution

Canadian Journal of Chemistry ◽

10.1139/v72-480 ◽

1972 ◽

Vol 50 (18) ◽

pp. 3017-3027 ◽

Cited By ~ 12

Author(s):

G. E. Dunn ◽

Gordon K. J. Lee ◽

Harold Thimm

Keyword(s):

Aqueous Solution ◽

Hydrogen Exchange ◽

Picolinic Acid ◽

Substituent Effects ◽

Kinetic Isotope ◽

Rate Maximum ◽

Isoelectric Ph ◽

Picolinic Acids ◽

Carboxyl Carbon ◽

Pyridinecarboxylic Acids

The first-order rate constants for decarboxylation of picolinic and five substituted picolinic acids in buffered aqueous solution at 150° and ionic strength 1.0 increase as pH increases above 0, go through a maximum at pH near 1, then level off at about half the maximum. For quinolinic acid at 95° the rate maximum occurs at the isoelectric pH. It is therefore concluded that the anion decarboxylates about half as fast as the isoelectric species. Anion and isoelectric species show similar carboxyl-carbon kinetic isotope and substituent effects, so they probably decarboxylate by similar mechanisms. The methyl betaine of picolinic acid decarboxylates about 200 times faster than the anion. From these facts it is concluded that the isoelectric species which decarboxylates is probably zwitterion rather than neutral acid, and that anion and zwitterion decarboxylate by loss of carbon dioxide to form 2-pyridyl carbanion and ylid, respectively. The slower decarboxylation of isoelectric species compared to betaine suggests that only a small fraction of the isoelectric species is zwitterion. However, qualitative estimates from spectra of the fraction of zwitterion present in the isoelectric species at 150° suggest that this cannot be the whole explanation. The relative reactivities of the 2-and 4-positions are similar for decarboxylation and hydrogen exchange in pyridinium ions, but not in the unprotonated species.

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Structures Formed by Cryoprotective Agents Used in Freezc-Etching

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100066292 ◽

1971 ◽

Vol 29 ◽

pp. 448-449

Author(s):

G. G. Cocks ◽

C. E. Cluthe

Keyword(s):

Aqueous Solution ◽

Polyethylene Oxide ◽

Liquid Nitrogen Temperature ◽

Ice Crystals ◽

Etching Technique ◽

Structural Constituent ◽

Cryoprotective Agents ◽

Quick Freezing ◽

Freeze Etching ◽

Fractured Surface

The freeze etching technique is potentially useful for examining dilute solutions or suspensions of macromolecular materials. Quick freezing of aqueous solutions in Freon or propane at or near liquid nitrogen temperature produces relatively large ice crystals and these crystals may damage the structures to be examined. Cryoprotective agents may reduce damage to the specimem, hut their use often results in the formation of a different set of specimem artifacts.In a study of the structure of polyethylene oxide gels glycerol and sucrose were used as cryoprotective agents. The experiments reported here show some of the structures which can appear when these cryoprotective agents are used.Figure 1 shows a fractured surface of a frozen 25% aqueous solution of sucrose. The branches of dendritic ice crystals surrounded hy ice-sucrose eutectic can be seen. When this fractured surface is etched the ice in the dendrites sublimes giving the type of structure shown in Figure 2. The ice-sucrose eutectic etches much more slowly. It is the smooth continuous structural constituent surrounding the branches of the dendrites.

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Preparation and characterisation of vanadium pentoxide supported rhodium catalyst

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s042482010010679x ◽

1988 ◽

Vol 46 ◽

pp. 946-947

Author(s):

A. Legrouri

Keyword(s):

Aqueous Solution ◽

Thermal Decomposition ◽

Physicochemical Properties ◽

Surface Area ◽

Vanadium Pentoxide ◽

High Purity ◽

Gas Adsorption ◽

Rhodium Catalyst ◽

Optical Methods ◽

Reducible Oxides

The industrial importance of metal catalysts supported on reducible oxides has stimulated considerable interest during the last few years. This presentation reports on the study of the physicochemical properties of metallic rhodium supported on vanadium pentoxide (Rh/V2O5). Electron optical methods, in conjunction with other techniques, were used to characterise the catalyst before its use in the hydrogenolysis of butane; a reaction for which Rh metal is known to be among the most active catalysts.V2O5 powder was prepared by thermal decomposition of high purity ammonium metavanadate in air at 400 °C for 2 hours. Previous studies of the microstructure of this compound, by HREM, SEM and gas adsorption, showed it to be non— porous with a very low surface area of 6m2/g3. The metal loading of the catalyst used was lwt%Rh on V2Q5. It was prepared by wet impregnating the support with an aqueous solution of RhCI3.3H2O.

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TEM observation of iron oxide pillars in montmorillonite

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100177544 ◽

1990 ◽

Vol 48 (4) ◽

pp. 882-883

Author(s):

H. Mori ◽

Y. Murata ◽

H. Yoneyama ◽

H. Fujita

Keyword(s):

Aqueous Solution ◽

Iron Oxide ◽

Fine Particles ◽

Aqueous Suspension ◽

Volume Ratio ◽

Exchange Capacity ◽

Nano Composites ◽

Pillared Montmorillonite ◽

Topographic Features ◽

Transmission Electron

Recently, a new sort of nano-composites has been prepared by incorporating such fine particles as metal oxide microcrystallites and organic polymers into the interlayer space of montmorillonite. Owing to their extremely large specific surface area, the nano-composites are finding wide application[1∼3]. However, the topographic features of the microstructures have not been elucidated as yet In the present work, the microstructures of iron oxide-pillared montmorillonite have been investigated by high-resolution transmission electron microscopy.Iron oxide-pillared montmorillonite was prepared through the procedure essentially the same as that reported by Yamanaka et al. Firstly, 0.125 M aqueous solution of trinuclear acetato-hydroxo iron(III) nitrate, [Fe3(OCOCH3)7 OH.2H2O]NO3, was prepared and then the solution was mixed with an aqueous suspension of 1 wt% clay by continuously stirring at 308 K. The final volume ratio of the latter aqueous solution to the former was 0.4. The clay used was sodium montmorillonite (Kunimine Industrial Co.), having a cation exchange capacity of 100 mequiv/100g. The montmorillonite in the mixed suspension was then centrifuged, followed by washing with deionized water. The washed samples were spread on glass plates, air dried, and then annealed at 673 K for 72 ks in air. The resultant film products were approximately 20 μm in thickness and brown in color.

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The microstructure of functionalized poLyacrylonitrile beads for bioseparations

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100178604 ◽

1990 ◽

Vol 48 (4) ◽

pp. 1094-1095

Author(s):

Eduardo A. Kamenetzky ◽

David A. Ley

Keyword(s):

Aqueous Solution ◽

Affinity Chromatography ◽

Pore Size Distribution ◽

Pore Size ◽

Surface Coverage ◽

Vacuum Impregnation ◽

Thin Sections ◽

Selective Staining ◽

Low Viscosity ◽

Diamond Knife

The microstructure of polyacrylonitrile (PAN) beads for affinity chromatography bioseparations was studied by TEM of stained ultramicrotomed thin-sections. Microstructural aspects such as overall pore size distribution, the distribution of pores within the beads, and surface coverage of functionalized beads affect performance properties. Stereological methods are used to quantify the internal structure of these chromatographic supports. Details of the process for making the PAN beads are given elsewhere. TEM specimens were obtained by vacuum impregnation with a low-viscosity epoxy and sectioning with a diamond knife. The beads can be observed unstained. However, different surface functionalities can be made evident by selective staining. Amide surface coverage was studied by staining in vapor of a 0.5.% RuO4 aqueous solution for 1 h. RuO4 does not stain PAN but stains, amongst many others, polymers containing an amide moiety.

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