Pyridines constants

To obtain k, one measures k, at several pyridine concentrations. To obtain the rate constant of carbene reaction with a quencher, Q, one holds the concentration of pyridine constant and varies the quencher concentration. A plot of k versus [Q] is now linear with slope k. For tert-butylchlorocarbene and trans-3-... 

Many chloroplatinates separate from aqueous solution with water of crystallisation. If this is suspected, the chloroplatinate should be dried to constant weight in the oven before analysis, to ensure elimination of water of crystallisation. Aniline, p-toluidine and pyridine all give anhydrous chloroplatinates, and can be conveniently used in the above determination no attempt should be made to recryrtallise their chloroplatinates. 

Pure pyridine may be prepared from technical coal-tar pyridine in the following manner. The technical pyridine is first dried over solid sodium hydroxide, distilled through an efficient fractionating column, and the fraction, b.p. 114 116° collected. Four hundred ml. of the redistilled p)rridine are added to a reagent prepared by dissolving 340 g. of anhydrous zinc chloride in a mixture of 210 ml. of concentrated hydrochloric acid and 1 litre of absolute ethyl alcohol. A crystalline precipitate of an addition compound (probable composition 2C5H5N,ZnCl2,HCl ) separates and some heat is evolved. When cold, this is collected by suction filtration and washed with a little absolute ethyl alcohol. The yield is about 680 g. It is recrystaUised from absolute ethyl alcohol to a constant m.p. (151-8°). The base is liberated by the addition of excess of concentrated... 

The value of the second-order rate constant for nitration of benzene-sulphonic acid in anhydrous sulphuric acid varies with the concentration of the aromatic substrate and with that of additives such as nitromethane and sulphuryl chloride. The effect seems to depend on the total concentration of non-electrolyte, moderate values of which (up to about 0-5 mol 1 ) depress the rate constant. More substantial concentrations of non-electrolytes can cause marked rate enhancements in this medium. Added hydrogen sulphate salts or bases such as pyridine... 

Experimental confirmation of the metal-nitrogen coordination of thiazole complexes was recently given by Pannell et al. (472), who studied the Cr(0), Mo(0), and W(0) pentacarbonyl complexes of thiazole (Th)M(CO)5. The infrared spectra are quite similar to those of the pyridine analogs the H-NMR resonance associated with 2- and 4-protons are sharper and possess fine structure, in contrast to the broad, featureless resonances of free thiazole ligands. This is expected since removal of electron density from nitrogen upon coordination reduces the N quad-rupole coupling constant that is responsible for the line broadening of the a protons. 

Two modified sigma constants have been formulated for situations in which the substituent enters into resonance with the reaction center in an electron-demanding transition state (cr+) or for an electron-rich transition state (cr ). cr constants give better correlations in reactions involving phenols, anilines, and pyridines and in nucleophilic substitutions. Values of some modified sigma constants are given in Table 9.4. 

The direct coupling constants (A/) of pyridine, obtained from a spectrum of the molecule in a nematic liquid crystal solvent, are listed in Batterham s monograph (B-73NMR, p. lo). These data provide information about the geometry of the molecule, as the couplings are proportional to the inverse cube of the distance (r,/) between the nuclei. 

Dicyclohexyl[18]crown-6, potassium complex equilibrium constant, 7, 742 (73M152101) Dicyclohexyl[18]crown-6, sodium complex equilibrium constant, 7, 742 (73MI52101) Dicyclopenta[6,e]pyridine... 

Pyridine, 3,5-dimethoxy-rate constants, 2, 190 <75JCS(P2)I600>... 

Pyridine, 3-(dimethylamino)-amination, 2, 236 methylation, 2, 342 nitration, 2, 192 iV-oxide synthesis, 2, 342 Pyridine, 4-(dimethylamino)-in acylation, 2, 180 alkyl derivatives pK, 2, 171 amination, 2, 234 Arrhenius parameters, 2, 172 as base catalysts, 1, 475 hydrogen-deuterium exchange, 2, 286 ionization constants, 2, 172 methylation, 2, 342 nitration, 2, 192 iV-oxide synthesis, 2, 342... 

Spinacine — see Imidazo[4,5-c]pyridine-6-carboxylic acid, 4,5,6,7-tetrahydro-Spiniferin-1, ( )-dihydro-occuiTence, 4, 705 synthesis, 4, 670 Spin-spin coupling constants pyridines, 2, 123 Spiramycin antibacterial veterinary use, I, 207 Spirans... 

In comparing the copper sulfate-pyridine method with the nitric acid method (Org. Syn. 1, 25) it should be pointed out that the constants on the samples are as follows ... 

M 604.5, m >250"(dec). Cl 16255, X,max 506nm. Dissolved in the minimum quantity of boiling water, filtered and enough EtOH was added to ppte ca 80% of the dye. This process was repeated until a soln of the dye in aqueous 20% pyridine had a constant extinction coefficient. 

The //NMR spectrum contains five signals with integral levels in the ratios 1 1 1 1 3 four lie in the shift range appropriate for aromaties or heteroaromaties and the fifth is evidently a methyl group. The large shift values (up to Sh = 9.18, aromaties) and typical coupling constants (8 and 5 Hz) indicate a pyridine ring, which accounts for four out of the total five double-bond equivalents. 

Hydroxypregna-5,16-dien-20-one Acetate Oxime 3j5-Hydroxy-pregna-5,16-dien-20-one acetate (30 g) is dissolved in 300 ml of anhydrous pyridine and 6.75 g of hydroxylamine hydrochloride is added. The mixture is stirred until solution is achieved, left in a tightly closed container at room temperature for 4 days and then poured into 1.5 liters of water. The flask is rinsed with water and the combined separated crystals are collected on a Buchner funnel, washed with water and dried to constant weight in an oven at 100°. Yield 30 g (96.5% of theory) mp 220-223°. 

The acetate prepared with acetic anhydride in pyridine has the following physical constants mp 174-176° [aj —98° (CHCl3). °... 

These rate constants are for the hydrolysis of cinnamic anhydride in carbonate buffer, pH 8.45, total buffer concentration 0.024 M, in the presence of the catalysts pyridine, A -methylimidazole (NMIM), or 4-dimethylaminopyridine (DMAP). In the absence of added catalyst, but the presence of buffer, the rate constant was 0.005 24 s . You may assume that only the conjugate base form of each catalyst is catalytically effective. Calculate the catalytic rate constant for the three catalysts. What is the catalytic power of NMIM and of DMAP relative to pyridine ... 

Berg et al. defined a different ortho steric constant. The model reaction is the quatemization of substituted pyridines with methyl iodide in acetonitrile solution. 

A plot of log (kiku) against pKa (this is called a Br nsted-type plot, as we will see in Section 7.4) for meta- and para-substituted pyridines gave an LFER describing normal behavior. On this same plot the or// o-substituted pyridines all fell below the m and p reference line. The steric constant was defined as the vertical distance between the reference line [whose equation was log (k plkn) = 0.35 pK — 1.73] and the result for the ortho compound, or... 

Similarity with cobalt is also apparent in the affinity of Rh and iH for ammonia and amines. The kinetic inertness of the ammines of Rh has led to the use of several of them in studies of the trans effect (p. 1163) in octahedral complexes, while the ammines of Ir are so stable as to withstand boiling in aqueous alkali. Stable complexes such as [M(C204)3], [M(acac)3] and [M(CN)5] are formed by all three metals. Force constants obtained from the infrared spectra of the hexacyano complexes indicate that the M--C bond strength increases in the order Co < Rh < [r. Like cobalt, rhodium too forms bridged superoxides such as the blue, paramagnetic, fCl(py)4Rh-02-Rh(py)4Cll produced by aerial oxidation of aqueous ethanolic solutions of RhCL and pyridine.In fact it seems likely that many of the species produced by oxidation of aqueous solutions of Rh and presumed to contain the metal in higher oxidation states, are actually superoxides of Rh . ... 

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