摘要:

Marcus theory provides a framework for correlating rate and equilibrium constants, and thus the possibility of predicting rate from structure. This had been successful for a number of simple reactions, including the addition of hydroxide to carbonyl compounds, enolate formation from carbonyl compounds, and aldol addition and elimination reactions. The theory can be extended, as Multidimensional Marcus Theory, to cover concerted reactions such as E2 eliminations, the water-mediated proton switch, or the breakdown of the tetrahedral intermediate in amide hydrolysis. We have now found that Marcus theory can be applied not only to inter-molecular reactions but also to intra-molecular reactions. This is illustrated for aldol condensation and ester/lactone formation.Keywords: Marcus theory, concerted reactions, intramolecular reactions, aldol condensation, ester formation, proton transfer.

ISSN:0008-4042    

DOI:10.1139/v96-144

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

The paper presents recent advances in solid phase microextraction (SPME). The method utilizes a small fused silica fibre coated with a suitable polymeric stationary phase for analyte extraction from various matrices. The fibre is mounted in a syringe-like holder for protection. SPME has been used for the characterization of polymeric coating properties. Infinite-dilution weight-fraction activity coefficients have been determined for five solutes in two different stationary phases. Their values correlate well with the structure of the analytes and the coating. Other SPME uses include simultaneous determination of 60 pesticides in liquid matrices. A comparison between SPME, liquid extraction, and Soxhlet extraction for the analysis of Atrazine, Simazine, and Metolachlor in well water, as well as Metolachlor in soil samples, produced similar results for all three methods. The solvent-free character of SPME enables its use as sample introduction technique for fast GC analysis. Separations of BTEX in less than 9 s, and of volatile compounds described in EPA method 624 in 2.5 min, are described. SPME has also been used for the analysis of tetraethyllead and Pb(II) in water. Before the analysis, lead is derivatized with sodium tetraethylborate, and the tetraethyllead formed is extracted from sample headspace. Sub-ppb detection limits have been achieved with FID. Finally, SPME has been coupled to HPLC with the use of a specially designed interface. The combination has been tested on samples of polyaromatic hydrocarbons and surfactants in water. Very good precision of retention times and peak areas has been achieved.Keywords: solid phase microextraction, coating characterization, pesticide analysis, fast GC, lead and organolead compounds, SPME/HPLC.

ISSN:0008-4042    

DOI:10.1139/v96-145

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

The adducts pyr•BF2Br and pyr•BFBr2(pyr = pyridine) form fluoroboron cations by displacement of Br−by excess pyridine, the ease of cation formation being pyr2BF2+» pyr2BFBr+» pyr3BF2+•Cl−can be displaced from pyr•BF2Cl and pyr•BFCl2, but much less readily, to form pyr2BF2+, pyr2BFCl+, and, under forcing conditions, a few percent of pyr3BF2+. Non-fluorine-containing mixed boron trihalide adducts of pyridine also form haloboron cations by heaviest-halide-ion displacement, for example pyr•BClI2giving pyr2BClI+, the ease of displacement always being I− > Br− > Cl−, and displacement always occurring more readily from mixed boron trihalide adducts than from unmixed-halogen adducts. The mechanistic implications of this are discussed.orthoSubstituents greatly reduce the ability of pyridine to displace heavy halide ion, so 2-methylpyridine gives 2-Mepyr2BF2+and 2-Mepyr2BFBr+but not 2-Mepyr2BFCl+or 2-Mepyr3BF2+, while 2,6-dimethylpyridine does not form any haloboron cations.19F spin-lattice relaxation times of the fluoroboron cations are much shorter than those of neutral boron trihalide adducts in the same solution, and provide a further diagnostic test for their presence.Keywords: fluoroboron cations, pyridines, mixed boron trihalide adducts, fluorine-19 NMR, boron-11 NMR.

ISSN:0008-4042    

DOI:10.1139/v96-146

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

The acid reactions of five arylhydroxylamines (Ar = 2,6-Me2C6H3, 2,5-Me2C6H3, 2-MeC6H4, 2-ClC6H4, and C6H5) have been studied at constant ionic strength (NaClO4) in the presence of varying amounts of NaBr and NaCl. Each system resulted in the correspondingp-aminophenol, the product of Bamberger rearrangement, as the only detectable product in the absence of halide. The addition of halide ion reduced the yield of this product, with the appearance of the correspondingp-haloaniline,o-haloaniline (where appropriate), and the parent aniline (predominantly with bromide). Rate constants for the reaction were measured in the case of the parent and 2,6-dimethyl systems and showed small decreases (chloride) or increases (bromide) with increasing halide concentration. These changes did not correlate with the change in products, implying that the rate variations were caused by specific salt effects. Product data were analyzed by a mechanism involving rate-limiting formation of the appropriate arylnitrenium ion followed by product-determining steps involving trapping by the solvent or by the added halide. The possibility that a portion of the halide-trapped products were derived from a pre-association mechanism was also included. Kinetic analyses then producedkBr:kwandkCl:kwratios for two limiting cases, one involving pre-association with an equilibrium constantkas = 0.3, and one ignoring pre-association. From an azide:water ratio (kAz:kw) previously determined for the 2,6-dimethylphenylnitrenium,kBrwas concluded to lie in the range (4–5) × 109 M−1s−1for all of the nitrenium ions of this study. This range forkBrthen led tokwvalues of (1–2) × 109 s−1(2,5-Me2), (2–3) × 109 s−1(2-Me), and (4–8) × 109 s−1(parent and 2-Cl), where the ranges reflect uncertainties in the exact value ofkBrand in the contribution from pre-association. The lifetime of the parent phenylnitrenium ion in water at one molar ionic strength is concluded to lie in the range 125–250 ps.Keywords: nitrenium ion lifetime, phenylhydroxylamine, phenylnitrenium ion, Bamberger rearrangement.

ISSN:0008-4042    

DOI:10.1139/v96-147

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

Carbon-13 chemical shifts of alkene carbons are observed in the ranges 78–109 ppm (Cα) and 154–164 ppm (Cβ) for a series of 11 2-acylidene-3,5-diaryl-2,3-dihydro-1,3,4-thiadiazoles, 3 2-acylidene-3-alkyl-2,3-dihydrobenzothiazoles, and 2 2-acylidene-3-alkyl-2,3-dihydrobenzoselenazoles of known geometry, indicating appreciable charge polarization in these compounds as in other push–pull olefins. Substitution that promotes more extensive charge delocalization results in the Cα signal shifting to the higher-frequency end of the chemical shift range. The observed shifts are compared with those calculated according to the Pretsch scheme.Keywords: carbon-13 NMR, chemical shifts, push–pull olefins, 1,3,4-thiadiazoles, benzothiazoles, benzoselenazoles.

ISSN:0008-4042    

DOI:10.1139/v96-148

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

Methoxy-(2-trimethylsilyl)ethoxycarbene reacts with two equivalents of dimethyl acetylene dicarboxylate, methyl propiolate, phenyl acetylene, or phenyl isocyanate without rearrangement of the carbene group.N-Phenylmaleimide captures the carbene with 1:1 -stoichiometry. The structure of the product implies that a migration of the trimethylsilylethyl group from oxygen to carbon accompanies that reaction. A mechanism for that complex rearrangement is proposed. Phenol andtert-butyl alcohol afford the orthoformates expected from overall insertion of the carbene into the OH bond.Keywords: dialkoxycarbene, insertion into O—H, C—H bonds, rearrangement,N-phenylmaleimide, dialkoxyoxadiazoline.

ISSN:0008-4042    

DOI:10.1139/v96-149

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

The reaction of some 2-amino-2-oxazolines with isocyanates and arylisothiocyanates yields 3-monosubstituted or 2,3-disubstituted 2-iminooxazolidines depending on the experimental conditions and on the nature of the electrophile reactants. The structures of a mono- and a disubstituted derivative were established by X-ray analysis. The chemical behaviour of the compounds was investigated by molecular and quantum mechanics calculations. An intramolecular transposition during the second step of the reaction was found.Keywords: 2-amino-2-oxazolines, iso(thio)cyanates, transposition. X-ray structure, MOPAC calculations.

ISSN:0008-4042    

DOI:10.1139/v96-150

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

The title compound (1) is prepared in satisfactory yield by reacting CH2N2with trifluoroacetic anhydride in Et2O; thed-analog was obtained by exchange with D2O.1H,13C,15N,17O, and19F NMR spectra of1were studied, as well as its IR spectrum. A single isomer is present corresponding to the more stableZconfiguration. The structural assignment was made on the basis of quantum mechanical calculations, which revealed that theZform is some 13.4 kJ mol−1more stable than theEform and the activation energy for theE → Z transition is 64.2 kJ mol−1. Mass spectra under different experimental conditions were recorded and breakdown pathways of the parent ion of1charted.Keywords: 3-diazo-1,1,1-trifluoro-2-propanone; 3-d-3-diazo-1,1,1-trifluoro-2-propanone; structure;1H,13C,15N,17O, and19F NMR; EI-MS.

ISSN:0008-4042    

DOI:10.1139/v96-151

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

Binding of cadmium(II) on soil fulvic acid (FA) was investigated over a range of fulvate-to-cadmium concentration ratios (8 – 59 equiv. mol−1) using113Cd NMR spectroscopy. The113Cd chemical shift of cadmium bound on fulvate was observed in a more downfield region (δ −20.4 to −15.6) than that bound on synthetic polymers, poly(acrylic acid) (PAA: δ −36.6 to −38.2), poly(methacrylic acid) (PMAA: δ −34.0 to −25.4), and poly(vinyl benzoic acid) (PVBA: δ −34.7 to −31.2). The calculated values of individual chemical shifts for the species CdL+and CdL2(L: carboxylate) formed in Cd(II)–carboxylate systems (e.g., acetate, benzoate) are δ −22 to −24 and δ −39 to −40, respectively. The relative downfield shift of cadmium(II)–fulvate suggests that functional groups (e.g., hydroxyl and neutral N donor) other than carboxylates may be involved in cadmium coordination. The chemical shifts of cadmium complexes of hydroxycarboxylates (e.g., glycolate) or carboxylates containing neutral N donor (e.g., picolinate) were generally observed in more downfield regions than their carboxylate counterparts.Keywords: fulvic acid, polyfunctionality, binding sites, chemical shift,113Cd NMR.

ISSN:0008-4042    

DOI:10.1139/v96-152

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

Rates of conversion of 1-(tri-n-butylstannyl)-2-phenylethyne to phenylethyne in H2O and D2O solutions of perchloric acid were found to be proportional to acid concentration, giving the hydronium ion rate constantand the isotope effect. The magnitude of this isotope effect suggests that the reaction occurs by rate-determining hydron transfer to the substrate, producing a vinyl carbocation; this carbocation then loses its tributylstannyl group, giving phenylacetylene as the only detectable aromatic product. The hydronium ion rate constant, when compared to the rates of protonation of phenylethyne and 1-(trimethylsilyl)-2-phenylethyne, gives a β-stannyl stabilizing effect of δΔG≠ = 6.6 kcal mol−1and a differential β-stannyl/β-silyl effect of δΔG≠ = 3.2 kcal mol−1. These stabilizations are very much smaller than previously reported β-stannyl effects. Possible reasons for the difference are suggested.Keywords: β-stannyl effect, β-silyl effect, carbocation stabilization, protodemetalation.

ISSN:0008-4042    

DOI:10.1139/v96-153

出版商:NRC Research Press    

年代:1996

数据来源: NRC