Showing papers on "Reactivity (chemistry) published in 1976"

Approach vector analysis: a stereochemical approach to reactivity

Abstract: A method of approximating preferred nucleophilic approach geometries to unsaturated functions, based on classical Lewis resonance structures, is described; the application of this approach vector analysis method to the stereochemistry of enone reductions is explained.

Rate constants for the reactions of OH with CH4 and fluorine, chlorine, and bromine substituted methanes at 296 K

Abstract: The absolute rate constants for the reactions of OH radicals with CH4 and fifteen fluorine, chlorine, and bromine substituted methane molecules have been measured using a discharge flow system and laser magnetic resonance detection of OH. Measurements were made at 296 K and at pressures between 107 and 1300 Pa. The results indicate that the reaction mechanism involves the abstraction of a hydrogen atom and formation of H2O and a methyl type radical product. Completely halogenated methane molecules are found to be relatively unreactive. Hydrogen containing molecules react with rate constants ranging from about 0.2 to 160×10−15 cm3/molecule⋅sec. The reactivity increases with decreasing carbon–hydrogen bond energies. Rough estimates are made of the Arrhenius parameters for the reactions.

Investigation of substituent effects of chalcones by 13C n.m.r. spectroscopy

Abstract: 13C chemical shifts for 23 para- and meta-substituted chalcones of the types 1 and 2 have been determined. The aromatic shieldings are compared with previous results for other aromatic derivatives. Correlations of the 13C chemical shifts of vinyl carbons and carbonyl carbons as well as ring carbons with Hammett σ parameters, π electron densities and the reactivity parameters of Swain and Lupton provide a consistent picture of electronic effects transmitted through the carbon framework of the compounds studied.

Reactivity scale for atmospheric hydrocarbons based on reaction with hydroxyl radical

Abstract: By use of relative and absolute rate constants for the reaction of the hydroxyl radical (OH) with a number of alkanes, alkenes, aromatics, and ketones, a reactivity scale is formulated based on the rate of removal of hydrocarbons and oxygenates by reaction with OH. In this five-class scale, each class spans an order of magnitude in reactivity relative to methane. Thus, assigned reactivities range from 10/sup 4/ for Class V containing the most reactive compounds (e.g., d-limonene). This scale differs in several significant ways from those presently utilized by air pollution control agencies and various industrial laboratories. For example, in contrast to other scales based on secondary manifestations such as yields of ozone and eye irritation, it focuses directly on initial rates of photooxidation. The proposed scale also provides a clearer understanding of the importance of alkanes in the generation of ozone during periods of prolonged irradiation. The present scale can be readily extended to include additional organic compounds (e.g., natural and anthropogenic hydrocarbons, oxygenates, chlorinated solvents), once their rate of reaction with OH is known.

Reciprocal reactivities of specific thiols when actin binds to myosin.

Abstract: We report measurements of the reactivity (degree of labeling, as mole of ligand per mole of protein, at constant exposure time) of the reactive thiol, "SH1", of a subfragment of myosin (S-1), and of Cys-10 of F-actin under various conditions, using N-iodo-[3H]acetyl-N-(1-sulfo-5-naphthyl)ethylenediamine, a fluorescent radioactive iodoacetamide analog When either ADP or adenyloyl imidodiphosphate (simulating unhydrolyzed ATP) is bound to the enzymatic site of S-1, the reactivity of "SH1" is slightly enhanced, but when active ATPase is going on, reactivity is reduced by about a third, presumably due to the species, (S-1) ADP,Pi The reactivity of Cys-10 alone is very low When the complex, (S-1)-F-actin, is formed, the reactivity of SH1 is strongly decreased, and the reactivity of Cys-10 is strongly increased The foregoing results explain our further observation (on glycerol-treated rabbit psoas fibers) that when fibers labeled in relaxation solution are compared with fibers labeled in rigor solution, myosin is more reactive and actin is less reactive, in the former case; alpha-actinin and C-protein are also less reactive in the former case

The Structure and Reactivity of [Zn(OMe)2·(EtZnOMe)6]

Abstract: The molecular structure of [Zn(OMe)2·(EtZnOMe)6], an enantiomorphic catalyst for the polymerisation of methyloxirane, has been determined.

Coupling of nonequivalent aromatic rings by soluble nickel catalysts. A general route to the 1,8-diarylnaphthalenes

Abstract: A facile route to the l,&diarylnaphthalene derivatives is described. The parent compound, l,8-diphenylnaphthalene, can be prepared in two steps with 35% overall yield from the commercial 1,8-diaminonaphthalene. The key step in these preparations is the direct joining of two nonequivalent aromatic molecules by an organonickel-catalyzed Grignard-aryl halide coupling reaction at -15 oC. The coupling reaction is regiospecific with respect to the position of substitution of both aromatic halves. The yields of thjs coupling reaction are high, even though the 1,8-diarylnaphthalenes are sterically crowded. The efficiency of the reaction under a variety of conditions is evaluated. The order of reactivity of aryl halides is found to be I > Br > Cl, and it is possible to effect coupling preferentially at an iodo group in the presence of a chloro group, the latter being available for further synthetic functionalization.

Reactivity of Organic Dye Triplet States in Electron Transfer Processes

Abstract: — 10 kcal/mol, kqT reaches the diffusion-controlled limit, decreases with increasing AG, and for AG > 4 kcal/mol kqT becomes proportional to exp (— AG/RT). The different reactivities of the acid and basic triplet forms of thionine and lumiflavin can be attributed to the difference in reduction potentials of these species. This difference is available from the pK values of triplet state and semireduced dye radical.

Kinetics of reaction of substituted pyridines and oxygen anions with methyl chloroformate in aqueous solution

Abstract: The rate constants for nucleophilic reaction of nine substituted pyridines with methyl chloroformate gives a sharply curved Bronsted plot, the slope changing from β 0.93 to 0.15 with increasing reactivity. This contrasts with the straight line (β 0.93) observed with p-nitrophenyl acetate, and is shown to be quantitatively consistent with a change in rate-determining step. It is argued that this is from breakdown to formation of a zwitterionic tetrahedral intermediate which is only just stable enough to exist. The hydrolytic stabilities of some of the methoxycarbonylpyridinium ions produced by this reaction, and the nucleophilic reactivities towards methyl cloroformate of imidazole, of phenolate, p-nitrophenolate, and acetate anions, and of phosphate dianion are reported and discussed.

Selective formation of ketone, diketone and aldehyde by the co insertion into nickel–alkyl bonds of dialkylnickel complexes. a novel nickel-catalyzed syntheses of ketones and tertiary alcohols from grignard reagents, aryl halides, and carbon monoxide

Abstract: Reaction of dialkylnickel complexes, NiR2L2 (R = CH3, C2H5, C3H7; L = 1/2 (2,2′-bipyridine), 1/2[1,2-bis(diphenylphosphino)ethane], triethylphosphine) with carbon monoxide give ketones, diketones and aldehydes depending on the reactivity of the dialkylnickel complexes and the reaction temperature. Nickel complexes NiY2L2 (Y = halogen or alkyl; L = ligand) catalyze formation of ketones and tertiary alcohols from Grignard reagents, aryl halides, and carbon monoxide.

Semicontinuous emulsion copolymerization of styrene and butyl acrylate

Abstract: Semicontinuous emulsion copolymerization of styrene and butyl acrylate with a constant rate of feed of monomer emulsion was investigated. The integral composition of the copolymer at the end of the feeding was different from the feed composition, and the difference was proportional to the monomer feeding rate. The closer the feed composition was to the composition at the azeotropic point, the lower was the sensitivity of the system to the feeding rate. At low feeding rates, the copolymerization proceeded at conversions of about 90–95%, and the composition of the copolymer was practically equal to that of the monomer feed. The reactivity ratios determined under these conditions were probably influenced by diffusion inside the growing polymer–monomer particles.

Nucleophilic Ion Pairs. I. Enhanced Esterolytic Reactivity of Hydrophobic Ion Pairs in Micellar Systems

Abstract: The reaction of the dodecyl(2-hydroxyimidophenethyl)dimethylammonium bromide(DHDB) and N-methylmyristohydroxamate (MMHA) anion with p-nitrophenyl acetate was studied mostly in the presence of aqueous nonionic micelles. Addition of a hydrophobic ammonium ion, TMAG, considerably enhanced the reactivity of MMHA but not that of DHDB (a zwitterion). The rate enhancement for MMHA-TMAC and DHDB in nonionic micelles were almost comparable to those in cationic(CTAB) micelles. It was proposed that the large enhancement observed for some anionic nucleophiles in cationic micelles is to a great extent caused by the formation of hydrophobic, desolvated ion pairs.