Relationship between Intrinsic Viscosity and Molecular Weight for Fractionated Cellulose

A relationship between intrinsic viscosity and molecular weight for fractionated cellulose is established. The exponent a of the Mark-Houwink equation is 0.80. The unperturbed dimensions of cellulose are discussed on the basis of the Stockmayer-Fixman and Kurata-Stockmayer viscometric theories, and it is determined that cellulose is not a very rigid macromolecule in nature. This conclusion disagrees with the results obtained by statistical mechanics procedures in which the restriction of β-glucose residues to the C1 conformation was kept. The steric hindrance factor, σ is about 2, and it seems to be independent of the substituents in cellulose if it is evaluated in both cellulose and cellulose derivatives from viscometric data. From statistical mechanics data reported in the literature, σ is estimated as 4.4 for cellulose and cellulose derivatives, such as cellulose nitrate, and this means that the steric hindrances of the substituents have little influence on the rigidity of cellulose derivatives. This view disagrees with the results obtained from the non-Newtonian behavior of cellulose and cellulose nitrate in dilute solutions. By applying the Noda-Hearst theory, the conclusion is reached that cellulose nitrate is a more extended macromolecule than cellulose.     

烯烃光敏氧化反应机理 II: 丙烯与单线态氧[2+2]环加成反应途径的解析

本文进一步报道丙烯与单线态氧[2+2]环加成反应途径的计算结果, 沿反应途径反应物间相互作用的详细情况和甲基在反应进程中的动态电子效应, 从而揭示烷基取代烯烃与单线态氧不易发生[2+2]环加成反应的原因. 为进一步探求单线态氧与烯烃[2+2]环加成反应的必要条件提供依据. 本文所用计算方法与前文相同.     

Effect of para substituents of 2-(N,N-dimethylamino)ethyl phenyl phenylphosphonates on inhibition of human pseudocholinesterase

Human serum pseudocholinesterase inhibition by nineteen of the title phosphonates was studied and the effect of para substituents of the phenoxy and phenyl groups on inhibition rates was compared. The effect of substituents of the phenoxy group on the reaction rate is in some cases even 60 times as great as the effect of substituents of the phenyl group. Logarithms of rate constants correlate linearly with sigma po substituent constants of the phenoxy group, but no linear dependence is observed for para substituents of the phenyl group. The role of the apicophilicity of substituents of phosphorus in the enzyme--inhibitor complex is discussed.     

Electron transfer to sulfides and disulfides: intrinsic barriers and relationship between heterogeneous and homogeneous electron-transfer kinetics

The electron-acceptor properties of series of related sulfides and disulfides were investigated in N,N-dimethylformamide with homogeneous (redox catalysis) and/or heterogeneous (cyclic voltammetry and convolution analysis) electrochemical techniques. The electron-transfer rate constants were determined as a function of the reaction free energy and the corresponding intrinsic barriers were determined. The dependence of relevant thermodynamic and kinetic parameters on substituents was assessed. The kinetic data were also analyzed in relation to corresponding data pertaining to reduction of diaryl disulfides. All investigated reductions take place by stepwise dissociative electron transfer (DET) which causes cleavage of the C(alkyl)--S or S--S bond. A generalized picture of how the intrinsic electron-transfer barrier depends on molecular features, ring substituents, and the presence of spacers between the frangible bond and aromatic groups was established. The reduction mechanism was found to undergo a progressive (and now predictable) transition between common stepwise DET and DET proceeding through formation of loose radical anions. The intrinsic barriers were compared with available results for ET to several classes of dissociative- and nondissociative-type acceptors, and this led to verification that the heterogeneous and the homogeneous data correlate as predicted by the Hush theory.     

Rhodium carbenoid-initiated Claisen rearrangement: scope and mechanistic observations

[formula: see text] It has been shown that alpha-diazoketones react with allylic alcohols in the presence of Rh(II) catalysts to furnish intermediate enols which subsequently undergo Claisen rearrangement to alpha-hydroxyketones. Herein we report (1) studies into the mechanism of this transformation which establish that Claisen rearrangement is neither rhodium- nor acid-catalyzed but a reaction intrinsic to the intermediate enols that proceeds at a rate governed by enol substituents (R3, R4, R5) and (2) the reaction of alpha-diazoketones with propargylic alcohols and preliminary investigations into its scope and mechanism.     

Reaction between 7,8-dihydropterins and hydrogen peroxide under physiological conditions

In vitiligo, a common skin disorder that produces white patches of depigmentation, 7,8-dihydropterins accumulate in the presence of high concentration of H₂O₂. In this work, we present a study of the reaction between 7,8-dihydropterins and H₂O₂. The rate of the reaction, as well as the products formed, strongly depend on the chemical structure of the substituents. Electron-donor groups as substituents are the most reactive derivatives and undergo oxidation of the pterin moiety. The corresponding bimolecular rate constants at 37 °C in neutral aqueous solutions are reported. The biological implications of the results obtained are also discussed.     

Fine tuning of sulfoalkylated cyclodextrin structures to improve their mass-transfer properties in an aqueous biphasic hydroformylation reaction

The structure of sulfoalkylated cyclodextrins (CDs) have been varied and optimised to improve their performances as mass-transfer promoters in an aqueous biphasic hydroformylation reaction. Their surface tensions have been measured using the Wilhelmy plate technique and compared. Their behaviour towards two hydrosoluble derivatives of triphenylphosphine has been evaluated by ³¹P{¹H} and ¹H NMR measurements and their catalytic activity has been assessed in a rhodium-catalyzed hydroformylation reaction of 1-decene. The best result was obtained using a β-CD sulfobutylated on the primary face and methylated on the secondary face. Indeed, this CD increased the reaction rate by a factor of 250 without inducing selectivity decrease. The accessibility to the secondary face of the CD appears to be determining in the catalytic process as it governs the approach between the CD-included substrate and the water-soluble catalyst. The impact of the nature of the CD substituents on the chemo- and regioselectivity of the reaction is also discussed.     

Polyesters. I. Rate of polyesterification of γ-arylitaconic acids with ethylene glycol

A kinetic study of the polyesterification reaction of γ-phenyl- γ-p-methoxyphenyl-, and γ-p-chlorophenylitaconic acids (1 mole) with ethylene glycol in the presence or in absence of p-toluenesulfonic acid as a catalyst has been carried out in order to show the effect of substituents on the rate and degree of polymerization. The reaction of 1 mole of the acid and an excess of ethylene glycol has also been studied. In all cases the reaction is found to follow the second-order rate equation. The mechanism of polyesterification has been discussed. In catalyzed polyesterification electron-withdrawing groups (CI) decrease the velocity of the reaction. The low values of ρ in both the auto-catalyzed and catalyzed reaction indicate that this polyesterification is slightly sensitive to the polar nature of the substituent.     

Cascade reactions of diazocarbonyl compounds with pyridinium aroylmethylides accompanied by water or benzoic acid elimination in the cyclocondensation step

The reaction of pyridinium aroylmethylides with diazocarbonyl compounds proceeds as a multistep process involving three ylide molecules and one diazo compound molecule. As a result, intermediate functionally substituted azines, if they contain carbonyl groups and reactive methylene fragment, undergo intramolecular cyclocondensation to form tri- and tetrasubstituted pyridazines. Depending on stereochemistry of cyclic ketols formed, water or benzoic acid is eliminated to form tetra- or trisubstituted pyridazines, respectively. The regularities of reactions of pyridinium ylides with diazo compounds are discussed depending on the nature of substituents in the both substrates and reaction conditions.     

Kinetics of reaction of sodium phenylarsonite with chloroacetic acid in aqueous alkaline medium

The reaction order with respect to its components, the rate constants, and thermodynamic parameters of the reaction of sodium phenylarsonite with chloroacetic acid in aqueous alkaline medium were elucidated. The effect of substituents at the arsenic atom on the nucleophilic reactivity of arsenite ion in the Meyer reaction is discussed.     

硫杂蒽酮光敏分解重氮盐及其用作光引发体系的研究

本工作对几种新型硫杂蒽酮化合物敏化光解重氮盐问题进行了研究。工作表明:敏化反应是通过电子转移过程实现的,重氮盐所带取代基以及溶剂对重氮盐的光解有较大影响。十分有趣的是该体系对甲基丙烯酸甲酯进行光敏自由基引发聚合的速率和其光解反应速率有很大的不同,本文对此现象进行了讨论。     

A study of the mechanism of the oxidative cyclization of benzaldehyde semicarbazones induced by cupric perchlorate in acetonitrile

Treatment of benzaldehyde semicarbazones 1a-i with cupric perchlorate in acetonitrile at 40 provided selectively the corresponding 1,2,4-triazolin-5-ones 2a-i . The relative rate constants for 2a-i formation were determined by the competitive method. The results obtained showed that electron-donating substituents (methyl and methoxy) increase the reaction rate, while the reverse was found for electron-withdrawing substituents (chloro and nitro group). The reactivity data are discussed on the grounds of two possible mechanisms.     

Polar Effects in Organic Reactions

Attempts to gauge the effect of polar substituents on organic reaction rates by means of “inductive substituent constants” are based on the assumption that these effects are independent of the type of reaction observed. The measured rate constants of nucleophilic substitution reactions show, however, that this assumption is only partly justified even for saturated molecules. It is invalid if the substituent and the reaction center are an electron donor and an electron acceptor, respectively, which are hyperconjugated by way of σ-bonds. In extreme cases the resulting polarization can lead to heterolytic fragmentation.     

从核磁共振的化学位移探讨取代苯亲电取代反应的活性和区域选择性

亲电取代反应是一类基本的有机反应,其中以苯环上的亲电取代反应最常见。该反应的反应活性及反应的位点与苯环上已有的取代基有很大的关系。本文从波谱学的角度,利用核磁数据,阐述了已有的取代基如何影响苯环的电子云密度,进而影响苯环亲电取代反应的活性和取代的位置。本文视角新颖,将对学生的学习及科研产生启发。     

Substituent effects in pericyclic reactions of radical cations: the ring opening of 3-substituted cyclobutene radical cations

The substituent effects on the ring-opening reaction of cyclobutene radical cations have been studied at the Becke3LYP/6-31G* level of theory. The effect on the reaction energies and activation energies of the concerted and stepwise pathways of electron-donating substituents such as methyl and methoxy as well as electron-withdrawing substituents such as nitrile and carboxaldehyde in the 3-position of the cyclobutene is discussed. The exothermicity of the reaction correlates well with the ability of the substituent to stabilize the 1,3-butadiene radical cation by electron donation or conjugation. The relative stability of the (E) and (Z) isomers of the resulting 1,3-butadiene radical cations depends largely on steric effects. Similarly, steric effects are responsible for the relative energies of the different diastereomeric transition structures. The cyclopropyl carbinyl intermediate of the stepwise pathway resembles the nonclassical carbocation and is stabilized by electron-donating substituents. In the case of electron-donating substituents, this species becomes a minimum on the potential energy hypersurface, whereas unstabilized or destabilized cyclopropyl carbinyl radical cations are not minima on the hypersurface. The stabilization of the cyclopropyl carbinyl radical cation by substituents correlates qualitatively with the Brown-Okamoto substituent parameter sigma+. However, in all cases studied here, the concerted mechanism is the lowest energy pathway.     

氮杂炔正离子与环烯反应的机理及取代基效应

用DFT方法研究了氮杂炔正离子[R-C≡N-CH3] 与二环[2,2,1]-2-庚烯的反应机理。所有几何构型用B3LYP/6-31G**基组进行优化,过渡态通过振动分析和内聚反应坐标进行了确认。并在相同基组上研究了当R为不同取代基时的反应变化规律。结果表明该反应为环加成反应,有两条反应途径,产物主要通过能量较低的过渡态TSa生成。当取代基为支链烷烃基团时,反应位垒较大;取代基为直链烷烃基团和苯环时,反应较易进行。     

Functional dependency of structures of ionic liquids: do substituents govern the selectivity of enzymatic glycerolysis?

The concept of regulating the preference of a reversible multi-step reaction by adjusting the substituents of ionic liquids (ILs) has been successfully exemplified with a group of tetraammonium-based ionic liquids as medium for the enzymatic glycerolysis. Simultaneous existence of long chain hydrophobic substituents and hydrophilic ethoxyl or hydroxyl moieties is found, respectively, to be essential for triglycerides (TG) dissolving and equilibrium shifting. The reactions in the ILs with cations consisting of long chain and free hydroxyl groups gave markedly higher conversion of TG and better preference to monoglyceride formation. Interestingly the predicted results from COSMO-RS (a quantum chemical model programme) achieved a good agreement with the experimental data, mapping out the specific solvation from the ILs as well as demonstrating the interaction between ILs, substrates and products being the intrinsic causes that govern reaction evolution and direct equilibrium shifting.     

Silicon-Directed Nazarov Cyclizations. Part VI. The anomalous cyclization of vinyl dienyl ketones

Reactions, of various vinyl dienyl ketones (see la – e , li ) with FeCl, give rise to β,γ-unsaturated α-vinyl-cyclo-pentenones (see 2a – e , 2i , Table 2). The reaction succeeds for vinyl dienyl ketones with substituents on either double bond. Aryl dienyl and alkyl dienyl ketones (see 1f – h ) do not cyclize cleanly. The effects of substituents on the rate of reaction is discussed in terms of the mechanism of the rearrangement. A ¹³C-labeling study establishes the pathway as an unusual 1-hydroxypentadienyl-cation electrocyclization to a cyclopentenyl cation which collapses via a pinacol rearrangement to the α-vinyl ketone.     

Chloracylierung und Bromacylierung von Carbonylverbindungen. IV. Bildungsmechanismus von Carbonsäure-(1-halogenalkyl)estern

Chloroacylation and bromoacylation of carbonyl compounds IV. Investigations of the reaction mechanism The kinetics of the Lewis acid catalyzed reaction between acyl chlorides and carbonyl compounds (especially aldehydes) is investigated by ¹H-NMR.-spectroscopy. With acetyl chloride as starting material a second order reaction is observed; the rate of the reaction increases in polar solvents as well as with increasing electron-donating capacity of the aldehyde. - With benzoyl chloride as starting material the reaction is first order with respect to benzoyl chloride, but zero order with respect to the aldehyde. The reaction rate is strongly influenced by the substituents of the benzoyl chloride. - Two polar reaction mechanisms which are in accord with these results are outlined and discussed.     

Composites based on phenyl substituted cobalt porphyrins with Nafion as catalysts for oxygen electroreduction

The electrochemical behavior of composite materials based on phenyl substituted cobalt porphyrins and Nafion is studied. Several cobalt porphyrins with presumably predictable variation of their hydrophilic/hydrophobic properties due to different donor and acceptor substituents in the para position of phenyl rings are synthesized and studied. It is shown that introduction of Nafion into a system with acceptor substituents results in a significant acceleration of the model oxygen reduction reaction. This allows assuming that a bond between a proton of the Nafion sulfogroup with the porphyrin active center is most probable in this very group of porphyrins, which facilitates the protonation step required for activation of the oxygen molecule. A certain correlation is found between the model reaction of oxygen electroreduction (halfwave potential, reaction rate constant) and Hammett constant varying as dependent on the nature of peripheric substituents.