Degree Of Methyl Substitution Research Articles

Effects of Methyl Substitution and Leaving Group on E2/SN2 Competition for Reactions of F- with RY (R = CH3, C2H5, iC3H7, tC4H9; Y = Cl, I).

The competition between base-induced elimination (E2) and bimolecular nucleophilic substitution (SN2) is of significant importance in organic chemistry and is influenced by many factors. The electronic structure calculations for the gas-phase reactions of F- + RY (R = CH3, C2H5, iC3H7, tC4H9, and Y = Cl, I) are executed at the MP2 level with aug-cc-pVDZ or ECP/d basis set to investigate the α-methyl substitution effect. The variation in barrier height, reaction enthalpy, and competition of SN2/E2 as a function of methyl-substitution and leaving group ability has been emphasized. And the nature of these rules has been explored. As the degree of methyl substitution on α-carbon increases, the E2 channel becomes more competitive and dominant with R varying from C2H5, iC3H7, to tC4H9. Energy decomposition analysis offers new insights into the competition between E2 and SN2 processes, which suggests that the drop in interaction energy with an increasing degree of substitution cannot compensate for the rapid growth of preparation energy, leading to a rapid increase in the SN2 energy barrier. By altering the leaving group from Cl to I, the barriers of both SN2 and E2 monotonically decrease, and, with the increased number of substituents, they reduce more dramatically, which is attributed to the looser transition state structures with the stronger leaving group ability. Interestingly, ∆E0‡ exhibits a positive linear correlation with reaction enthalpy (∆H) and halogen electronegativity. With the added number of substituents, the differences in ∆E0‡ and ∆H between Y = Cl and I likewise exhibit good linearity.

Effects of methyl substitution on the auto-ignition of C16 alkanes

The auto-ignition quality of diesel fuels, quantified by their cetane number or derived cetane number (DCN), is a critical design property to consider when producing and upgrading synthetic paraffinic fuels. It is well known that auto-ignition characteristics of paraffinic fuels depend on their degree of methyl substitution. However, there remains a need to study the governing chemical functionalities contributing to such ignition characteristics, especially in the case of methyl substitutions, which have not been studied in detail. In this work, the auto-ignition of 2,6,10-trimethyltridecane has been compared with the reference hydrocarbons used for cetane number determination, i.e. n-hexadecane and heptamethylnonane, all of them being C16 isomers. Results from a constant-volume combustion chamber under different pressure and temperature initial conditions showed that the ignition delay time for both cool flame and main combustion events increased less from n-hexadecane to trimethyltridecane than from trimethyltridecane to heptamethylnonane. Additional experimental results from blends of these hydrocarbons, together with kinetic modelling, showed that auto-ignition times and combustion rates were correlated to the concentration of the functional groups indicative of methyl substitution, although not in a linear manner. When the concentration of these functional groups decreased, the first stage OH radical concentration increased and ignition delay times decreased, whereas when their concentration increased, H2O2 production was slower and ignition was retarded. Contrary to the ignition delay times, DCN was correlated linearly with functional groups, thus homogenizing the range of values and clarifying the differences between fuels.

Coagulation of methylated arsenic from drinking water: Influence of methyl substitution

Methylated arsenic can be found in virtually all earth surface environments. So far, however, little information has been collected regarding their removal by coagulation. In this study, the removal of monomethylarsenate (MMA) and dimethylarsenate (DMA) from drinking water by coagulation was investigated from the viewpoint of methyl substitution. Results indicated that FeCl3 was more efficient than AlCl3 and polyaluminum chloride (PACl) in methylated As removal. For the initial arsenic concentration of 200μg/L, an FeCl3 dosage of 0.2mmol Fe/L was sufficient to attain about 95% removal of MMA, while a dosage of 0.6mmol Fe/L achieved about 57% removal of DMA. Arsenic removal efficiency was negatively correlated with the degree of methyl substitution. With the increase in methyl group number, the quantity of negatively charged arsenic species decreased and molecular size increased, leading to the decrease of methylated As removal by coagulation. Adsorption on preformed hydroxide flocs was the major mechanism during coagulation. Both FTIR and XPS results indicated that the AsO group of As might substitute the OH group of Fe/Al hydroxide to form a Fe/AlOAs complex. Furthermore, the use of traditional oxidants and coagulation aids exhibited limited help for improving coagulation removal of DMA.

Pyrolysis of phenolic impregnated carbon ablator (PICA).

Molar yields of the pyrolysis products of thermal protection systems (TPSs) are needed in order to improve high fidelity material response models. The volatile chemical species evolved during the pyrolysis of a TPS composite, phenolic impregnated carbon ablator (PICA), have been probed in situ by mass spectrometry in the temperature range 100 to 935 °C. The relative molar yields of the desorbing species as a function of temperature were derived by fitting the mass spectra, and the observed trends are interpreted in light of the results of earlier mechanistic studies on the pyrolysis of phenolic resins. The temperature-dependent product evolution was consistent with earlier descriptions of three stages of pyrolysis, with each stage corresponding to a temperature range. The two main products observed were H2O and CO, with their maximum yields occurring at ∼350 °C and ∼450 °C, respectively. Other significant products were CH4, CO2, and phenol and its methylated derivatives; these products tended to desorb concurrently with H2O and CO, over the range from about 200 to 600 °C. H2 is presumed to be the main product, especially at the highest pyrolysis temperatures used, but the relative molar yield of H2 was not quantified. The observation of a much higher yield of CO than CH4 suggests the presence of significant hydroxyl group substitution on phenol prior to the synthesis of the phenolic resin used in PICA. The detection of CH4 in combination with the methylated derivatives of phenol suggests that the phenol also has some degree of methyl substitution. The methodology developed is suitable for real-time measurements of PICA pyrolysis and should lend itself well to the validation of nonequilibrium models whose aim is to simulate the response of TPS materials during atmospheric entry of spacecraft.

Synthesis of a series of phenylacetic acid 1-β-O-acyl glucosides and comparison of their acyl migration and hydrolysis kinetics with the corresponding acyl glucuronides

We report the synthesis of the 1-β-O-acyl glucoside conjugates of phenylacetic acid (PAA), R- and S-α-methyl-PAA and α,α'-dimethyl-PAA, and measurement of their transacylation and hydrolysis reactivity by NMR methods. These are analogues of acyl glucuronides, the transacylation kinetics of which could be important in adverse drug effects. One aim of this work was to investigate whether, as previously postulated, the free carboxylate group of the acyl glucuronides plays a part in the mechanism of the internal acyl migration. In addition, such acyl glucosides are known to be endogenous biochemicals in their own right and investigation of their acyl migration propensities is novel. Our previously described selective acylation procedure has proved highly successful for 1-β-O-acyl glucuronide synthesis and when subsequently applied to 6-O-trityl glucose, it gave good yields and excellent anomeric selectivity. Mild acidolysis of the O-trityl intermediates gave the desired acyl glucosides in excellent yield with essentially complete β-selectivity. Measurement of the acyl glucoside transacylation kinetics by (1)H NMR spectroscopy, based simply on the disappearance of the 1-β-isomer in aqueous buffer at pH 7.4, showed marked differences depending on the degree of methyl substitution. Further kinetic modelling of the isomerisation and hydrolysis reactions of the acyl glucosides showed considerable differences in kinetics for the various isomeric reactions. Reactions involving the -CH(2)OH group, presumably via a 6-membered ring ortho-ester intermediate, are facile and the α-glucoside anomers are significantly more reactive than their β-counterparts. By comparison with degradation rates for the corresponding acyl glucuronides, it can be inferred that substitution of the carboxylate by -CH(2)OH in the acyl glucosides has a significant effect on acyl migration for those compounds, especially for rapidly transacylating molecules, and that thus the charged glucuronide carboxylate is a factor in the kinetics.

Photophysics of Methyl-Substituted Uracils and Thymines and Their Water Complexes in the Gas Phase

We report studies on several methylated uracils and thymines and thymine−water complexes in the gas phase using resonantly enhanced multiphoton ionization (REMPI) and laser-induced fluorescence (LIF) spectroscopy. Results from two different REMPI experiments provided strong evidence that, after photoexcitation to the S1 state, bare molecules were funneled into and trapped in a dark state via fast internal conversion. Lifetimes of this dark state were determined to be tens to hundreds of nanoseconds, depending on the internal energy and the degree of methyl substitution. The mass spectra of hydrated thymine clusters demonstrated dependence on the excitation wavelength, and the gradual loss of the ion signal with increasing water content across the absorption region of the S1 state indicated a reduced lifetime of the S1 state by the water solvent. In addition, the lifetime of the dark state also decreased gradually as thymine became more hydrated. On the basis of these results, we conclude that, in water so...

Photoaddition Reactions of Silyl Ketene Acetals with Aromatic Carbonyl Compounds: A New Procedure for β‐Hydroxyester Synthesis.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Photoaddition Reactions of Silyl Ketene Acetals with Aromatic Carbonyl Compounds: A New Procedure for ?-Hydroxyester Synthesis

Photochemical reactions of aromatic carbonyl compounds with silyl ketene acetals have been explored. Irradiation of acetonitrile or benzene solutions containing aryl aldehydes or ketones in the presence of silyl ketene acetals is observed to promo te formation of <TEX>${\beta}-hydroxyester$</TEX>, 2,2-dioxyoxetane and 3,3-dioxyoxetane products. The ratios of these photoproducts, which arise by competitive single electron transfer (SET) and classical Paterno-Buchi mechanistic pathways, is found to be dependent on the degree of methyl-substitution on the vinyl moieties of the ketene acetals in a manner which reflects expected alkyl substituent effects on the oxidation potentials of these electron rich donors. An analysis of the product distribution arising by irradiation of a solution containing butyrophenone (6) and the silyl ketene acetal 9, derived from methyl isobutyrate, provides an estimate of the rate constants for the competitive Norrish type Ⅱ, SET and Paterno-Buchi processes occuring. Finally, sequences involving silyl ketene acetal-aryl aldehyde or ketone photoaddition followed by 2,2-dioxyoxetane hydrolysis represent useful procedures for Claisen-condensation type, <TEX>${\beta}-hydroxyester$</TEX> synthesis.

Surface-Mediated Segregation and Transport Processes in Mixed Hydrocarbon Multilayer Assemblies

The transport and structural phase dynamics exhibited by multilayer assemblies comprised of cyclic and linear alkanes are analyzed with reflection absorption infrared (RAIR) and temperature-programmed desorption spectroscopies. Infrared spectroscopy reveals that methyl group substitutions have a significant effect on the nature of the mode softening seen in the C-H stretching region for surface-bound cyclohexanes. The magnitude of the red-shifts seen increases with the degree of methyl substitution. The energetics of the surface binding do not correlate in a simple way with the magnitude of red-shifts seen in the RAIR spectra, however. We find instead that the strengths of the surface interactions are more directly correlated with both the size and shape of the molecule (with the latter presumably reflecting its ability to form a densely packed structure). We also find that the diffusion of molecules in a mixed hydrocarbon multilayer assembly is weakly activated, with substantial interlayer mixing being seen at temperatures significantly below the threshold for the desorption of the multilayer. The mixing, while driven by mass action, shows a pronounced bias for the surface binding of n-alkanes over cycloalkanes of similar molecular weight (e.g., n-octane is more strongly bound on Pt(111) than is cis-1,3-dimethylcyclohexane). The data strongly suggest that attractive lateral interactions in the adsorbed layers lead to the biases seen in this surface-induced segregation. Thermal desorption spectra confirm this sensitivity and interestingly show multiple monolayer desorption features for cyclic alkane adsorbates when mixed in a monolayer assembly with an appropriate linear n-alkane. We suggest that the attractive lateral interactions in the monolayer lead to the formation of island domains and that the desorption kinetics appear to sensitively reflect this underlying rate/structure sensitivity.

Effects of alkyl substitution on ionization energies of alkanes and haloalkanes and on heats of formation of their molecular cations Part 1. Simple methyl derivatives

It has been found that the differences between adiabatic ionization energies (IEs) of methyl derivatives, ΔIE(CH 3X/C(CH 3 m X), are linearly correlated with l/ n 2, where n is the principal quantum number of the valence electron of the atom X; X represents the univalent atoms, H, Cl, Br and I; and m is the degree of methyl substitution and equals 1, 2 or 3. The heats of formation of methylated cations, C(CH 3) m- m X +, have been estimated. The IEs of alkyl fluorides and the heats of formation of the fluoride cations, C(CH 3) m H 3- m F +, have also been discussed.

13C [formula omitted] n.m.r. study of coke formation on HZSM-5

The carbonaceous residue (coke) formed on HZSM-5 during the conversion of methanol to gasoline was studied by 13C n.m.r. spectroscopy using cross-polarization and magic angle spinning and by e.p.r. spectroscopy. Measurements were made on samples with different coke contents and on samples from which some external coke had been removed by treatment with an oxygen plasma. It was found that the amount of carbon visible by n.m.r. differed from that expected from the measured coke contents and that the ratio of these amounts varied with the degree of coking. We concluded that the n.m.r.-visible carbon was most probably composed of methyl-substituted polycyclic aromatic hydrocarbons, the degree of methyl substitution and ring condensation changing with the coke content. It was inferred that these methyl-substituted polycyclic aromatics were present in the channels of the zeolite whereas the coke invisible to n.m.r. could be highly condensed polycyclic aromatics or “graphitic” structures on the external surfaces of the zeolite crystals. It appears that initial coke formation is internal and that external coke forms at latter stages in the deactivation process.

Reductive addition of glutathione to p-benzoquinone, 2-hydroxy- p-benzoquinone, and p-benzoquinone epoxides. Effect of the hydroxy- and glutathionyl substituents on p-benzohydroquinone autoxidation

The reductive addition of GSH to p-benzoquinones, 2-hydroxy- p-benzoquinone, and 2,3-epoxy- p-benzoquinones with different degree of methyl substitution was studied in terms of absorption spectral changes and autoxidation reactions. The nucleophilic addition of GSH to p-benzoquinone yields a glutathionyl- p-benzohydroquinone product with maximal absorption at λ 303nm. This compound autoxidizes slowly - but at a rate 8-fold higher than the parent hydroquinone - to glutathionyl- p-benzoquinone, which reveals maximal absorption at λ 367nm. The autoxidation of the glutathionyl derivative is accompanied by O 2 consumption and H 2O 2 formation. The nucleophilic addition of GSH to either 2-hydroxy- p-benzoquinone or 2,3-epoxy- p-benzoquinone yields the same primary molecular product, 2-hydroxy-5-glutathionyl- p-benzohydroquinone, a compound that shows maximal absorption at λ 300nm and autoxidizes at rates substantially higher (44-fold) than the parent glutathionylhydroquinone lacking a −OH substituent. The autoxidation product, 2-hydroxy-5-glutathionyl- p-benzoquinone, reveals maximal absorbance at λ 343nm as well as a resolved absorption band at longer wavelengths ( λ 520nm), the latter contributed by the −OH substituent. The glutathionyl substituent exerted only minor changes in the reduction potential of the quinones, whereas the −OH substituent lowered significantly the half-wave reduction potential, as measured in aqueous solutions. The rate of autoxidation was markedly enhanced by both substituents as follows: hydroxy-glutathionyl- p-benzohydroquinone ⪢ hydroxy- p-benzohydroquinone ⪢ glutathionyl- p-benzohydroquinone > p-benzohydroquinone. Superoxide dismutase enhanced the rate of autoxidation of p-benzohydroquinone and its glutathionyl adduct, whereas it inhibited autoxidation of the hydroxy derivatives with or without glutathionyl substitution. The biochemical significance of these results is discussed in terms of the pro-oxidant character of the reductive addition of GSH to p-benzoquinones, α-hydroxyquinones, and quinone epoxides.

Spaltung aromatischer und vinylischer CH-Bindungen durch Cyclopentadienylruthenium(II)-Komplexe

Zusammenfassung Neopentylruthenium complexes of the type Cp(Me n Ph 3- n P) 2 RuCH 2 CMe 3 (Cp  C 5 H 5 , C 5 Me 5 ; n = 0−3) react with aromatic CH bonds in benzene or toluene to give the corresponding phenyl- or tolyl-ruthenium compounds and neopentane. With toluene, a mixture of the m - and p -tolyl complexes is formed. The reactivity of the neopentylruthenium compounds decreases with increasing degree of methyl substitution at the phosphorus atom, and with bidentate phosphane ligands. A permethylated Cp group increases the reactivity. (C 5 Me 5 )(Me 3 P) 2 RuCH 2 CMe 3 reacts also with the vinylic CH bond of ethylene to form an (η 2 -ethylene)vinylruthenium complex.

ChemInform Abstract: Synthesis, Stereochemical Analysis, and Ligand‐Transformation Reactions of New Classes of Bicapped Tricobalt Clusters, (Co3(η5‐C5H5‐xMex)3(μ3‐X)(μ3‐Y))n (x = 0, 1, 5), Containing Mixed π‐Acceptor X and π‐Donor Y Capping Ligands (X: CO, NO; Y: NSiMe3, NC(O)NH2, NH): A Rational Preparative Route to Nitrene (μ3‐NH) Clusters and the Unprecedented Direct Exchange of an Isoelectronic μ3‐NO+ Ligand for a μ3‐CO Ligand.

AbstractThe reactions of trimethylsilyl azide with the starting compounds (I) lead to different products depending on the degree of methyl substitution at the cyclopentadienyl ring.

Argentation Constants of Styrene Derivatives

Abstract Equilibrium constants for the formation of silver ion complexes (argentation constants) of styrene derivatives in water were measured at 0 and 25°C by Andrews and Keefer’s solubility method. The argentation constants for the 1 : 1 complex (K1) decreased in this order: cis-β-alkylstyrene&amp;gt;α-alkylstyrene&amp;gt;trans-β-alkylstyrene, for a given alkyl group. This order was related to differences in the resonance stabilization of the double bond. The K1 values of the α- and β-alkylstyrene series did not seem to correlate with Taft’s steric substituent constant, Es, within the respective series. However, when the hyper conjugation effect of alkyl substituents was considered, a good linearity between logK1 and Hancock’s parameter, Esc, was obtained. Thus, the steric effect was separated from the hyperconjugation effect, and it was found that the steric effects for the three series are nearly identical. Thermodynamic data indicated that an isokinetic relationship holds between ΔH and ΔS and that the argentation equilibrium is controlled mainly by the entropy factor. The degree of methyl substitution at the double bond of styrene slightly influenced the K1 values, although 1,2-dihydronaphthalene had an abnormally large K1 value.