Methyl-substituted Cyclohexanes Research Articles

Utilization of 13C NMR Carbon Shifts for the Attribution of Diastereomers in Methyl-Substituted Cyclohexanes.

In this report, we address the challenge of assigning diastereomers for methyl cyclohexanes, particularly those with quaternary centers, which remains nontrivial despite modern NMR techniques. By utilizing a HSQC NMR experiment to identify methyl-carbons coupled with a simple conformational analysis, we identified an effective and quite general method for assigning stereochemistry, even in cases where diastereomeric mixtures are inseparable.

Hydroconversion of lignin-derived platform compound guaiacol to fuel additives and value-added chemicals over alumina-supported Ni catalysts

Hydroconversion of guaiacol (GUA) over γ-Al2O3 and phosphatized-γ-Al2O3 (γ-Al2O3(P)) supported Ni catalysts was initiated by Lewis-sites and active Ni sites. Conversion proceeded via transmethylation and hydrodemethylation/hydrodemethoxylation as major and minor pathways, respectively, resulting in mainly catechol and methylcatechols, and via series of consecutive hydrodehydroxylation (HDHY) and ring hydrogenation (HYD) reactions leading to partially and fully deoxygenated, saturated, and unsaturated products. High Ni-loading and high H2 pressure promoted the formation of cyclohexane and methyl-substituted cyclohexanes; however, above 300 °C the hydrogenation–dehydrogenation equilibrium favored the formation of benzene and methyl-substituted benzenes. Ni/γ-Al2O3(P) showed suppressed HYD/HDHY activity resulting in pronounced formation of catechol and/or phenol and their methyl-substituted derivatives. Surface phenolate species were substantiated as surface intermediates of hydrodeoxygenation. Phosphatizing reduced the concentration of both basic OH and Lewis acid (Al+) – Lewis base (O–) pair surface sites of the γ-Al2O3 support and, thereby, suppressed phenolate formation and hydrodeoxgenation.

Coadsorption of cyclohexanes and CO on Ni(755): promoting effect of CO and increase of decomposition fraction by methyl substituent

Coadsorption of methyl-substituted cyclohexanes and CO has been investigated by temperature-programmed desorption (TPD). Coadsorbed CO promotes decomposition of cyclohexanes. Decomposition fraction (ratio of decomposed molecules to adsorbed ones) of cyclohexanes was determined with changing coverages of CO and cyclohexanes. Decomposition fraction increases sharply when a tertiary CH bond is introduced into a cyclohexane ring by methyl substitution. This is consistent with the fact that dissociation energy of a tertiary CH bond is smaller than those of primary and secondary CH bonds.

Conformational analysis of six- and twelve-membered ring compounds by molecular dynamics.

A molecular dynamics (MD)-based conformational analysis has been performed on a number of cycloalkanes in order to demonstrate the reliability and generality of MD as a tool for conformational analysis. MD simulations on cyclohexane and a series of methyl-substituted cyclohexanes were performed at temperatures between 400 and 1200 K. Depending on the simulation temperature, different types of interconversions (twist-boat-twist-boat, twist-boat-chair and chair-chair) could be observed, and the MD simulations demonstrated the expected correlation between simulation temperature and ring inversion barriers. A series of methyl-substituted 1,3-dioxanes were investigated at 1000 K, and the number of chair-chair interconversions could be quantitatively correlated to the experimentally determined ring inversion barrier. Similarly, the distribution of sampled minimum-energy conformations correlated with the energy-derived Boltzmann distribution. The macrocyclic ring system cyclododecane was subjected to an MD simulation at 1000 K and 71 different conformations could be sampled. These conformations were compared with the results of previously reported conformational analyses using stochastic search methods, and the MD method provided 19 out of the 20 most stable conformations found in the MM2 force field. Finally, the general performance of the MD method for conformational analysis is discussed.

Stereochemical Analysis of Methyl-Substituted Cyclohexanes Using 2 + 1 Resonance-Enhanced Multiphoton Ionization Spectroscopy

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTStereochemical Analysis of Methyl-Substituted Cyclohexanes Using 2 + 1 Resonance-Enhanced Multiphoton Ionization SpectroscopyDale R. Nesselrodt, Alan R. Potts, and Tomas. BaerCite this: Anal. Chem. 1995, 67, 23, 4322–4329Publication Date (Print):December 1, 1995Publication History Published online1 May 2002Published inissue 1 December 1995https://doi.org/10.1021/ac00119a019RIGHTS & PERMISSIONSArticle Views72Altmetric-Citations7LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (974 KB) Get e-Alerts Get e-Alerts

Kinetics of abstraction reactions of tert-butoxyl radicals with cyclohexane and methyl-substituted cyclohexanes in the gas phase

The reactions of tert-butoxyl radicals, generated by the pyrolysis of di-tert-butyl peroxide with cyclohexane and a number of methyl-substituted cyclohexanes have been studied. The Arrhenius parameters corresponding to overall hydrogen abstraction have been determined in the temperature range 399–434 K based on the values of the Arrhenius parameters for pressure-dependent decomposition of tert-butoxyl radicals obtained by Batt and Robinson (L. Batt and G. N. Robinson, Int. J. Chem. Kinet., 1982, 14, 1053). The compounds studied were cyclohexane, methylcyclohexane, 1,2-dimethylcyclohexane, 1,3-dimethylcyclohexane, 1,1-dimethylcyclohexane, 1,1,3-trimethylcyclohexane and 1,2,4-trimethylcyclohexane. Arrhenius parameters for the abstraction of secondary and tertiary hydrogen atoms by tert-butoxyl radicals have been determined to be log(A/dm3 mol–1 s–1)= 9.23 ± 0.07, E/kJ mol–1= 25.4 ± 0.9 and log(A/dm3 mol–1 s–1)= 9.90 ± 0.11, E/kJ mol–1= 22.2 ± 1.1, respectively.

ChemInform Abstract: Identification of Conformational Isomers of Methyl‐Substituted Cyclohexanone and Tetrahydropyran Frozen in a Molecular Beam.

Examination of the n→3s Rydberg transition in several methyl-substituted cyclohexanes and tetrahydropyrans (THP's) cooled in a free-jet expansion reveals the presence of two distinct molecular species. These have been assigned as pairs of conformational isomers that freeze out in the molecular beam at their room temperature populations as a result of the substantial inversion barrier of the saturated six-membered ring system

Identification of conformational isomers of methyl-substituted cyclohexanone and tetrahydropyran frozen in a molecular beam

Examination of the n→3s Rydberg transition in several methyl-substituted cyclohexanes and tetrahydropyrans (THP's) cooled in a free-jet expansion reveals the presence of two distinct molecular species. These have been assigned as pairs of conformational isomers that freeze out in the molecular beam at their room temperature populations as a result of the substantial inversion barrier of the saturated six-membered ring system

ChemInform Abstract: Reactions of Methyl Radicals with Cyclohexane and Methyl‐Substituted Cyclohexanes

The reactions of methyl radicals, generated by the photolysis of acetone, with cyclohexane and a number of methyl-substituted cyclohexanes have been studied. The Arrhenius parameters corresponding to overall hydrogen abstraction have been obtained from experiments in the temperature range 100–200 °C based on the value of the rate constant for recombination of methyl radicals. The overall rate constants for hydrogen abstraction by methyl radicals could be adequately expressed as the sum of contributions from each tertiary, secondary and primary hydrogen atom for all of the following nine compounds which were studied: cyclohexane, methylcy clohexane, 1,2-dimethylcyclohexane, 1,3-dimethylcyclohexane, 1,4-dimethylcyclohexane, 1,1-dimethylcyclohexane, 1,3,5-trimethylcyclohexane, 1,2,4-trimethylcyclohexane and 1,1,3-trimethylcyclohexane. The Arrhenius parameters for the abstraction of secondary hydrogen atoms are log (A/cm3 mol–1s–1)= 10.83, E/kJ mol–1= 37.4, and those for the abstraction of tertiary hydrogen atoms are log (A/cm3 mol–1 s–1)= 11.78, E/kJ mol–1= 35.7.

Reactions of methyl radicals with cyclohexane and methyl-substituted cyclohexanes

The reactions of methyl radicals, generated by the photolysis of acetone, with cyclohexane and a number of methyl-substituted cyclohexanes have been studied. The Arrhenius parameters corresponding to overall hydrogen abstraction have been obtained from experiments in the temperature range 100–200 °C based on the value of the rate constant for recombination of methyl radicals. The overall rate constants for hydrogen abstraction by methyl radicals could be adequately expressed as the sum of contributions from each tertiary, secondary and primary hydrogen atom for all of the following nine compounds which were studied: cyclohexane, methylcy clohexane, 1,2-dimethylcyclohexane, 1,3-dimethylcyclohexane, 1,4-dimethylcyclohexane, 1,1-dimethylcyclohexane, 1,3,5-trimethylcyclohexane, 1,2,4-trimethylcyclohexane and 1,1,3-trimethylcyclohexane. The Arrhenius parameters for the abstraction of secondary hydrogen atoms are log (A/cm3 mol–1s–1)= 10.83, E/kJ mol–1= 37.4, and those for the abstraction of tertiary hydrogen atoms are log (A/cm3 mol–1 s–1)= 11.78, E/kJ mol–1= 35.7.

Gas chromatographic separation of isomeric hydrocarbons of the norbornane series on graphitized carbon-black

The possibility of using graphitized thermal carbon black for the separation of hydrocarbons, such as methyl-substituted cyclohexanes and alkyl-substituted bicyclo[2.2.1]heptane, has been investigated. The retention sequence found for different isomers makes it possible to draw a conclusion about their structure.

Linear free energy relationships in heterogeneous catalysis: IX. Kinetic study of catalytic dehydrogenation of cyclohexanes by the pulse technique

The dehydrogenation of methyl-substituted cyclohexanes catalyzed by chromiaalumina and molybdena-alumina was studied by means of the pulse technique. In the case, where the rate is expressed as v = αβp (1 + αp) , the relation between the conversion, x, and the maximum partial pressure, p m, of a triangular pulse can be formulated as follows, under the ideal conditions: xF 2W = β αp m − ln(αp m + 1) αp m 2 By using this equation, α and β of all the reactants used were obtained at a few temperatures. From the fact that α increases with increasing temperature as well as from the results of some supplementary experiments, it is demonstrated that αβ stands for the rate constant of the first hydrogen abstraction and β for that of the second one. It was found that αβ varied remarkably with the reactants used and was correlated to the delocalizability, a quantum-chemical reactivity index, though β was almost invariant. Therefore, it is concluded that the slowest step is the dehydrogenation to cyclomonoolefin, where each of two hydrogens is abstracted stepwise, and that the first hydrogen abstraction from an original hydrocarbon determines the relative reactivity.

Linear free energy relationships in heterogeneous catalysis: VII. Reactivity of ring hydrogens in catalytic dehydrogenation of cyclohexanes

The rates of dehydrogenation of various methyl-substituted cyclohexanes catalyzed by chromia-alumina and molybdena-alumina were numerically analyzed in terms of the varying reactivities of the ring hydrogens classified into several groups, from the viewpoint of the LFER. It is demonstrated that the rate, v(R, T), can be represented as the sum of the reactivities of the ring hydrogens, v H( m, T), as v(R, T) = ∑ w(R, m) v H( m, T), where w(R, m) is the number of mth hydrogens in a reactant R. The logarithm of the reactivity of ring hydrogen is, in turn, linearly related to the delocalizability, D r R(H), a quantum-chemical reactivity index for hydrogen abstraction reactions, as log v H (m,T) = log v H (O,T) + γ(T) ΔD r R ( H m) 2.303RT , where v H( O, T) is the reactivity of an imaginary hydrogen whose delocalizability is 1.0, and γ( T) is a proportional constant. Furthermore, from the temperature dependence, γ( T) has been correlated to the isokinetic temperature, T 8, and as the result the rates are expressed as follows: v( R,T) = v H (O, ∞) exp ( −E A (0) RT) ∑ w( R,m) exp γ D (1 − T T 8 ) ΔD r R (H m) RT where γ D is a constant independent of temperature. Thus, the rate of dehydrogenation of any reactant at any temperature can be calculated with the knowledge of delocalizabilities inherent to ring hydrogens and four parameters characteristic for a catalyst, i.e., v H( O, ∞), E A( O), γ d, and T 8. This equation is in accordance with the reaction scheme where the dehydrogenation to monoolefin is the slow step.