Methyl-substituted Hydrocarbons Research Articles

Odd-even alternations in helical propensity of a homologous series of hydrocarbons.

Odd and even homologues of some n-alkane-based systems are known to exhibit notably different trends in solid-state properties; a well-known illustration is the zigzag plot of their melting point versus chain length. Odd-even effects in the solid state often arise from intermolecular interactions that involve fully extended molecules. These effects have also been observed in less condensed phases, such as self-assembled monolayers; however, the origins of these effects in such systems can be difficult to determine. Here we combined NMR and computational analysis to show that all-syn contiguously methyl-substituted hydrocarbons, with chain lengths from C6 to C11, exhibit a dramatic odd-even effect in helical propensity. The even- and odd-numbered hydrocarbons populate regular and less-controlled helical conformations, respectively. This knowledge will guide the design of helical hydrocarbons as rigid scaffolds or as hydrophobic components in soft materials.

A Barrierless Pathway Accessing the C9H9 and C9H8 Potential Energy Surfaces via the Elementary Reaction of Benzene with 1-Propynyl

The crossed molecular beams reactions of the 1-propynyl radical (CH3CC; X2A1) with benzene (C6H6; X1A1g) and D6-benzene (C6D6; X1A1g) were conducted to explore the formation of C9H8 isomers under single-collision conditions. The underlying reaction mechanisms were unravelled through the combination of the experimental data with electronic structure and statistical RRKM calculations. These data suggest the formation of 1-phenyl-1-propyne (C6H5CCCH3) via the barrierless addition of 1-propynyl to benzene forming a low-lying doublet C9H9 intermediate that dissociates by hydrogen atom emission via a tight transition state. In accordance with our experiments, RRKM calculations predict that the thermodynamically most stable isomer – the polycyclic aromatic hydrocarbon (PAH) indene – is not formed via this reaction. With all barriers lying below the energy of the reactants, this reaction is viable in the cold interstellar medium where several methyl-substituted molecules have been detected. Its underlying mechanism therefore advances our understanding of how methyl-substituted hydrocarbons can be formed under extreme conditions such as those found in the molecular cloud TMC-1. Implications for the chemistry of the 1-propynyl radical in astrophysical environments are also discussed.

The Structural Basis for the Production of Cancer and Detoxification by Oxidized Metabolites of Mesoanthracenic Methylated and Non-Methylated Polynuclear Hydrocarbons: a Paradigm Shift

Although many attempts have been made since 1930 to explain the production of cancer in experimental animals by oxidized metabolites of polynuclear aromatic hydrocarbons (PAH) with either molecular structure types ArCH2X or ArX, the simplest interpretation is that one type must function in experimental animals to produce cancer, whereas the other structural type prevents the production of cancer by formation of detoxification products. The specific predictions of the Unified Theory of carcinogenesis by PAH or PACs (Polynuclear Aromatic Compounds) depends on the validity of the predictions for the production of cancer by PACs with structural types ArCH2X or ArX. These two structural types have been adopted as crucial models for PAC metabolites by Flesher, Sydnor and colleagues since the 1970s. We were aided by the experiments and strong inferences of Professors Fieser, Fried, Boyland, Sims, and Huggins, and the theoretical studies of a possible structure of the ultimate carcinogen for 7-methylbenz[a]anthracene and closely related hydrocarbons by Brookes and Dipple together with the calculations of Dewar that predict the most reactive centers for substitution reactions in unsubstituted hydrocarbons. Shear in the late 1930s and 1940s was the first to show that meso-aldehydes and meso-methyl-substituted hydrocarbons function as precursors to active compounds with structure type ArCH2X. The discoveries by Flesher, Sydnor and coworkers demonstrated that biological methylation must take place at the most reactive center in non-methylated hydrocarbons catalyzed by cytosolic SAM-dependent methyltransferases, and the methyl group must subsequently be hydroxylated and conjugated with a good leaving group, if the PAC is to be carcinogenic. This is precisely the same site of special reactivity required for methylation by chemical synthesis as calculated by the Dewar molecular orbital approach. Blocking the reactive center in the aromatic nucleus (in the form of products as ArX) with the same groups attached to the meso-methyl group of methyl-substituted hydrocarbons (in the form ArCH2X) which produces cancer typically abolishes the production of cancer. Consequently, no theory relating ring oxidation to the production of cancer can account for the fact that substitution of reactive centers in non-methylated hydrocarbons with hydroxy or acetoxy groups, while simultaneously blocking methylation, also abolish its carcinogenic activity. If alternative hypotheses or theories do not cite the fundamental studies of Fieser and of Shear and Leiter there is little possibility that the alternative theory is correct. It seems obvious that most, if not all, hypotheses or theories that attempt to relate ring oxidized metabolites of PAH to the production of cancer have been disproved.

Synthesis of alternating hydroxy- and methyl-substituted hydrocarbons by oxymercuration of cyclopropylcarbinols

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTSynthesis of alternating hydroxy- and methyl-substituted hydrocarbons by oxymercuration of cyclopropylcarbinolsDavid B. Collum, W. Clark. Still, and Fariborz. MohamadiCite this: J. Am. Chem. Soc. 1986, 108, 8, 2094–2096Publication Date (Print):April 1, 1986Publication History Published online1 May 2002Published inissue 1 April 1986https://doi.org/10.1021/ja00268a062RIGHTS & PERMISSIONSArticle Views396Altmetric-Citations276LEARN 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 (430 KB) Get e-Alerts Get e-Alerts

The effect of steric strain in the bay-region of polycyclic aromatic hydrocarbons: Tumorigenicity of alkyl-substituted benz[ a]anthracenes

3,6-Dimethylcholanthrene (3,6-DMC) and 7,11,12-trimethylbenz[ a]anthrene (7,11,12-TMBA) were tested for tumor-initiating activity on mouse skin as an approach to evaluate the potential role of steric strain in the bay-region on tumorigenic potency. Methyl-substitution at the 6-position of 3-methylcholanthrene (3-MC) increases steric strain in the bay-region of the hydrocarbon as it does at the 12-position of benz[ a]anthracene (BA) causing both hydrocarbons to become non-planar. 3,6-DMC had at least 2- to 3-fold higher tumor-initiating activity than did 3-MC. Introduction of an 11-methyl group in 7,12-dimethylbenz[ a]anthracene (7,12-DMBA) results in the formation of a more highly hindered (buttressing effect) hydrocarbon. 7,11,12-TMBA had 5% or less of the tumor-initiating activity of 7,12-DMBA, although the hydrocarbon still had relatively high tumorigenic activity on mouse skin. The results obtained with 3,6-DMC and studies reported previously with other methyl-substituted hydrocarbons, show that hydrocarbons possessing steric strain in the bay-region of the molecule can have enhanced tumorigenic activity. The basis of this steric effect remains unclear, however, as a result of the decreased tumorigenic activity of the 11-methyl-substituted derivative of 7,12-DMBA. The weak tumor-initiating activity of BA was enhanced at least 4- to 8-fold as a result of methyl-substitution at the 6- and 8-positions (6,8-dimethylbenz[ a]anthracene). The higher tumorigenic activity of 6,8-dimethylbenz[ a]anthracene compared to BA is consistent with a presumed decrease in metabolic detoxification of the dimethyl-substituted derivative at the 5,6- and 8,9-double bonds.

Structure-resonance theory and electrochemical oxidation potentials of π-electron hydrocarbons

The polarographic half-wave oxidation potentials of 51 benzenoid hydrocarbons are correlated with the logarithms of the structure counts of the neutral molecule and the radical cation, ln SC R and ln SC R + , respectively. For example, for 25 unsubstituted hydrocarbons and 26 methylsubstituted hydrocarbons, the following relationship was obtained: E 1 2 (ox) = 4.018 + 1.607 ln SC R −1.027 ln SC R + ; r = 0.955, s = ± 0.07. Such an excellent correlation indicates that effects associated with delocalization of π-electrons and the π-electron charge distribution in the neutral and charged molecular species, respectively, are significant in electrochemical oxidation, contrary to an earlier literature report which claimed that they are not. The results also show that the structure counts provide a simple working method for correlations of the electrochemical reactivities of polynuclear aromatic hydrocarbons and their derivatives.

Action of 7,8-benzoflavone on incidence of skin tumors induced by polycyclic hydrocarbons

Tumors were induced in CBA mice or (C57BL×CBA)F1 hybrids by application of benzo-(a) pyrene (BP), 6-methyl-BP, 6-formyl-BP, or 7,12-dimethylbenz (a) anthracene (DMBA) to the dorsal skin. If 7,8-benzoflavone (BF) was mixed with the above compounds it reduced the carcinogenic action of the methyl-substituted hydrocarbons (DMBA and 6-methyl-BP) but did not affect the carcinogenic action of BP itself or of 6-formyl-BP. The inhibitory action of BF on carcinogenesis induced by methyl-substituted hydrocarbons is due, it can tentatively be suggested, to inhibition of oxidation of the methyl group.

Action of mouse antiserum against isologous aggregated immunoglobulins on transplantation and antitumor immunity

Tumours were induced in CBA mice or (C57BL X CBA)F1 hybrids by the application of benz(a)pyrene (BP), 6-methylBP, 6-formylBP or 7,12-dimethylbenzanthracene (DMBA) to the skin. 7,8-benzoflavone (BF) used in combination with the mentioned agents decreased the carcinogenic action of methyl-substituted hydrocarbons--DMBA and 6-methylBP and had no effect on the carcinogenic action of BP and 6-formylBP. It is suggested that the inhibitory action of BF on carcinogenesis induced by methyl-substituted hydrocarbons was due to the inhibited oxidation of the methyl group.