The mechanism and rate parameters for the pyrolysis of n-hexane in the range 723–823 K

The pyrolysis of n-hexane has been investigated in the ranges 723–823 K and 10–100 Torr at up to 3% decomposition. The reaction is homogeneous and free from the self-inhibition by olefin products observed for several other alkanes. The products of the reaction are hydrogen, methane, ethane, ethene, propene, but-1-ene, and pent-1-ene, with smaller amounts of propane. It is shown that the results are in quantitative agreement with a conventional Rice-Herzfeld chain mechanism terminated by the combination and disproportionation of ethyl radicals, but with the mechanism extended so as to include the unimolecular isomerizations via a six-membered cyclic transition state between 1-hexyl and 2-hexyl (1-methylpentyl) radicals. The overall rate constant of initiation is estimated to be given by The rate constant for the reaction is given by which when combined with published data gives an Arrhenius plot curved upwards at low values of 1/T as has been observed for several other hydrogen abstraction reactions of methyl and of ethyl. Estimates are made of rate constants and ratios of rate constants for several reactions of the free radicals involved in the reaction. It is suggested that the minor product propane arises mainly from a hydrogen abstraction by 1-propyl from hexane with a contribution from a minor termination process involving ethyl and methyl.     

Further experiments on pyrolysis of 2,2-dimethylpropane behind shock waves

Pyrolysis of a dilute mixture of neopentane (2,2-dimethylpropane) has been studied behind incident shock waves at an average pressure of 0.35 atm; the reaction was followed by absorption spectroscopy for H atoms. In the temperature range 1230–1455 K, the rate constant for dissociation of neopentane to t-butyl and methyl radicals is 1.1 E 13 exp(?62 kcal/RT) s^?1. These data and some of the literature results, between 1000 and 1450 K, can be fitted by an RRKM model of the hindered Gorin type, with five active internal rotors in the complex. To match our data with other literature data down to 800 K, a vibrational model was more satisfactory, but this did not fit very low pressure pyrolysis data in the 1000–1100 K range. Apparently, the VLPP data are too high because of heterogeneous processes or chain reactions.     

The kinetics and mechanism of thermal decomposition of alkyl isocyanates

The kinetics of the gas-phase decomposition of ethyl, isopropyl, and t-butyl isocyanates have been studied in the temperature range of 380–530°C. t-Butyl isocyanate decomposes almost exclusively by a unimolecular route to isobutene and HNCO, but in EtNCO and i-PrNCO this route competes with a free-radical chain which produces CO, CH₄, and HCN or CH₃CN. In i-PrNCO, however, the chain process is very rapidly inhibited by the propene formed in the parallel unimolecular route. A minor heterogeneous bimolecular decomposition in each case gives rise to carbon dioxide and a carbodiimide. Mechanisms and trends in the alkyl isocyanates from methyl through t-butyl are discussed.     

Thermal degradation of poly(alkyl acrylates). II. Primary esters: Ethyl,n-propyl,n-butyl,and 2-ethylhexyl

Quantitative analyses of the products of thermal degradation of poly(ethyl acrylate), poly(n-propyl acrylate), poly(n-butyl acrylate) and poly(2-ethylhexyl acrylate) have been made, principally by the combined application of GLC and mass and infrared spectroscopy. Data are recorded in mass balance tables. The major gaseous products are carbon dioxide and the olefin corresponding to the ester group. The minor gaseous products include the corresponding alkane, the alkane/olefin ratio being of the order of 10^?2–10^?3, and traces of carbon monoxide and hydrogen. The alcohol corresponding to the alkyl group is the major liquid product but there are also traces of monomer and the corresponding methacrylate. Alcohol production exhibits autocatalytic properties. The chain fragment fractions of the products are colored yellow and have average chain lengths of 3.2, 3.3, 3.6, and 5.6 for the ethyl, n-propyl, n-butyl and 2-ethylhexyl esters, respectively. The infrared spectra are similar to those of the parent polymers but with well defined differences. Insolubility develops in the ethyl, n-propyl, and n-butyl esters, but the residual material from poly(2-ethylhexyl acrylate) remains soluble even at very advanced stages of degradation. All of these products and reaction characteristics are accounted for in terms of radical reactions with a unique initiation step.     

Glass transformation temperatures of polymers of olefin oxides and olefin sulfides

A number of polymers belonging to poly(olefin oxide) and poly(olefin sulfide) series have been prepared and their glass transformation temperatures determined by dilatometry and differential thermal analysis. In the poly(olefin oxide) series, the T_g remained practically unchanged as the length of the pendent alkyl group was increased from methyl to n-hexyl. However, a 20°C decrease in T_g was observed when the pendent group was changed from ethoxymethyl to n-hexoxymethyl. In the poly-(olefin sulfide) series, the T_g value decreased as the pendent alkyl group changes from methyl to ethyl. Replacement of ether oxygen in the polymer main chain by sulfide sulfur increased the T_g value. In some polymers, first-order transitions were observed, but their significance has not been assessed.     

Host–Guest Complexation Using Pillar[5]arene Crystals: Crystal-Structure Dependent Uptake,Release, and Molecular Dynamics of an Alkane Guest

Host–guest complexation has been mainly investigated in solution, and it is unclear how guest molecules access the assembled structures of host and dynamics of guest molecules in the crystal state. In this study, we studied the uptake, release, and molecular dynamics of n-hexane vapor in the crystal state of pillar[5]arenes bearing different substituents. Pillar[5]arene bearing 10 ethyl groups yielded a crystal structure of herringbone-type 1:1 complexes with n-hexane, whereas pillar[5]arene with 10 allyl groups formed 1:1 complexes featuring a one-dimensional (1D) channel structure. For pillar[5]arene bearing 10 benzyl groups, one molecule of n-hexane was located in the cavity of pillar[5]arene, and another n-hexane molecule was located outside of the cavity between two pillar[5]arenes. The substituent-dependent differences in molecular arrangement influenced the uptake, release, and molecular dynamics of the n-hexane guest. The substituent effects were not observed in host–guest chemistry in solution, and these features are unique for the crystal state host–guest chemistry of pillar[5]arenes.     

Rate constants for the reaction of OH radicals with a series of alkanes and alkenes at 299 ± 2 K

Using methyl nitrite photolysis in air as a source of hydroxyl radicals, relative rate constants for the reaction of OH radicals with a series of alkanes and alkenes have been determined at 299 ± 2 K. The rate constant ratios obtained are: relative to n-hexane = 1.00, neopentane 0.135 ± 0.007, n-butane 0.453 ± 0.007, cyclohexane 1.32 ± 0.04; relative to cyclohexane = 1.00, n-butane 0.341 ± 0.002, cyclopentane 0.704 ± 0.007, 2,3-dimethylbutane 0.827 ± 0.004, ethene 1.12 ± 0.05; relative to propene = 1.00, 2-methyl-2-butene 3.43 ± 0.13, isoprene 3.81 ± 0.17, 2,3-dimethyl-2-butene 4.28 ± 0.21. These relative rate constants are placed on an absolute basis using previous absolute rate constant data and are compared and discussed with literature data.     

Investigations on poly(vinyl chloride). I. Evolution of aromatics on pyrolysis of poly(vinyl chloride) and its mechanism

Poly(vinyl chloride) (PVC) alone or mixed with 10 wt-% and 50 wt-% TiO₂, SnO₂, ZnO, and Al₂O₃ were pyrolyzed by using a pyrolysis gas chromatograph. Benzene, toluene, ethylbenzene, o-xylene, styrene, naphthalene, and various chlorobenzenes were identified. No hydrocarbons could be detected in pyrolysis products of any samples at 200°C. More aromatic hydrocarbons than aliphatic hydrocarbons are released from the PVC–TiO₂ system and in preheated PVC. The contrary result is observed in the PVC–ZnO and PVC–SnO₂ systems. Aromatics having methyl endgroups are easily released from the PVC–ZnO and PVC–SnO₂ systems and at elevated pyrolysis temperature, because methylene groups are easily isolated along the chain by ZnO, SnO₂ and the heating. The release of ethylbenzene o-xylene, and chlorobenzenes suggests a repeated dehydrochlorination and recombination of HCl and Cl₂ to double bonds along the chain. Possible decomposition mechanisms of PVC are discussed.     

Investigations on poly(vinyl chloride). II. Pyrolysis of chlorinated polybutadienes as model compounds for poly(vinyl chloride)

The pyrolysis of chlorinated polybutadienes (CPB) was investigated by using a pyrolysis gas chromatograph. CPB corresponds to poly(vinyl chloride) (PVC) constructed with head–head and tail–tail linkages of the vinyl chloride unit. Benzene, toluene, ethyl-benzene, o-xylene, styrene, vinyltoluene, chlorobenzenes, naphthalene, and methylnaphthalenes were detected in the pyrolysis products from CPB above 300°C, and no hydrocarbons could be detected at 200°C. The pyrolysis products from CPB were similar to those from PVC and new products could not be detected. Lower aliphatics, toluene, ethylbenzene, o-xylene, chlorobenzenes, and methylnaphthalenes were released more easily from pyrolysis of CPB than from PVC; amounts of benzene, styrene, and naphthalene formed were small. These results support the conclusion that recombination of chlorine atoms with the double bonds in the polyene chain takes place and that scission of the main chain may depend on the location of methylene groups isolated along the polyene chain during the thermal decomposition of PVC.     

The pyrolysis of neopentane at small extents of reaction

The pyrolysis of neopentane, at small extents of reaction, was studied by gas chromatography, in Pyrex reaction vessels between 450° and 530°C and in the initial pressure range 25–200 mm Hg. At initial time, this thermal decomposition can be essentially represented by a homogeneous long-chain radical mechanism. The rate constant of the unimolecular initiation process is approximately given by the expression The initial rate constant of the global reaction (order 3/2) is nearly equal to This reaction is strongly inhibited by propene or isobutene and self-inhibited by the isobutene formed; an interpretation of all these inhibition phenomena of the neopentane pyrolysis is proposed. Our observations and conclusions, which have been summarized in communications during 1968 and 1969, are compared to those of other authors, particularly to the recent ones of Purnell and colleagues [13] and of Taylor and colleagues [14], [15].     

Anti-inflammatory and antiproliferative activities of Ixora brevifolia Benth. (Rubiaceae)

The crude extract and fractions from the branches of Ixora brevifolia, a tree found in the Brazilian Cerrado, were tested for anti-inflammatory and in vitro antiproliferative effects. The crude extract and n-hexane fraction exhibited significant inhibition of ear oedema in mice, while n-hexane-precipitated and chloroform fractions strongly inhibited the myeloperoxidase activity in ear tissue. The n-hexane and n-hexane-precipitated fractions showed strong growth inhibition for glioma cell line and the hydromethanolic fraction inhibited the growth of leukaemia cell line.     

Polymerization of coordinated monomers. VI. Molecular complex formation of methyl methacrylate coordinated to stannic chloride with styrene or toluene

The equilibrium constants for the complex formation between stannic chloride and methyl methacrylate were determined in n-hexane–toluene solution at 0, ?20, and ?30°C by using the absorption band at 350 nm. Continuous variation plots at ?20°C in n-hexane based on the ¹H-chemical shifts definitely show a 1:1 interaction between the coordinated methyl methacrylate and styrene or toluene. The magnitudes of the shifts for the four groups of protons in methyl methacrylate are found to be in a specific ratio in common with the 1:2 complex–styrene or -toluene system. The equilibrium constants for the ternary molecular complex formation between the 1:2 complex and styrene or toluene were determined in n-hexane in the temperature range ?50 to +20°C by use of the chemical shifts. The concentrations of the complex species in the alternating copolymerization solutions were estimated by use of the equilibrium constants. There is a linear relationship between the enthalpy and the entropy changes for the ternary molecular complex formation, which is governed by the enthalpy factor. The specificity of the interactions indicates a specific time-averaged orientation of benzene ring to the coordinated methyl methacrylate. The effects of the coordination of methyl methacrylate to stannic chloride were discussed on the basis of results of ¹³C-NMR spectroscopy.     

Modelling of the homogeneously catalyzed and uncatalyzed pyrolysis of neopentane: Thermochemistry of the neopentyl radical

The mechanism for neopentane (NpH) pyrolysis in the absence and presence of additives isobutene, HCl and HBr, in the temperature range 750–800 K, has been reinvestigated with the aid of computer simulation and sensitivity analysis techniques. With best values assigned to all rate constants in the kinetic chain, a basic mechanism comprising 18 reversible reactions involving 19 atomic, radical, and molecular species has been used to simulate pure neopentane pyrolysis data. Predictions of major and minor product yields provided quantitative agreement with experimental data against which the model was tested. The mechanism was supplemented by additional species and reactions in order to simulate experimental neopentane pyrolysis data in the presence of HCl and HBr additives. An apparent discrepancy between a recent direct measurement of k₅, the rate constant for thermal decomposition of the neopentyl radical [1], and that reported from studies of neopentane pyrolysis in the presence and absence of HCl [2], has been identified as being due to the use of an incomplete mechanism in the latter determination. Simulations of hydrogen halide catalyzed pyrolyses exhibit a high sensitivity to the thermochemical parameters associated with the neopentyl radical (Np). The influence of uncertainties in ΔH(Np) and S(Np) are evaluated and lead to suggested values ΔH(Np) = 8.7 ± 0.8 kcal mol^?1 and S(Np) = 78.8 ± 1.0 cal mol^?1 K^?1. © 1993 John Wiley & Sons, Inc.     

Influences of ClH and BrH on the pyrolyses of neopentane and ethane at small extents of reaction

A symbolic mechanism “μH, YH” has been proposed to account for the homogeneous chain pyrolysis of an organic compound μH in the presence of a hydrogenated additive YH at small extents of reaction. An analysis of this mechanism leads to two limiting cases: the thermal decomposition of neopentane corresponds to the first one (A), that of ethane to the second one (B). Previous experimental work has shown that this mechanism seems to account for a number of experimental observations, especially the inhibition of alkane pyrolyses by alkenes. Experimental investigations were extended by examining the influences oftwo hydrogen halides (ClH and BrH) upon the pyrolyses of neopentane (at 480°C) and ethane (around 540°C). The experiments have been performed in a conventional static Pyrex apparatus and reaction products have been analyzed by gas-liquid chromatography. The study shows that ClH and BrH accelerate the pyrolysis of neopentane (into i-C₄H₈ + CH₄). The experimental results are interpreted by reaction schemes which appear as examples of the mechanism “μH, YH” in the first limiting case (A). The proposed schemes enable one to understand why the accelerating influence of ClH is lower or higher than that of BrH, depending on the concentration of the additive. An evaluation of the rate constant of the elementary steps neo-C₅H₁₁ · → i-C₄H₈ + CH₃ · is discussed. In the case of ethane pyrolysis, BrH inhibits the formation of the majorproducts (C₂H₄ + H₂) and, even more, that of n-butane traces. The experimental results are interpreted by a reaction scheme which appears as an example of the mechanism “μH, YH” in the second limiting case (B). On the contrary, ClH has no noticeable influence on the reaction kinetics. This result inessentially due to the fact that the bond dissociation energy of Cl? H(?103 kcal/mol) is higher than that of C₂H₅—H (?98 kcal/mol), whereas that of Br—H (?88 kcal/mol) is lower.     

Radical equilibrium studies; the thermodynamic parameters of n-propyl

A novel method for the investigation of the thermodynamic properties of free radicals is described. It involves the establishment of an equilibrium of the form where R₁ and R₂ are free radicals, and the measurement of the recombination products of R₁ and R₂. The method is applied to the case where R₁ is n-propyl, R₂ is methyl, and the olefin is ethene, using the thermal decomposition of azomethane (a source of methyl) in the presence of ethene in the temperature range of 581–649°K. Using the best available thermodynamic parameters for methyl, it is concluded that those for n-propyl are in need of adjustment. We recommend the values ΔH_f°(300°K) = 22.6 ± 1 kcal/mol and S₃₀₀° = 67.4 ± 3 cal/mol · K together with either the heat capacity data of O'Neal and Benson or the essentially identical data derivable from the results of Purnell and Quinn.     

Further studies of the spur process of positronium formation in mixtures of organic liquids

To test some predictions of the spur model of positronium (Ps) formation, positron lifetime studies were made of the following binary organic mixtures: (a) carbondisulphide mixtures with n-tetradecane, n-hexane, isooctane, neopentane, and tetramethylsilane (TMS); (b) neopentane mixtures with methanol, ethanol, cyclohexanol, and methylcyclohexane; (c) cis-2-butene/trans-2-butene, and benzene/ethanol. The results were in agreement with the model. A minimum in the Ps yield versus CS₂ concentration, explained as being caused by electron localization on CS₂ at low and delocalization on several CS₂ molecules at higher CS₂ concentration, depended on the electron work function V_o of the solvent. This minimum was pronounced (shallow or absent) at high (low) V_o. Solvation of electrons and positrons in alcohol clusters strongly influenced the Ps yield for the neopentane mixtures. The Ps yield was higher in cis- than in trans-2-butene. The Ps formation process in polar liquids is discussed. Experiment facts do not preclude that Ps is also formed by the encounter pair process of fully solvated particles in the positron spur.     

Origin of the [C4H9O]+ ions in the mass spectrum of n-butyl ethyl ether

The mechanisms of formation of m/z 73 ions in the mass spectrum of the ionized title compound were investigated by deuterium substitution and by examining the decompositions of metastable ions. Two routes to the [C₄H₉O]⁺ ions were found in the normal spectrum. The ethyl lost by the major pathway contains the α- and β-hydrogens and a γ-hydrogen from the butyl group. The minor route involves the loss of ethylene from the [M? H]⁺ ion. There were metastable peaks for losses of ethyl, ethanol and methyl from the molecular ion. The ethyl contains the α- and β-methylenes and a γ-hydrogen, while the methyl is the δ-methyl of the butyl group. The labeling data rule out a previous mechanistic proposal for the loss of ethyl and support a mechanism involving stepwise isomerization to the sec-butyl ethyl ether molecular ion. However, the metastable ion chemistries of the molecular ions from the n- and sec-butyl ethyl ethers are highly dissimilar, perhaps due to decompositions from different electronic states. The n-pentyl methyl ether ions loses both ethyl and propyl, apparently following rearrangements to the 3-pentyl and 2-pentyl ether ions. Di n-butyl and n-butyl methyl ethers also give metastable peaks for loss of methyl, ethyl and the shorter chain alcohol.     

The mass spectrometric fragmentation of n-alkanes

The fragmentation of n-hexane, n-nonane and n-tetradecane under electron impact has been investigated, using ¹³C labelled compounds. The mechanism of the formation of the alkyl radical ions is quantitatively explained by using a method of calculation developed in an earlier publication for n-heptane. It is assumed that these ions are formed either by a direct C-C bond cleaveage or by a secondary olefin loss from an alkyl radical ion. In the latter case the probability for a particular carbon to be lost in the neutral fragment is assumed to be random. The probability for a direct cleavage to an alkyl ion is about 80% for an ion containing at least half of the number of carbon atoms of the molecular ion and 15% for the smaller ions. The [M? H]⁺ ion seems to be a special case not yet clearly understood. Former results about the loss of methyl from the molecular ion are confirmed.     

Studies on the polymerization of ethyl acrylate. II. Chain transfer studies

Transfer constants for different solvents representing hydrocarbons, halogenated compounds, alcohols, ketones, acids, and esters were determined in the thermal polymerization of ethyl acrylate at 80°C and they are compared with the available data on methyl acrylate and ethyl methacrylate. It was observed from the values of transfer constants that ethyl acrylate radicals are a little more effective than methyl acrylate or ethyl methacrylate in abstracting hydrogen atom from hydrocarbons and alcohols. In acetic and n-butyric acid media, it has been found, by the aid of endgroup analysis, that the derived solvent radicals from transfer reactions are not too efficient to start a new chain.     

Silver ion chromatography enables the separation of 2-methylalkylresorcinols from alkylresorcinols

Alkylresorcinols (∑ARs) is the generic term for a highly varied class of lipids found mainly in cereals. These bioactive compounds consist mainly of 5-alkylresorcinols (ARs), which differ in length, unsaturation, and substituents on the alkyl side chain on C-5. In addition, 2-methyl-5-alkylresorcinols (mARs) are scarcely studied minor compounds that are supposed to exist with the same structural diversity. In the first step, ∑ARs were enriched by solid-phase extraction from wheat grain and quinoa seed extracts. The subsequent application of silver ion chromatography (SIC), silica gel, coated with 20% AgNO₃, then deactivated with 1% water) enabled an unprecedented full separation of saturated mARs from conventional ARs. Specifically, saturated mARs were eluted with n-hexane/ethyl acetate (92:8, v/v), and conventional ARs with n-hexane/ethyl acetate (80:20, v/v). The unpreceded separation indicated that the SIC method could be useful not only for separations according to the degree of unsaturation, but also in the case of steric hindrance by additional (alkyl) substituents. Continued fractionation enabled the collection of unsaturated ARs in wheat and quinoa extracts. In this way, 35 ∑ARs (including five mARs) were detected by gas chromatography/mass spectrometry analysis in wheat and 45 ∑ARs (including 21 mARs) in quinoa. These included several low abundant and partly unknown ∑ARs such as 1,3-dihydroxy-5-tricosadienylbenzene.