Alkane Systems Research Articles

Excess heat capacities of cyclohexane + alkane systems and orientational order of n-alkanes

Excess heat capacities of cyclohexane + alkane systems and orientational order of n-alkanes

Spectroscopic studies of the conformations of n-decamethyltetrasilane

The conformational properties of n-decamethyltetrasilane, n-Si 4(CH 3) 10, have been studied by variable temperature NMR ( 1H and 13C) and vibrational (IR and Raman) spectroscopy. As in related alkane systems, the anti conformer is more stable than the gauche conformer. Values of △ H 0 and △ S 0 for the anti → gauche reaction are 2.26 ± 0.15 kJ mol -1 and △ S 0 = 7.82± 0.86 J K -1mol -1, respectively. Thus, the two conformers are present in comparable quantities at room temperature. At very low temperatures, broadening of the two proton resonances ( SiMe 2, SiMe 3) occurs, but phase changes precluded the observation of decoalescence. By approximate lineshape anaylysis, the free energy of activation, △ G‡, for rotation about the central SiSi bond is estimated to be ∼25 kJ mol -1.

ChemInform Abstract: VIBRATIONAL SPECTRA AND CARBON‐HYDROGEN STRETCHING MODE ASSIGNMENTS FOR A SERIES OF N‐ALKYL CARBOXYLIC ACIDS

The Raman and infrared spectra for the C4, C8, C12, and C16 carboxylic acid series were obtained at liquid nitrogen temperatures, in the 2900 cm−1 spectral region and were compared to spectra for selected branched and straight chain alkane systems. For hexadecanoic acid‐d3 the Fermi resonance interaction between the CD3 symmetric stretching mode and the overtone level of the CD3 asymmetric deformation vibration was examined as a function of physical state. In this example of a negative Fermi resonance perturbation, in which the lower frequency overtone component is more intense than the higher frequency fundamental, the coupling increases as the system passes from either the matrix isolated species or solution phase to the polycrystalline state. The spectra of the carboxylic acid series, as well as the spectra of 2,2,4‐trimethyl pentane and n‐octane, suggest that in the 2900 cm−1 region an analogous Fermi resonance coupling exists between the CH3 symmetric stretching vibration at ∼2938 cm−1 and the 2871 cm−1 overtone transition of the CH3 asymmetric deformation. Unfortunately, the 2930–2940 cm−1 region for the solid systems is further complicated by transitions attributed to crystal interactions. Although the Fermi interaction for the methyl group has been recognized in the simpler halomethane system, for example, it has not been previously discussed in the more general context of significantly larger branched and straight chain molecules. Comparisons of the carboxylic acid spectra in the 2900 cm−1 region have also led to assignments for the α‐CH2 (adjacent to the carbonyl group) and the ω‐CH2 (adjacent to the methyl group) C–H stretching modes.

Vibrational spectra and carbon–hydrogen stretching mode assignments for a series of n-alkyl carboxylic acids

The Raman and infrared spectra for the C4, C8, C12, and C16 carboxylic acid series were obtained at liquid nitrogen temperatures, in the 2900 cm−1 spectral region and were compared to spectra for selected branched and straight chain alkane systems. For hexadecanoic acid-d3 the Fermi resonance interaction between the CD3 symmetric stretching mode and the overtone level of the CD3 asymmetric deformation vibration was examined as a function of physical state. In this example of a negative Fermi resonance perturbation, in which the lower frequency overtone component is more intense than the higher frequency fundamental, the coupling increases as the system passes from either the matrix isolated species or solution phase to the polycrystalline state. The spectra of the carboxylic acid series, as well as the spectra of 2,2,4-trimethyl pentane and n-octane, suggest that in the 2900 cm−1 region an analogous Fermi resonance coupling exists between the CH3 symmetric stretching vibration at ∼2938 cm−1 and the 2871 cm−1 overtone transition of the CH3 asymmetric deformation. Unfortunately, the 2930–2940 cm−1 region for the solid systems is further complicated by transitions attributed to crystal interactions. Although the Fermi interaction for the methyl group has been recognized in the simpler halomethane system, for example, it has not been previously discussed in the more general context of significantly larger branched and straight chain molecules. Comparisons of the carboxylic acid spectra in the 2900 cm−1 region have also led to assignments for the α-CH2 (adjacent to the carbonyl group) and the ω-CH2 (adjacent to the methyl group) C–H stretching modes.

Effects of molecular flexibility and shape on the excess enthalpies and heat capacities of alkane systems

Using the Picker flow microcalorimeter, molar excess enthalpies (hE) and molar excess heat capacities (cEP) have been obtained at 25°C for nonane isomers of different degrees of chain branching mixed with n-octane, and for 2,2,5-trimethylhexane and 3,3-diethylpentane (3,3-DEP) mixed with other normal alkanes and with cycloalkanes. As the degree of steric hindrance within the nonane isomer is increased, hE with n-octane changes from positive to negative and cEP from negative to positive. Values of hE are also anomalously negative for 3,3-DEP + n-Cn with n⩽ 14 and for 3,3-DEP + cyclopentane. Correspondingly, cEP is anomalously positive for 3,3-DEP + n-C6 and for 3,3-DEP + cyclopentane. The results are interpreted in terms of two contributions: (1) destruction of orientational order in the pure n-alkane giving hE > 0, cEP < 0, and (2) a “condensation” of the more freely moving molecule, e.g., n-Cn or cyclopentane onto the sterically-hindered 3,3-DEP resulting in a restriction of movement giving hE < 0, cEP > 0, i.e., a creation of order in the solution.

Lifetime saturation of SO2 (3B1) molecules: evaluation of chemical and physical quenching channels in alkanes

Lifetime data at 25 °C for the decay of SO2 (3B1) molecules have been obtained in the presence of high pressures of CH4, C2H6, and C3H8. At high pressures, the physical quenching channel saturates and a kinetic model which incorporates this effect has been applied to the SO2, alkane systems studied. This model permits one to evaluate the partitioning of the total quenching rate constants into their chemical and physical components. The value (in units of 103 torr−1 s−1) of the chemical and physical quenching rate constants (zero pressure limit), respectively, are (0.18 and 4.70), (CH4); (1.66 and 7.70),(C2H6);and (12.5 and 6.5), (C3H8).

A comparison of gas-liquid chromatographic enthalpies of solution with calorimetric values for four alkane systems

Using a gas-liquid-chromatography technique, specific retention volumes v g were measured in the range 293 to 303 K, for n-hexane + n-hexadecane, 2, 2-dimethyl butane + n-hexadecane, cyclohexane + n-hexadecane, and n-hexane + n-tetracosane. ΔH m ∞ for the liquids was calculated from the variation of specific retention volume with temperature for each system and from calorimetrically measured enthalpies of vaporization of the solutes. The same quantity was derived, by extrapolation, from calorimetrically measured enthalpies of mixing of the liquids. Difference between the values of ΔH m ∞ obtained in these two ways were about the same as the differences between calorimetrically determined enthalpies of mixing from different laboratories. ΔG o, ΔS o, and ΔH o of solution are also reported for each system and ΔC p ∞ is reported for n-hexane dissolving in n-tetracosane.

13C NMR studies of molecular relaxation processes in polar fluids

A study of the dynamical molecular structure of a dilute polar fluid is reported, in which 13C spin-lattice relaxation times of decanol in deuterated cyclohexane are presented for the individual carbon atoms, and the results are discussed in the context of viscosity data of decanol in alkane systems. The two techniques provide complementary information about the mobility of the alcohol chains and the onset of multimer formation, which is also pertinent to the dynamics of electron solvation in the same systems.

Calculation of the average energy of recoil hot reactions. Hot hydrogen replacement reactions in alkane systems moderated with noble gases

The average energy of reaction for recoil hot species in a well scavenged system is defined explicitly and calculated for recoil tritium for hydrogen replacement reactions with cyclohexane and n-butane as a function of noble gas moderation. In general this average reaction energy is relatively independent of the composition of the system. However, in cases of high reactivity or low threshold energies for hot reaction where the primary reactive process competes significantly with thermal scavenging, more significant composition dependencies may be manifest. The generality of the method outlined is specifically dependent on the availability of information on the energy dependence of the cross sections for the reactions of interest.

Partial molar heat capacities at infinite dilution for alkane systems using gas-liquid chromatography

Measurement of the specific retention volume of n-octane and cycloheptane in n-C 36H 74 as a function of temperature using a high-precision gas chromatograph has allowed estimation of the change in enthalpy of solution with temperature. Coupled with heat capacities for the pure solute vapor, this provides the partial molar heat capacity at infinite dilution. Comparison with calorimetric data for a similar system supports the validity of the present results.

ChemInform Abstract: PROPERTIES OF N‐ALCOHOL + N‐ALKANE MIXTURES

AbstractDie polaren Gesamtbeiträge zu den drei Eigenschaften IR‐Absorption, Mischenthalpie und Excessenergie von Alkohol‐ Alkan‐Gemischen werden aufgrund theoretischer Überlegungen in zwei Anteile (direkte H‐Bindung, Dipol‐Dipol‐Wechselwirkung) aufgetrennt.

Interactions in alkane systems by depolarized Rayleigh scattering and calorimetry. Part 1.—Orientational order and condensation effects in n-hexadecane + hexane and nonane isomers

Molecular optical anisotropies (γ2) have been obtained from depolarized Rayleigh scattering experiments on the 5 hexane isomers and 15 nonane isomers of widely varying degrees of branching. Values of γ2 have also been obtained for these molecules at high dilution in carbon tetrachloride. The latter values reflect the anisotropies of the molecular shapes of the isomers while the pure state values also contain a contribution due to correlation of the orientations of neighbouring molecules. Thus, for highly branched, isotropic isomers, the pure and dilute solution γ2 values are the same and are small. With increasing molecular anisotropy, the pure state value increases more rapidly than the dilute, the divergence reflecting the increasing correlation of molecular orientations. Values of γ2 have been found for n-hexadecane at high dilution in the hexanes and nonanes. They increase with the anisotropy of the solvent molecules and yield increasing values of the J12 parameter, which characterizes the correlation of orientations of the n-hexadecane and solvent molecules. The J12 values are given approximately by the geometric mean, (J11J22)½, of the order parameters of the pure liquids, indicating that the mixing of two liquids results in a net destruction of orientational order, i.e., ΔJ12=½(J11+J22)–J12 is positive. This change explains the variation in the heats of mixing of n-C16+ hexane systems. Molar heats of mixing have been measured at 25°C through the concentration range for n-C16 with 21 nonane isomers. With increasing nonane branching, ΔhM increases proportionally to ΔJ12. However, when nonane isomers have high degrees of steric hindrance, e.g., 3,3-diethylpentane or 2,3,3,4-tetramethylpentane, a new and large negative contribution is encountered in ΔhM. For example, ΔhM for n-C16+ 3,3-diethylpentane is S-shaped and negative at high nonane concentrations. Values of ΔvM and ΔsM also contain negative contributions for the few systems studied. The effect resembles a “condensation” of the less hindered alkane onto the large sterically hindered nonane, similar to the condensation effect found in cholesterol + lipid systems.

Interactions in alkane systems by depolarized Rayleigh scattering and calorimetry. Part 2.—Orientational order in systems containing hexadecane isomers of different structure

Depolarized Rayleigh scattering has been used to obtain values of the molecular optical anisotropy (γ2) for the following hexadecane isomers at high dilution in carbon tetrachloride : 2,2,4,4,6,8,8-heptamethylnonane, 6-, 4- and 2-methylpentadecane, 6-pentylundecane and n-hexadecane. The values increase moderately through the series corresponding to increasingly anisotropic molecular shapes. Values of γ2 have also been obtained for the hexadecane series in their pure liquid states. These increase rapidly relative to the dilute solution values, indicating the development of short-range orientational order or correlations of molecular orientations. The pure liquid results with 6-, 4- and 2-methylpentadecane show that orientational order depends strongly on the methyl position and length of the straight-chain part of the alkane molecule. Values of γ2 for the hexadecane isomers have been found for high dilution in cyclohexane, 2,2-dimethylbutane and n-hexane which indicates that the hexadecanes can correlate their orientations to only a limited extent with these hexane solvents. Heats of mixing have been measured at 25°C through the concentration range for the hexadecane isomers mixed with cyclohexane, 2,2-dimethylbutane and n-hexane. For each hexane, the heats increase through the series of hexadecane isomers. This corresponds to the increase of orientational order in the pure hexadecane isomers and the destruction of order in the hexadecane on mixing with the hexane. The magnitudes and concentration dependences of the heats for the n-C6+ C16 isomer systems are similar to those for n-C6+ n-C16 heats at different temperatures. This is explained by changing temperature having the same effect on the order of n-C16 as changing molecular shape on the order of the C16 isomers at 25°C.

Properties of n-alcohol + n-alkane mixtures

The total polar contributions (AP) to three properties [infrared absorbance, mixing enthalpies (HM) and excess free energies (GE)] of alcohol + alkane (alp) systems are separated into a direct hydrogenbond contribution ( AB) from the formation of isolated imers and a dipole-dipole contribution (AD) resulting from dipolar correlation between these transient imers. Dilute concentration range data giving the AB contributions to these properties were found dependent only on OH group concentration (c) and are used to show the serious inadequacies of previous theories. A new proposed association model having only two parameters, that are fixed for all systems, does give good results for the AB contributions and further is quite compatible with the effect of temperature change and with the n.m.r. chemical shift (ε) and apparent mean square dipole moment (p2) data that are also studied. Thus association theory has been made quantitative for the AB contributions to three properties of a/p systems and the approach given for deriving models appears capable of wider application. The model was used to extrapolate the AB contributions into the concentrated alcohol range to thus give the AD contributions by difference. The latter are then shown to be the origin of the distinctive behaviour shown by lower alcohols in their pure and binary mixture properties either with alkanes or with other alcohols where for the latter the principle of congruence is shown to be completely misleading. Two contributions (Ag and AD) explain the different c dependence shown by the i.r., HM and the δ data for a/p systems and, qualitatively, the HM data for alcohol+alcohol systems while the existence of a significant dipole term is strongly supported by the remarkable similarities found between the p2(c) data and the derived dipole-dipole contribution to the entropy of a/p systems. A method is given for predicting latent heats and partial molar enthalpies of higher alcohols from the HM data for one a/p system and a refined estimate is made of the enthalpy of formation of a hydrogen bond. Polar structure and non-linear dielectric effects are also discussed.

Enthalpy-entropy compensation and order in alkane and aqueous systems

Enthalpy-entropy compensation and order in alkane and aqueous systems

Charge transfer reactions in alkane and cycloalkane systems. Estimated ionization potentials

AbstractRate constants of change transfer reactions kCT, involving C3C9 alkanes and cycloalkanes, have been determined in an ion cyclotron resonance mass spectrometer. The rate constants are significantly lower than the corresponding rate constants for collision when the reaction is less than about 0.5 eV exothermic for linear alkane ions, or less than about 0.2 eV exothermic for cycloalkane ions. In this region of low reaction efficiency, the efficiency of reaction with linear or branched alkanes seems to depend primarily on reaction exothermicity. (The efficiencies of reaction of a given ion with cyclic alkanes also depend on ΔHrn, but are higher than for reactions with other compounds). Although the lowered reaction efficiencies probably result, at least in part, from unfavorable Franck–Condon factors in the energy range near the ionization onset, quantitative correlations between reaction efficiency and estimated relative Franck–Condon factors were not observed. When the enthalpy of reaction is small (less than about −0.15 eV), it is seen that the reverse charge transfer can also occur, and equilibrium is established under the conditions of these experiments. From the observed equilibrium constants, values for the standard free energy change are derived, and assuming that ΔS is small for electron transfer equilibria, values of ΔHrn are estimated. In the case of the equilibria involving cyclohexane ion, these values of ΔHrn lead to estimates of the ionization potentials of methylcyclopentane, 3‐methylpentane, n‐octane, 2,2‐dimethylbutane, and 2,3‐dimethylbutane, which are lower than the ionization potentials of cyclohexane, that is, &lt;9.88 eV, although all these compounds had previously been reported to have ionization potentials above 10.03 eV. That the ionization potentials are indeed lower than 10.03 eV is confirmed by determining the quantum yields of ionization with 10.03‐eV photons.It is pointed out that the conclusions reached here apparently also apply to the charge transfer reactions of alkane ions in the liquid phase.

Conduction state energy of excess electrons in condensed media. Liquid methane, ethane, and argon and glassy matrices

The temperature dependence of the conduction electron energy V/sub 0/ in liquid methane and ethane has been measured, and the theoretical model of Springett, Jortner, and Cohen (SJC) for the temperature dependence of V/sub 0/ is shown to apply to liquid alkanes as well as rare gases. Criteria for electron localization in liquid rare gases, molecular gases, and alkanes are critically discussed. The SJC criterion for electron localization in liquid rare gases is shown not be extendable to liquid alkanes or to molecular gases. In alkanes it is assumed that transient localization arises from rotational and translational fluctuations leading to cavities of molecular size. Then consideration of the kinetic and electronic polarization energies with a cavity radius given by Wigner-Seitz radius leads to a new localization criterion that seems generally applicable to alkane systems. Electrons in molecular gases probably move as molecular anions so neither of the above criteria apply to them. An empirical relation between the electron mobility and V/sub 0/ is found that seems valid for all alkanes at room temperature for both localized and quasi-free electron states. Values of V/sub 0/ for polar and nonpolar glassy matrices at 77/sup 0/K are derived from experimental data by twomore » independent methods. It is found that V/sub 0/ becomes more positive as the glassy matrix polarity decreases.« less

Effects of orientaltional order in solution thermodynamics: mixtures containing a liquid crystal

The thermodynamic properties, nematic order parameters and nematic–isotropic transition temperatures of p-methoxybenzylidene-p-n-butylaniline (MBBA)–alkane systems are highly sensitive to the alkane shape and indicate that MBBA molecules correlate their orientations with normal, but not with highly branched isomers.

ChemInform Abstract: HEATS OF MIXING OF TETRA‐ALKYLTIN COMPOUNDS WITH NORMAL AND BRANCHED ALKANES. ORDER IN LONG ALKANES

Heats of mixing at 25°C of SnR4(R = CH3, C2H5, C4H10) with seven n-alkanes from C5 to C16, three very branched alkanes 2,2,4-trimethylpentane, 2,2,4,6,6-pentamethylheptane, 2,2,4,4,6,8,8-heptamethylnonane and two lightly branched alkanes 2-methylpentane and 2-methylundecane are reported. Except for SnBu4 with n-C5 and n-C6, and SnR4 with the n-Cn series gives positive Δhm values which increase rapidly with n. In contrast, any SnR4 with the very branched alkanes give small or negative ΔhM values which are independent of the number of carbon atoms in the alkane. Intermediate results are obtained with the lightly branched alkanes. The heats indicate that only the alkane exterior of the SnR4 molecules interacts with the alkane component. We associate the positive contribution in the normal alkane systems with destruction, by the spherical SnR4 molecules, of the orientational order existing in the n-Cn components. The order being absent in the globular branched alkanes, the ΔhM are small and negative as required by the difference in free volumes of the components. A quantitative treatment is given in terms of recent free volume theories coupled with an estimate of the orientational order obtained from depolarized Rayleigh scattering results. It is possible that part of the highly negative heats found with tetrabutyltin and the highly branched alkanes, which are unexplained by free volume differences, reflects preferential packing, an effect observed with spherical molecules of different size.

Sodium hypochlorite as a selective oxidant for organic compounds

A study of the reactions of sodium hypochlorite with a wide variety of compounds has been carried out. The data indicate that a structure capable of producing a carbanion stable enough to undergo halogenation is oxidized by sodium hypochlorite to a ketone or a carboxylic acid. How far the oxidation will proceed is determined by enolateion resonance stabilization and by inductive and steric effects of the activating group (e.g.CO, –NO2, –CN, or –CO2H). Unactivated aromatic, hydroaromatic, alicyclic, and alkane systems are not oxidized. Heteroaromatic ring systems capable of undergoing azomethine-type addition can readily be cleaved, whereas benzenoid ring systems undergo no change. Oxidation of an activated alicyclic compound containing a tertiary sp3 carbon system may result in loss of one, two, or three carbon atoms as CO2 or CHCl3via 1,3- and 1,4-halogenation steps. Based on this investigation, a new methodology of oxidation has been developed for structural elucidation of complex materials such as coal.