G. Pilcher

Enthalpies of combustion of 1,2-dihydroxybenzene and of six alkylsubstituted 1,2-dihydroxybenzenes

Abstract The standard ( p o = 101.325 kPa) molar enthalpies of combustion in oxygen at 298.15 K were measured by static-bomb calorimetry and the standard molar enthalpies of sublimation at 298.15 K were measured by microcalorimetry for 1,2-dihydroxybenzene (catechol) and six alkylsubstituted catechols: −Δ c H m o (cr)/(kJ·mol −1 ) Δ sub H m o /(kJ·mol −1 ) Catechol 2864.5 ± 0.8 86.6 ± 1.6 3-Methylcatechol 3505.4 ± 0.5 93.2 ± 1.0 4-Methylcatechol 3504.6 ± 0.6 94.9 ± 1.0 3-isoPropylcatechol 4808.8 ± 1.1 97.8 ± 1.7 4-terButylcatechol 5461.9 ± 0.9 99.3 ± 1.4 3-Methyl-6-isopropylcatechol 5460.2 ± 0.9 96.6 ± 0.9 3,5-diterButylcatechol 8082.7 ± 1.8 100.1 ± 0.6 The increment in the molar enthalpy of formation of the gaseous compound for substitution of alkyl-groups into catechol was found to be approximately the same as the corresponding increment for substitution into benzene.

Enthalpies of combustion of the three aminopyridines and the three cyanopyridines

Abstract The standard ( p o = 101.325 kPa) enthalpies of combustion in oxygen at 298.15 K were measured in a static-bomb calorimeter and the standard enthalpies of sublimation at 298.15 K were measured by microcalorimetry for the three aminopyridines, the three cyanopyridines, and 3-pyridinecarboxylic acid: Δ c H o (cr)/(kJ · mol −1 ) Δ sub H m o /(kJ · mol −1 ) 2-(C 5 H 4 N)NH 2 2864.4±0.5 78.7±0.8 3-(C 5 H 4 N)NH 2 2885.2±0.5 84.0±1.4 4-(C 5 H 4 N)NH 2 2866.8±0.5 88.1±1.1 2-(C 5 H 4 N)CN 3142.7±0.5 70.7±1.2 3-(C 5 H 4 N)CN 3138.5±0.5 72.1±1.8 4-(C 5 H 4 N)CN 3143.0±0.5 73.2±0.6 3-(C 5 H 4 N)CO 2 H 123.4±1.2 Some differences between the derived enthalpies of formation of the gaseous compounds are compared with theoretically predicted values for certain of these compounds, and the enthalpy increments for substitution into pyridine are compared with those for benzene.

Enthalpy of micellization I. Sodium n-dodecylsulphate☆

Abstract The enthalpy of micellization of sodium n -dodecylsulphate in aqueous and in 0.023 mol dm −3 sodium chloride solution, as a function of concentration of surfactant and of temperature has been measured using a Beckman 190B microcalorimeter. The dilution of the micellar solutions from a concentration 150 times the critical micelle concentration was found to be endothermic. The micellization process becomes more exothermic with increasing concentration of surfactant, with increasing temperature, and on addition of salt.

Enthalpies of combustion of the three hydroxy-pyridines and the four hydroxy-2-methylpyridines

Abstract The enthalpies of combustion in oxygen at 298.15 K were measured in a static-bomb calorimeter and the enthalpies of sublimation at 298.15 K were measured by microcalorimetry for the following crystalline compounds: −ΔH c o (c)/(kJ·mol −1 ) ΔH o (sub.)/(kJ·mol −1 ) 2-Hydroxypyridine (2515.8 ± 0.4) (86.6 ± 1.3) 3-Hydroxypyridine (2550.1 ± 0.9) (88.3 ± 1.3) 4-Hydroxypyridine (2537.5 ± 1.1) (103.8 ± 1.7) 2-Methyl-3-hydroxypyridine (3187.9 ± 0.9) (89.1 ± 1.3) 2-Methyl-4-hydroxypyridine (3176.8 ± 0.7) (113.0 ± 1.3) 2-Methyl-5-hydroxypyridine (3195.5 ± 1.3) (96.2 ± 2.1) 2-Methyl-6-hydroxypyridine (3149.2 ± 1.9) (92.0 ± 1.3) The derived enthalpies of formation of the gaseous compounds are compared with theoretically predicted values for certain of these compounds.

Enthalpy of micellization II. n-dodecyltrimethylammonium bromide☆

The enthalpy of micellization of n-dodecyltrimethylammonium bromide has been measured using a Beckman 190B microcalorimeter at 23, 25, and 30 °C, in water and in sodium bromide solution as a function of surfactant concentration up to 20 times the critical micelle concentration. Dilution of these micellar solutions was found to be endothermic, the endothermicity decreasing with increasing salt concentration. Micellization was found to be exothermic, and the exothermicity increased with sodium bromide concentration and with temperature. The change in enthalpy of micellization between 23 and 30 °C is in accord with enthalpies deduced from the variation of the critical micelle concentration with temperature.

Enthalpies of combustion of thiobenzamide, N, N-dimethylthiobenzamide, and N, N-diethylthiobenzamide

The standard (po = 0.1 MPa) molar enthalpies of combustion in oxygen at 298.15 K of crystalline thiobenzamide, N, N-dimethylthiobenzamide, and N, N-diethylthiobenzamide to produce CO2(g), N2(g), and H2SO4·115H2O(l) were measured by rotating-bomb calorimetry. The standard molar enthalpies of sublimation at 298.15 K were measured by microcalorimetry. −ΔcHmo(cr)(kJ·mol−1) ΔcrgHmo(kJ·mol−1) Thiobenzamide 4401.4±1.1 103.4±2.2 N, N-Dimethylthiobenzamide 5757.0±1.4 94.8±2.0 N, N-Diethylthiobenzamide 7064.0±1.5 91.4±3.2 These results form the basis of a bond-energy scheme to estimate ΔfHmo(C6H5CSNr2, g).

Enthalpies of formation of cis-azobenzene and trans-azobenzene

The standard ( p ° = 0.1 MPa) molar enthalpy of formation of crystalline trans -azobenzene was determined from its enthalpy of combustion in oxygen at 298.15 K measured by static-bomb calorimetry. The enthalpy of isomerization: Δ isom H ° m (cr, cis → trans )/(kJ · mol −1 ) = -(49.1 ± 1.0), was measured by reaction-solution calorimetry: the isomerization was catalysed by dicyclopentadienyltitanium diiodide in toluene solution. The enthalpy of isomerization in heptane solution: Δ isom H ° m ( trans → cis )/(kJ · mol −1 ) = -(48.9 ± 2.3) when coupled with the enthalpies of solution of the two forms in heptane confirmed the reaction-solution calorimetric value. For the crystalline solids:Δ f H ∘ m ( trans -azobenzene, cr)/(k/J · mol -1 ) = (308.6 ± 1.9),Δ f H ∘ m ( cis -azobenzene, cr)/(k/J · mol -1 ) = (357.7 ± 2.1) The enthalpy of sublimation of trans -azobenzene was determined from the variation of vapour pressure with temperature measured by the Knudsen method: Δ g cr H ° m ( trans -azobenzene)/ (kJ · mol −1 ) = (93.6±1.9).

Enthalpies of combustion of γ-butyrolactone, γ-valerolactone, and δ-valerolactone

Abstract The standard ( p o = 0.1 MPa) molar enthalpies of combustion in oxygen at 298.15 K were measured by static-bomb calorimetry and the standard molar enthalpies of vaporization at 298.15 K were measured by microcalorimetry for three lactones. −Δ c H m o /(kJ·mol −1 Δ l g H m o /(kJ·mol −1 ) γ-butyrolactone 2010.6±0.5 54.4±0.4 γ-valerolactone 2649.6±0.8 54.8±0.4 δ-valerolactone 2673.3±0.5 58.0±0.4 The derived standard molar enthalpies of formation of the gaseous compounds were compared with values expected for strain-free structures. The conventional strain-energy in the six-membered ring of δ-valerolactone is greater than that in the five-membered-ring lactones, and this is in reverse order of strain-energies generally observed in alicyclic compounds.

The Dissociation Enthalpies of Terminal (N–O) Bonds in Organic Compounds

Dissociation enthalpies of terminal (N–O) bonds, DH∘(N–O), in amine N-oxides, nitrile N-oxides, pyridine N-oxides, quinoxaline 1,4-dioxides, furoxans, nitrones, azoxy-derivatives, azo-N,N-dioxides, nitro compounds, nitramines, and alkyl nitrates are calculated from published enthalpy of formation, enthalpy of sublimation, and enthalpy of vaporization data. For each class of organic compounds, the calculated DH∘(N–O) values are critically evaluated. The derived DH∘(N–O) values can be used to estimate enthalpies of formation of other molecules in each of these classes of organic compounds.

Enthalpies of combustion of 1,4-naphthoquinone, 9,10-anthraquinone, 9,10-phenanthraquinone, 1,4,9,10-anthradiquinone, 5,8-dihydroxy-1,4-naphthoquinone, and 1,4-dihydroxy-9,10-anthraquinone

Abstract The standard ( p o = 0.1 MPa) molar enthalpies of combustion in oxygen at 298.15 K were measured by static-bomb calorimetry for some quinones and dihydroxyquinones. The standard molar enthalpies of sublimation at 298.15 K were measured by microcalorimetry for 1,4-naphthoquinone and 9,10-phenanthraquinone; values were selected from the literature for the remaining compounds in order to derive the standard molar enthalpies of formation in the gaseous state. −Δ c H m o (cr) ( kJ·mo l −1 ) Δ f H m o (g) ( kJ·mol −1 ) 1,4-Naphthoquinone 4604.1 ± 1.1 −97.5 ± 1.9 9,10-Anthraquinone 6464.0 ± 2.1 −75.7 ± 2.9 9,10-Phenanthraquinone 6497.8 ± 1.6 −49.3 ± 2.8 1,4,9,10-Anthradiquinone 6033.6 ± 2.4 −209.0 ± 4.0 5,8-Dihydroxy-1,4-naphthoquinone 4196.8 ± 0.5 −499.1 ± 3.2 1,4-Dihydroxy-9,10-anthraquinone 6057.4 ± 1.0 −471.7 ± 2.3 The energies of the intramolecular hydrogen bonds in the dihydroxyquinones were assessed as (25 ± 3) kJ·mol −1 . 1,4,9,10-Anthradiquinone is apparently considerably strained, and although its reaction with water produces 1,4-dihydroxy-9,10-anthraquinone, a concomitant formation of hydrogen peroxide is shown to be thermodynamically improbable.