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Thermolabile Kohlenwasserstoffe, 30 Thermische Stabilität und Bildungsenthalpie von 1,1,2,2‐ und 1,1,1,2‐Tetraphenylethan. – Die Stabilisierungsenergien von Benzhydryl‐ und Triphenylmethyl‐Radikalen
Author(s) -
Beckhaus HansDieter,
Dogan Barbara,
Schaetzer Jürgen,
Hellmann Siegrid,
Rüchardt Christoph
Publication year - 1990
Publication title -
chemische berichte
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 0009-2940
DOI - 10.1002/cber.19901230126
Subject(s) - chemistry , standard enthalpy of formation , substituent , medicinal chemistry , sublimation (psychology) , thermolabile , enthalpy , enthalpy of sublimation , thiophenol , endothermic process , alkyl , calorimetry , thermal decomposition , organic chemistry , thermodynamics , psychology , physics , adsorption , psychotherapist , enzyme
Thermolabile Hydrocarbons, 30. ‐ Thermal Stability and Heat of Formation of 1,1,2,2‐ and 1.1,1,2‐Tetraphenylethane. ‐Energy of Stabilisation of Benzhydryl and Triphenylmethyl Radicals Heats of combustion of symmetrical ( 3 ) and unsymmetrical tetraphenylethane ( 4 ) were determined by combustion calorimetry: ΔH c 0 (c)( 3 ) = ‐ 3249.5 ± 0.8;( 4 ) = ‐ 3252.0 ± 0.6 kcal/mol. Enthalpies of sublimation of 3 (ΔH sub,298K 0= 32.7 kcal/mol) and 4 (31.7 kcal/mol) were obtained from vapour‐pressure‐temperature relations, which were determined by a gassaturation method. The resulting heats of formation ΔH f 0 (g) ( 3 ) = 85.4 ± 1.3 and ΔH f 0 (g)( 4 ) = 87.3 ± 0.8 kcal/mol correspond to low strain energies: H , ( 3 ) = 1.5 and H s ( 4 ) = 4.6 kcal/mol. The thermolysis reactions above 200°C in solution with thiophenol as a scavenger are clean first order processes. The following activation parameters are obtained from kinetic measurements: ΔH # ( 3 ) = 47.3 ± 0.3, ( 4 ) = 45.0 ± 0.3 kcal/mol, ΔS # ( 3 ) = 12.8 ± 0.5 and ( 4 ) = 14.0 ± 0.6 e.u. From the presented thermodynamic and kinetic data the stabilisation energies of the benzhydryl ( 1 ) and the trityl radical ( 2 ) have been derived by comparison with the corresponding data for the alkanes 7 and 9 . As a result 1 is stabilysed by 12.9 ± 0.9 and 2 by 20.0 ± 1.7 kcal/mol, compared with a secondary or tertiary alkyl radical, respectively. Thus the substituent effect of two (or three) phenyl rings in 1 (or 2 ) is smaller than twice (or thrice) the value for one phenyl group 14) : 7.8 ± 2.0 for a secondary and 8.4 ± 1.2 kcal/mol for a teriary benzyl radical. This can be explained by steric repulsion between the phenyl rings and steric inhibition of resonance.