ISSN:
0009-2940
Keywords:
Bond cleavage, C-C, kinetics of / Heats of combustion / Heats of sublimation / Radicals, stability of
;
Chemistry
;
Inorganic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
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 RadicalsHeats of combustion of symmetrical (3) and unsymmetrical tetraphenylethane (4) were determined by combustion calorimetry: ΔHc0(c)(3) = - 3249.5 ± 0.8;(4) = - 3252.0 ± 0.6 kcal/mol. Enthalpies of sublimation of 3 (ΔHsub,298K0 = 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 ΔHf0(g) (3) = 85.4 ± 1.3 and ΔHf0(g)(4) = 87.3 ± 0.8 kcal/mol correspond to low strain energies: H, (3) = 1.5 and Hs (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 group14): 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.
Additional Material:
1 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/cber.19901230126
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