Browsing by Author "Wang, Chengrong"

Now showing 1 - 1 of 1
  • Thumbnail Image
    Item
    I. Dissociation and aromatization of a semibenzene. Reactions of triphenylmethyl and methyl isobutyryl radicals. II. The chemistry of alpha- and beta- azo radicals
    (1993) Wang, Chengrong; Engel, Paul S.
    Part I. Semibenzene methyl 7,7-diphenyl-p-mentha-1(7),2,5-triene-8-carboxylate (1), which can be regarded as the para recombination product of triphenylmethyl and methyl isobutyryl radicals (5), affords exactly these intermediates on thermolysis or, inefficiently, on direct photolysis. Whereas thiophenol or triplet 9-fluorenone aromatize 1, thermolysis in the presence of 1,4-cyclohexadiene allows trapping of 5 and oligomeric radicals. From the measured heat of aromatization, the C-C bond dissociation enthalpy of 1 and its analog lacking the side chain ("p-isotriphenylmethane") is calculated to be the lowest value known for any closed shell neutral hydrocarbon. Exposure of 1 to the atmosphere causes rapid autoxidation to the hydroperoxide. Part II. The thermolysis of 2,3-dimethyl-2,3-bis(phenylazo)butane (16a) leads exclusively to cleavage of the central C-C bond forming 2-(phenylazo)isopropyl radicals (17a). These radicals recombine to the C-N dimer 19a about 40 times faster than to the more stable C-C dimer 16a. The thermolysis of 2,3-dimethyl-2,3-bis(tert-butylazo)butane (16b) leads to C-N homolysis via a short lived $\beta$-azo radical 18b. A mechanistic study of 16b shows that C-N homolysis greatly dominates over C-C homolysis. Nevertheless, we estimate that the latter process would occur with $\Delta$G$\dagger$ (150$\sp\circ$C) $>$ 40.3 kcal/mol, which implies that the RSE of aliphatic hydrazonyl radical 17b is at least 6 kcal/mol below that of 17a, and it is little or no more than the RSE of the 1,1-dimethylallyl radical. The nature of the decomposition products of 2-cyclopropylazo-2-(phenylazo)-propane (26) suggests that the thermolysis of 26 mainly causes cleavage of one C-N bond on the cyclopropyl side, while photolysis of 26 leads to the predominant cleavage of the C-N bond on the phenyl side. Attempted determination of the heat of hydrogenation of the N=N double bond via catalytic hydrogenation of azo-n-butane (ANB) and oxidation of di-n-butyl hydrazine (DNBH) by HgO and O$\sb2$ gave varying values of $\Delta$H$\sb{\rm H}$ centering around 20 kcal/mol. 3-Methyl-1-phenyl-3-(phenylazo)-butene-1 (20) was prepared to study the thermolysis kinetics. The similarity of the BDH of 2a and 1a implies that the radical stabilization provided by the azo group is no better than that of the allyl group. (Abstract shortened by UMI.)