`FeS'-assisted scission of strong bonds in phenoxydiphenylmethanes. Competition between hydrogen atom transfer and free radical rearrangement pathways

Tom Autrey, John Linehan, Lauren Kaune, Tess R. Powers, Eric F. McMillan, Carrie Stearns, James A. Franz

Research output: Contribution to journalArticle

4 Citations (Scopus)

Abstract

Model compound studies comparing rates of decomposition and product distributions from ortho- and para-phenoxydiphenylmethanes [(PhO)PhCH2Ph] suggest that hydrogen atom abstraction from the model compounds, to yield a benzylic radical intermediate, competes with hydrogen atom transfer to the aryl rings from the reduced `FeS' catalyst. A free-radical rearrangement pathway involving o-phenoxydiphenylmethane, facilitated by the presence of the `FeS' catalyst, generated in situ from ferric oxyhydroxysulfate (OHS) and sulfur, leads to apparent Ar-OAr bond scission at temperatures significantly lower than expected for homolytic scission pathways. Thermolysis of the ortho isomer proceeds predominately through a pathway involving an intramolecular addition of the benzylic radical to the 1-position of the appended diphenyl ether, Ar1-5 participation, forming a spirodienyl radical intermediate. Scission of the C-O bond, followed by hydrogen atom abstraction, yields thermally labile o-(hydroxyphenyl)phenylmethane (oHPPM). Under the reaction conditions, at 390 °C, tautomerism of oHPPM to the keto isomer followed by homolysis of the weak C-C bond in the keto intermediate yields diphenylmethane and phenol. To unambiguously demonstrate the importance of the free-radical rearrangement pathway, products from the thermolysis of o-(4-methylphenoxy)diphenylmethane were quantitatively determined. Decomposition of this labeled diaryl ether at 390 °C in 9,10-dihydrophenanthrene containing OHS/sulfur yields 4-methyldiphenylmethane and phenol as the major products. Catalytic decomposition of the corresponding para isomer, p-(4-methylphenoxy)diphenylmethane, where the intramolecular free-radical rearrangement pathway is hindered, shows that the rate of decomposition is significantly slower than observed for the corresponding ortho isomer, and 4-methyldiphenyl ether and toluene are the major products. The selectivity observed for the product distribution in the catalytic thermolysis of the para isomer is consistent with a reversible hydrogen atom transfer pathway from the `FeS' catalyst.

Original languageEnglish
Pages (from-to)927-933
Number of pages7
JournalEnergy and Fuels
Volume13
Issue number4
DOIs
Publication statusPublished - Jul 1999

ASJC Scopus subject areas

  • Chemical Engineering(all)
  • Energy Engineering and Power Technology
  • Fuel Technology

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