Model Compound Studies of the Beta-O-4 Linkage in Lignin: Absolute Rate Expressions for Beta-Scission of Phenoxyl Radical from 1-Phenyl-2-phenoxyethanol-1-yl Radical.
- ASSOC WESTERN UNIVERSITY
- BATTELLE (PACIFIC NW LAB)
The lifetime of the lignin radical model compound, 1-phenyl-2-phenoxyethanol-1-yl, PhC?(OH)CH2-OPh is several orders of magnitude greater than suggested by the previous experimental studies. The ketyl radical is competitively trapped by thiophenol to yield PhCH(OH)CH2-OPh in competition with? -scission to yield phenoxyl radical and acetophenone. A basis rate expression, hydrogen atom abstraction by sec-phenethyl alcohol, PhC?(OH)CH3, from thiophenol, log(kabs/M -1 s -1 )=(8.88?0.24)? (6.07?0.34)/? ,?= 2.303RT,was determined by competing reduction and radical termination pathways. The Smoluchowski equation was used to calculate the temperature dependent rate of PhC?(OH)CH3 termination. The hydrogen abstraction pathway was used as a basis reaction to determine the activation barrier for the? -scission of phenoxyl from 1-phenyl-2-phenoxyethanol-1-yl: log(k? -sci/s -1 )= (12.85?0.22)?(15.06?0.38)/?, k? -sci(298K) ca. (64.0s -1 in benzene), log(k? -sci/s -1 )= (12.50?0.18)?(14.46?0.30)/? , k? -sc i(298K) ca. (78.7s -1 in benzene containing 0.8 M 2-propanol).B3LYP/cc-PVZT methods predict intramolecular hydrogen bonding between the? -OH on the ketyl radical and the?OPh leaving group stabilize both the ground state and transition state structures. At this level of theory the activation barrier (14.9 kcal/mol) is in fair agreement with the experimentally determined activation barrier.
- Research Organization:
- Pacific Northwest National Lab., Richland, WA (US)
- Sponsoring Organization:
- US Department of Energy (US)
- DOE Contract Number:
- AC06-76RL01830
- OSTI ID:
- 15002622
- Report Number(s):
- PNNL-SA-35970; JOCEAH; KC0302010; TRN: US200416%%192
- Journal Information:
- Journal of Organic Chemistry, Vol. 67, Issue 23; Other Information: PBD: 15 Nov 2002; ISSN 0022-3263
- Country of Publication:
- United States
- Language:
- English
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