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Radical sensitization of acetylene pyrolysis
Author(s) -
Durán R. P.,
Amorebieta V. T.,
Colussi A. J.
Publication year - 1989
Publication title -
international journal of chemical kinetics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.341
H-Index - 68
eISSN - 1097-4601
pISSN - 0538-8066
DOI - 10.1002/kin.550211006
Subject(s) - chemistry , radical , acetylene , neopentane , chain propagation , intramolecular force , reaction rate constant , hydrogen atom , photochemistry , hydrogen , toluene , methyl radical , atmospheric temperature range , medicinal chemistry , polymerization , kinetics , stereochemistry , molecule , organic chemistry , polymer , physics , alkyl , quantum mechanics , meteorology
The kinetics of acetylene polymerization initiated by neopentane (Np) or acetone (Ac) decompositions has been investigated in a static reactor dynamically coupled to a modulated beam mass spectrometer between 850–950 K. Overall rates follow the expression: R = − d [C 2 H 2 ]/ dt = k s [X ] 1/2 [C 2 H 2 ] + k u [C 2 H 2 ] 2 (I), where X represents Np or Ac and k s , k u the rate constants of the sensitized and unsensitized reactions, respectively. The rate law of the sensitized reaction clearly suggests a chain polymerization mechanism with k s = k p ( k i / k t ) 1/2 ( i , t , and p stand for initiation, termination, and propagation, respectively). Remarkably, the derived values of k p are nearly independent of the sensitizer, although Ac acts as a source of methyl radicals whereas Np also produces hydrogen atoms, and fall in the expected range for the addition of vinylic radicals to acetylene. It is shown that a chain transfer process involving the fast [1,5] intramolecular hydrogen atom shift in 4‐methyl‐buta‐1,3‐dien‐1‐yl radicals (CH 3 CH CHCH ĊH) followed by further addition to C 2 H 2 and aromatization, transforms methyl radicals into hydrogen atoms and is able to account for the presence of toluene among the products of the sensitized reactions. Based on current thermochemical data for the but‐1‐en‐3‐yn‐2‐yl radical (CH 2 ĊCCH) and present rates of propagation it is argued that if the unsensitized polymerization of acetylene also proceeded by a vinyl radical chain, then even the most favorable self‐initiation reaction: 2C 2 H 2 = C 4 H 3 + H ( a ), would be far too slow. Finally, present results also show that acetone at impurity levels (⩽ 0.1%) can not provide fast enough spurious initiation rates in chain mechanisms for the “unsensitized” acetylene pyrolysis at pressures above 10 torr.