ISSN:
0538-8066
Keywords:
Chemistry
;
Physical Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
The pyrolysis of 2-methylbut-1-ene-3-yne (C5H6) has been studied from 375 to 450°C in a quartz reaction vessel in the absence and presence of O2 or NO. From 375 to 425°C, the rates of disappearance of reactant and of formation of dimers are second order in C5H6. The major product is polymer, with the dimers accounting for about 3% of the C5H6 consumed. In addition, toluene and p-xylene are produced, their production coming, at least in part, from decomposition of the C5H6 dimers (C10H12). Also, trace amounts of CH4, C2H4, C2H6, are formed. The rate coefficients for C5H6 removal and C10H12 formation in the absence of O2 or NO are \documentclass{article}\pagestyle{empty}\begin{document}$$ \begin{array}{l}{\rm\rm\rm log\{ }k\{ {\rm C}_{\rm 5} {\rm H}_{\rm 6} {\rm \},M}^{ - 1} {\rm - s}^{ - 1} \} = 6.79 \pm 2.39 - (4179 \pm 1603)/T \\{\rm log\{ }k\{ {\rm C}_{{\rm 10}} {\rm H}_{{\rm 12}} \},{\rm M}^{ - {\rm 1}} {\rm - s}^{ - 1} \} = 7.40 \pm 2.32 - (5793 \pm 1555)/T \\\end{array} $$\end{document} where the uncertainties are one standard deviation. The reaction mechanism for dimer formation is analogous to that in vinyl acetylene (C4H4) pyrolysis (5), except that in the C4H4 system cyclooctatetraene is seen as an unstable product that isomerizes to styrene, whereas in the C5H6 system, the dimethylcyclooctatetraene apparently is too unstable to be detected. The dimers detected were 2,6-dimethylstyrene (P4), p-isopropenyltoluene (P5), and two other unidentified dimers (P3) with nearly identical gas chromatographic retention times. From the effect of the radical scavengers and by comparison of the C4H4 and C5H6 systems, the following mechanistic characteristics were determined: (1) The direct formation of styrene in the C4H4 system comes from a head-to-head modified Diels-Alder six-member cycloaddition that proceeds through a diradical intermediate. (2) There is no positive evidence for a direct head-to-tail modified Diels-Alder six-member cycloaddition. However, if it does occur, it does not involve diradicals but must be concerted. (3) Cyclooctatetraene is formed in concerted, non-free-radical mechanisms that may proceed both by head-to-head and head-to-tail eight-member cycloadditions. For the C5H6 system, the head-to-head adduct isomerizes to P3, whereas the head-to-tail adduct isomerizes to P3, P4, and/or P5. and/or P5. Kinetic data suggest that P3 is not produced from the cyclooctatetraene intermediate, in which case, head-to-head addition would not occur. It appears that the head-to-head additions are free radical in nature and proceed mainly through a six-membered ring intermediate, while head-to-tail additions are a concerted molecular process and proceed mainly through an eight-membered ring intermediate.
Additional Material:
12 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/kin.550170808
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