The Products of the Reactions of o-Benzyne with Ethene, Propene, and Acetylene: A Combined Mass Spectrometric and Quantum Chemical Study

Abstract

The primary products of the bimolecular reactions of ortho-benzyne, o-C6H4 (1,2-dehydrobenzene), with ethene, propene, and acetylene have been detected by molecular beam mass spectrometry at a combustion relevant temperature of T = 1475 K. o-Benzyne was produced by flash pyrolysis of phthalic anhydride in the absence and presence of the respective reactant. Potential reaction pathways of the addition reactions were investigated by quantum chemical calculations. Channels with biradical intermediates were found to be energetically more favorable than alternative quasi-concerted [2+1] cycloaddition and concerted H-transfer pathways. Bicyclic benzocyclobutene and benzocyclobutadiene were identified as the main products of the reactions with C2H4 and C2H2, respectively. At combustion temperatures, however, these cyclic products are likely to undergo sequential ring opening. In the case of propene, the presence of an allylic H atom initiates a favorable ene-type reaction sequence yielding the open-chain product allylbenzene. Overall, hydrocarbon reactivity was found to increase in the order C2H2, C2H4 to 3H8. The range of the estimated bimolecular rate constants is comparable to the rate constants of the corresponding phenyl radical reactions and hence point out a potentially important role of o-C6H4 reactions in flame and soot formation chemistry.

Isomer-Specific Product Detection of Gas-Phase Xylyl Radical Rearrangement and Decomposition Using VUV Synchrotron Photoionization

“…Although we are unaware of any previous studies that have considered benzyne as a C 8 H 8 decomposition product, the reverse process has been investigated before using both experimental and theoretical methods. 48,49 Benzocyclobutene can also lose a H atom on the four-membered ring to form another C 8 H 7 radical (6) after surmounting a barrier of 80.7 kcal/mol relative to o-xylylene. This species can then lose a further H atom to form benzocyclobutadiene (with barrier of 60.7 kcal mol -1 ), thus providing a possible explanation for the presence of benzocyclobutadiene in the experiments.…”

Initiation reactions in the high temperature decomposition of styrene

“…Similarly, ortho -benzyne + ethylene = styrene, as considered by Friedrichs et al , 89 is another potential pathway; although, in this study, the reaction is viewed in the reverse direction. Note that in the Friedrichs et al 89 study this was only a potential product of this reaction, and at their conditions bicyclic benzocyclobutene was considered the more likely product.…”

“…Similarly, ortho -benzyne + ethylene = styrene, as considered by Friedrichs et al , 89 is another potential pathway; although, in this study, the reaction is viewed in the reverse direction. Note that in the Friedrichs et al 89 study this was only a potential product of this reaction, and at their conditions bicyclic benzocyclobutene was considered the more likely product. This reaction has not been included because: (1) there is a substantial barrier (∼120 kcal mol −1 ) for the direct elimination of C 2 H 4 from styrene, (2) there is little evidence for ethylene in the mass spectra at short reaction times, and (3) a mechanism based upon this reaction could not simulate the observed density gradients in the present LS data.…”

The Reaction of o‐Benzyne with Vinylacetylene: An Unexplored Way to Produce Naphthalene

“…On the next reaction step, the three‐membered ring in i2 further expands to a four‐membered ring in i4 via a barrier of 9.1 kcal/mol relative to the reactants. The much more stable bicyclic structure i4 (−71.1 kcal/mol) has the six‐ and four‐membered rings with a common edge and represents a substituted analog of benzocyclobutene identified as the main product of the reaction of o ‐C 6 H 4 with C 2 H 4 [19] . Further reaction steps take place in the exothermic region of the C 10 H 8 PES with the reactants considered as the zero energy level.…”

“…The reaction mechanism was explained based on the biradical addition (1,4‐cycloaddition followed by fragmentation) pathway to PAH molecules proposed by Comandini et al [14] . A detailed experimental and theoretical study of the primary products of the bimolecular reactions of o ‐C 6 H 4 with small prototype unsaturated hydrocarbons ethylene, propene, and acetylene was performed by Friedrichs et al [19] . who utilized molecular beam mass spectrometry at a combustion relevant temperature of T=1475 K, with possible reaction pathways explored by quantum chemical calculations.…”