Addition and cyclization reactions in the thermal conversion of hydrocarbons with enyne structure, II. Analysis of radicals and carbenes from ethynylbenzene

Abstract

The pyrolysis of ethynylbenzene (1) in helium was studied in a tubular flow reactor at 10.7 mbar/103O0C and reaction times ranging from 5 to 30 ms. Reactive intermediates such as radicals and carbenes were scavenged with dimethyl disulfide (DMDS). Qualitative and quantitative analysis of the scavenging products and of the stable pyrolysis products were carried out by GC-MS analysis. -The radicals phenyl In the preceding paperll] we have reported on the products of the pyrolysis of ethynylbenzene (1) which is also a component in fuel-rich flames, and have discussed it as a starter in mechanisms of formation of polycyclic aromatic hydrocarbons. The formation of the complex mixture of stable products from this pyrolysis between 700 and 1000°C has been interpreted by specified radical reactions only. From the structures of the products, however, it has not always been possible to decide whether carbenes may also be involved. To obtain a better insight into the complex reaction events, reactive intermediates such as radicals and carbenes are analyzed directly in the study presented here.For this purpose, a new scavenging method using dimethyl disulfide (DMDS) has been applied for the first time to the pyrolysis of hydrocarbons. This method is superior to gas-phase scavenging since it takes place at low temperature in the condensed phase under conditions where disturbing consecutive reactions of radicals and carbenes are negligible or can easily be recognized. It is particularly well suited to the analysis of medium-sized and large hydrocarbon radicals[2a,bl and has already been successfully applied to hydrocarbon f l a m e~ [~,~] .To investigate the influence of ethyne, which isa pyrolysis product and is also a major component in fuel-rich hydrocarbon flames, mixtures of 1 with various concentrations of ethyne were pyrolyzed separately. Results RadicalsThe radicals detected in the pyrolysis of 1 are given in Scheme 1. The major radical by far is phenyl (2a) followed in concentration by the three isomeric radicals 0-, m-, and p-ethynylphenyl (lc, d, e)lsl which in total make about a fifth of the amount of 2a. For quantitative data, see the concentration-time curves in the Figures 1 and 2. For the two isomeric radicals l a and lb, which were detected in still lower concentrations than the ethynylphenyl radicals, it was confirmed by the mass spectra of their scavenging products that the radical site is at the side chain, not at the ring. It was generally found from the fragmentation pattern of methylthio compounds that the SCH3 group is more strongly bound to the aromatic ring than to an aliphatic group, resulting in a relatively weak parent peak in the latter case. From a similarly weak parent peak in the mass spectra of the scavenging products of l a and l b the structure shown in Scheme 1 was deduced.Furthermore, 1 -and 2-naphthyl radicals (3a, b) occurred in about five percent relative to phenyl radicals. Using the solvent split technique, we also detected radicals such as 4a and 5a. These radicals are probably for...

Fullerenes and Soot Formation— New Pathways to Large Particles in Flames

“…[26] Under the conditions that prevail when soot and fullerene formation sets in within C 2 H 2 / O 2 flames the concentration of C 2 H 2 drops by a factor of about 50 from the burner to the end of the oxidation zone. [24] This loss of a growth component is more than compensated for by the increase in the rate constants of the growth reactions, which grow by several orders of magnitude.…”

Cycloaromatization of Open and Masked 1,3-Hexadien-5-ynes − Mechanistic and Synthetic Aspects

“…[15,54,62,63] Additionally, hexadienynes of this type have been synthesized and used as precursors for the thermal generation of 1,2,4-cyclohexatriene derivatives by the Hopf cyclization (see Section 4) and high-temperature syntheses of specially designed PAHs by FP and FVP techniques (see Sections 5Ϫ7). Finally, hexadienynes, as well as other enynes, [43a] have been used as model compounds to develop building blocks for kinetically based combustion models.…”

“…Finally, hexadienynes, as well as other enynes, [43a] have been used as model compounds to develop building blocks for kinetically based combustion models. [4,54,62,63] The previously studied cycloaromatizations during which the carbon skeleton of each considered 1,3-hexadien-5-yne substructure is converted into members of the newly formed aromatic six-membered ring, can essentially take place by exothermic, and slightly endothermic steps. To trigger such reaction clusters, chain-carrier radicals from other sources are needed.…”

Thermal Rearrangement of Ethynylarenes to Cyclopentafused Polycyclic Aromatic Hydrocarbons: An Electronic Structure Study

“…Some indirect empirical evidence against the free-radical mechanisms is also available. The absence of phenylethynyl among the radicals produced by pyrolysis of ethynylbenzene at 1030 °C and the formation of large quantities of C 6 H 5 • under such conditions seem to preclude the reaction pathways initiated by hydrogen abstraction (mechanisms III and IV). Partial degradation of the starting material due to the loss of the C⋮C fragment (presumably from the arylethynyl radicals) is also evident in some FVP experiments, especially those carried out at higher temperatures …”