Addition and cyclization reactions in the thermal conversion of hydrocarbons with enyne structure, I. Detailed analysis of the reaction products of ethynylbenzene
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E-Article
Title:
Addition and cyclization reactions in the thermal conversion of hydrocarbons with enyne structure, I. Detailed analysis of the reaction products of ethynylbenzene
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<jats:title>Abstract</jats:title><jats:p>The pyrolysis of ethynylbenzene (C<jats:sub>8</jats:sub>H<jats:sub>6</jats:sub>, <jats:bold>1</jats:bold>) was studied in a flow system between 700 and 1100°C (reaction time about 0.3 s) by using a mixture of 5 mol‐% of <jats:bold>1</jats:bold> in nitrogen and also in hydrogen at 700°C. The products were analyzed gas chromatographically. At 700°C in nitrogen, the main products were 1‐ and 2‐phenylnaphthalene (<jats:bold>2, 3</jats:bold>), 1‐methylene‐2‐phenyl‐1<jats:italic>H</jats:italic>‐indene (<jats:bold>4</jats:bold>), 1‐methylene‐3‐phenyl‐1<jats:italic>H</jats:italic>‐indene (<jats:bold>5</jats:bold>), and 5,10‐dihydroindeno[2,1‐α]indene (<jats:bold>6</jats:bold>). At higher temperatures, ethynylaromatics and more stable aromatics such as benzene, naphthalene, acenaphthylene, biphenyl, pyrene, fluoranthene, and six further C<jats:sub>16</jats:sub>H<jats:sub>10</jats:sub> isomers where detected. With hydrogen as diluent, the dimer formation was reduced, mainly in favor of styrene. – The complex mixture of reaction products and the dependence of its composition on the pyrolysis temperature cannot be explained in terms of one reaction scheme only. It is suggested that H atoms act as important chain carriers. At temperatures around 700°C they mainly add to <jats:bold>1</jats:bold> yielding the phenylvinyl radicals <jats:bold>1a</jats:bold> and <jats:bold>1b</jats:bold>. These add to <jats:bold>1</jats:bold> forming dimers C<jats:sub>16</jats:sub>H<jats:sub>12</jats:sub> via radicalic intermediates C<jats:sub>16</jats:sub>H<jats:sub>13</jats:sub>. With increasing temperature the 2‐phenylvinyl radical <jats:bold>1a</jats:bold> not only reacts back to H + <jats:bold>1</jats:bold> but also decomposes by β(CC) cleavage into phenyl and ethyne. The latter channel is more endothermic by 33 kJ/mol. Additionally, isomeric ethynylphenyl radicals are increasingly formed by bimolecular H abstraction. Thus, with increasing temperature product formation is controlled by reactions of phenyl‐type radicals.</jats:p>