Formation of 6‑Methyl-1,4-dihydronaphthalene in the Reaction of the <i>p</i>‑Tolyl Radical with 1,3-Butadiene under Single-Collision Conditions

Abstract

Crossed molecular beam reactions of <i>p</i>-tolyl (C₇H₇) plus 1,3-butadiene (C₄H₆), <i>p</i>-tolyl (C₇H₇) plus 1,3-butadiene-<i>d</i>₆ (C₄D₆), and <i>p</i>-tolyl-<i>d</i>₇ (C₇D₇) plus 1,3-butadiene (C₄H₆) were carried out under single-collision conditions at collision energies of about 55 kJ mol^–1. 6-Methyl-1,4-dihydronaphthalene was identified as the major reaction product formed at fractions of about 94% with the monocyclic isomer (<i>trans</i>-1-<i>p</i>-tolyl-1,3-butadiene) contributing only about 6%. The reaction is initiated by <i>barrierless</i> addition of the <i>p</i>-tolyl radical to the terminal carbon atom of the 1,3-butadiene via a van der Waals complex. The collision complex isomerizes via cyclization to a bicyclic intermediate, which then ejects a hydrogen atom from the bridging carbon to form 6-methyl-1,4-dihydronaphthalene through a tight exit transition state located about 27 kJ mol^–1 above the separated products. This is the dominant channel under the present experimental conditions. Alternatively, the collision complex can also undergo hydrogen ejection to form <i>trans</i>-1-<i>p</i>-tolyl-1,3-butadiene; this is a minor contributor to the present experiment. The de facto barrierless formation of a methyl-substituted aromatic hydrocarbons by dehydrogenation via a single event represents an important step in the formation of polycyclic aromatic hydrocarbons (PAHs) and their partially hydrogenated analogues in combustion flames and the interstellar medium