Department of Chemistry, University of Hawaii at Manoa, Honolulu, HI 96822, (USA) http://www.chem.hawaii.edu/Bil301/welcome.html.
Angew Chem Int Ed Engl. 2015 Apr 27;54(18):5421-4. doi: 10.1002/anie.201411987. Epub 2015 Mar 5.
The hydrogen abstraction/acetylene addition (HACA) mechanism has long been viewed as a key route to aromatic ring growth of polycyclic aromatic hydrocarbons (PAHs) in combustion systems. However, doubt has been drawn on the ubiquity of the mechanism by recent electronic structure calculations which predict that the HACA mechanism starting from the naphthyl radical preferentially forms acenaphthylene, thereby blocking cyclization to a third six-membered ring. Here, by probing the products formed in the reaction of 1- and 2-naphthyl radicals in excess acetylene under combustion-like conditions with the help of photoionization mass spectrometry, we provide experimental evidence that this reaction produces 1- and 2-ethynylnaphthalenes (C12 H8 ), acenaphthylene (C12 H8 ) and diethynylnaphthalenes (C14 H8 ). Importantly, neither phenanthrene nor anthracene (C14 H10 ) was found, which indicates that the HACA mechanism does not lead to cyclization of the third aromatic ring as expected but rather undergoes ethynyl substitution reactions instead.
氢提取/乙炔加成（HACA）机制长期以来一直被认为是燃烧系统中多环芳烃（PAHs）芳香环生长的关键途径。然而，最近的电子结构计算对该机制的普遍性提出了质疑，这些计算预测，从萘基自由基开始的 HACA 机制优先形成苊烯，从而阻止了第三个六元环的环化。在这里，我们通过在燃烧条件下用光电离质谱法探测过量乙炔中 1-和 2-萘基自由基反应生成的产物，提供了实验证据，证明该反应生成了 1-和 2-乙炔基萘（C12 H8 ）、苊烯（C12 H8 ）和二乙炔基萘（C14 H8 ）。重要的是，没有发现菲和蒽（C14 H10 ），这表明 HACA 机制不会导致第三个芳香环的环化，而是预期的而是发生乙炔基取代反应。
