Synthesis of Polycyclic Aromatic Hydrocarbons by Phenyl Addition–Dehydrocyclization: The Third Way

Abstract

Polycyclic aromatic hydrocarbons (PAHs) represent the link between resonance‐stabilized free radicals and carbonaceous nanoparticles generated in incomplete combustion processes and in circumstellar envelopes of carbon rich asymptotic giant branch (AGB) stars. Although these PAHs resemble building blocks of complex carbonaceous nanostructures, their fundamental formation mechanisms have remained elusive. By exploring these reaction mechanisms of the phenyl radical with biphenyl/naphthalene theoretically and experimentally, we provide compelling evidence on a novel phenyl‐addition/dehydrocyclization (PAC) pathway leading to prototype PAHs: triphenylene and fluoranthene. PAC operates efficiently at high temperatures leading through rapid molecular mass growth processes to complex aromatic structures, which are difficult to synthesize by traditional pathways such as hydrogen‐abstraction/acetylene‐addition. The elucidation of the fundamental reactions leading to PAHs is necessary to facilitate an understanding of the origin and evolution of the molecular universe and of carbon in our galaxy.

Reaction kinetics of 1,4-cyclohexadienes with OH radicals: an experimental and theoretical study

“…The formation of the first aromatic ring is key to understanding the growth rates of polycyclic aromatic hydrocarbons (PAHs) and soot formation processes. 1,2 Several mechanisms, such as hydrogen abstraction acetylene addition (HACA), [3][4][5] hydrogen abstraction vinylacetylene addition (HAVA), 6 and phenyl addition dehydrocyclization (PAC), 7 have been proposed in the literature for the growth of PAHs. In these mechanisms, phenyl radicals derived from benzene are ubiquitous.…”

Inception of Carbonaceous Nanostructures via Hydrogen-Abstraction Phenylacetylene-Addition Mechanism

“…A number of mechanisms were proposed to explain the formation and growth of PAHs starting from small hydrocarbons. − The hydrogen-abstraction acetylene-addition (HACA) mechanism , is widely suggested as an important formation pathway of PAHs in the combustion of hydrocarbons , and in the circumstellar envelops of carbon-rich stars . A repetitive sequence of hydrogen abstraction from an aromatic hydrocarbon followed by acetylene addition, cyclization, and aromatization leads to PAHs with multiple fused benzene rings. , Similarly, the hydrogen-abstraction vinylacetylene-addition (HAVA) mechanism has been invoked since naphthalene, phenanthrene, and anthracene were shown to be synthesized via barrier-less collisions of phenyl (C 6 H 5 ) and naphthyl (C 10 H 7 ) radicals with vinylacetylene.…”

“…A repetitive sequence of hydrogen abstraction from an aromatic hydrocarbon followed by acetylene addition, cyclization, and aromatization leads to PAHs with multiple fused benzene rings. , Similarly, the hydrogen-abstraction vinylacetylene-addition (HAVA) mechanism has been invoked since naphthalene, phenanthrene, and anthracene were shown to be synthesized via barrier-less collisions of phenyl (C 6 H 5 ) and naphthyl (C 10 H 7 ) radicals with vinylacetylene. The phenyl addition-dehydrocyclization (PAC) mechanism hypothesizes the addition of phenyl to an aromatic hydrocarbon followed by hydrogen abstraction, cyclization, and dehydrogenation processes. , Two prototype PAHsfluoranthene and triphenylenewere synthesized by the reactions of phenyl with naphthalene and biphenyl, respectively, at high temperatures, which provided a systematic elucidation of the validity of the PAC mechanism . Recent studies revealed a PAH formation mechanism involving resonance-stabilized radicals (RSRs). , The dimerization of RSRs with other large hydrocarbons stabilizes to covalently bonded clusters at high temperatures through a rapid radical-driven hydrocarbon clustering, and thus bypassing the stepwise addition of small hydrocarbon species; this process leads to a quick inception of oligomers of aromatic compounds …”

Continuous Butadiyne Addition to Propargyl: A Radical-Efficient Pathway for Polycyclic Aromatic Hydrocarbons

“…Four chemical pathways have been proposed to explain the formation of PAHs by bonding small hydrocarbons and monocyclic and polycyclic aromatics (Figure a). These include hydrogen-abstraction acetylene-addition (HACA), hydrogen-abstraction vinylacetylene-addition (HAVA), phenyl-addition dehydrocyclization (PAC), and the clustering of hydrocarbons via radical chain reactions (CHRCR) …”