# Synergistic Effects of Fuel Components on Aromatics Formation in Combustion: A Review
## 2 Formation Mechanisms of Benzene and PAHs  
### Formation Mechanisms of Benzene  
In the formation of polycyclic aromatic hydrocarbons (PAHs) and soot from aliphatic hydrocarbons, the initial step of forming the first aromatic ring is often considered the rate-controlling step [5]. Benzene, a fundamental aromatic compound, is typically produced through several reaction pathways, including the C\({}_{2}\) + C\({}_{1}\), C\({}_{3}\) + C\({}_{3}\), C\({}_{4}\) + C\({}_{2}\), and C\({}_{5}\) + C\({}_{1}\) pathways. These pathways illustrate the sequential combination of carbon-containing species to yield benzene, as depicted in Figure 3.  
The mechanism of benzene (A1) formation varies depending on the fuel type, composition, and combustion conditions. Numerous studies have investigated benzene formation across different fuels and combustion setups. [PERSON] et al. [19] explored benzene formation during the reaction of acetylene with formyl cation under high temperatures and ionization rates. Their findings revealed an efficient mechanism of benzene formation facilitated by the abundance of acetylene (C\({}_{2}\)H\({}_{2}\)) and formyl cations in the system. [PERSON] et al. [22] observed benzene formation in the acetylene flame and pyrolysis process under low-pressure conditions, emphasizing the significant impact of propargyl recombination reactions on benzene and fulvene formation. [PERSON] et al. [17] explored potential benzene formation pathways via vinyl addition to 1,3-butadiene using experimental studies in MIT laser photolysis, predicting rate coefficients for this reaction across various kinetic models. [PERSON] et al. [21] conducted molecular beam experiments and RRKM calculations on the reaction of diatomic carbon with 1,3-butadiene, where the signal \(m/z=77\) indicated the formation of phenyl radical, subsequently converted to benzene through the hydrogen addition reaction. They also discussed the formation of the first aromatic ring during the experimental study of acetylene. The kinetic mechanism revealed benzene formation during the reaction of acetylene with the 1,3-butadieneyl radical and, subsequently, hydrogen abstraction [23]. Additionally, [PERSON] et al. [20] theoretically reviewed the pathway to benzene formation via the reaction of 1,3-butadienyl with propargyl radical. [PERSON] et al. [18] employed density functional theory to calculate the energy barrier for benzene formation via the reaction of cyclopentadiene and ethylene, revealing a significantly lower energy barrier for this reaction pathway. Furthermore, the decomposition of carbon-carbon bonds during alkane combustion facilitates the generation of methyl radicals (CH\({}_{3}\)), consequently augmenting propargyl radical (C\({}_{3}\)H\({}_{3}\)) formation through reactions (1) and (2). The involvement of produced propargyl radicals in reactions with propyne (pC\({}_{3}\)H\({}_{4}\)) (3) and alkene (aC\({}_{3}\)H\({}_{4}\)) (4) further contributed to the formation of benzene.  
\[\mathrm{C}_{2}\mathrm{H}_{2}+\mathrm{CH}_{3}=\mathrm{p}\mathrm{C}_{3} \mathrm{H}_{4}+\mathrm{H} \tag{1}\] \[\mathrm{p}\mathrm{C}_{3}\mathrm{H}_{4}+\mathrm{H}=\mathrm{C}_{3} \mathrm{H}_{3}+\mathrm{H}_{2}\] (2) \[\mathrm{p}\mathrm{C}_{3}\mathrm{H}_{4}+\mathrm{C}_{3}\mathrm{H}_{ 3}=\mathrm{A}1+\mathrm{H}\] (3) \[\mathrm{aC}_{3}\mathrm{H}_{4}+\mathrm{C}_{3}\mathrm{H}_{3}=\mathrm{ A}1+\mathrm{H}\]