# Synergistic Effects of Fuel Components on Aromatics Formation in Combustion: A Review
## 2 Formation Mechanisms of Benzene and PAHs
### Formation Mechanisms of PAHs
#### 2.2.3 Formation Mechanism with Radicals  
Radicals have a vital role in enhancing molecular weight, thereby facilitating the formation of aromatic compounds during the combustion process [59; 60; 61]. Interactions between radicals and between radicals and other hydrocarbons can lead to the formation of polycyclic aromatic hydrocarbons [59; 60; 61; 62]. This section delves into the contributions of two types of radicals in PAH formation: resonantly stabilized radicals and non-resonantly stabilized radicals, which participate in additional pathways that contribute to PAH formation. Resonantly stabilized radicals, such as propargyl, cyclopentadienyl, benzyl, indenyl, and allyl radicals, possess the same nuclear structure but exhibit different electronic sys  
Figure 6: HAVA mechanism pathway [48; 50; 58]. tems [63; 64; 65; 66; 67; 68; 69; 70; 71; 72; 73; 74]. For instance, propagyl and allyl radicals have two electronic structures with identical numbers of carbon and hydrogen atoms but different arrangements of radical sites. Aromatic hydrocarbons, owing to the presence of a benzenoid ring, exhibit several resonance structures, exemplified by the benzyl radical, as depicted in Figure 7. The first two structures are analogous to benzene, demonstrating how different structures can be generated based on the radical site. Thermodynamically, resonantly stabilized radicals are more stable than non-resonant radicals. This thermodynamic stability has significant implications; for example, aromatic hydrocarbons tend to decompose into resonantly stabilized radicals rather than non-resonant radicals. Theoretical studies by [PERSON] et al. [75] have shown that allyl radicals are 3 kcal/mole more stable than benzyl radicals. The resonantly stabilized radicals also exhibit slower dissociation and reaction with oxygen compared to non-resonant radicals [76; 77]. Additionally, the recombination reactions of propagyl and its reaction with allyl contribute to benzene formation. Moreover, the reaction of the propagyl radical with A2 CH\({}_{2}\) leads to the formation of phenanthrene (8). Similarly, the recombination reaction of resonantly stabilized cyclopentadienyl (9) and their reaction with methyl radicals lead to the formation of naphthalene and benzene, respectively. The hydrogen abstraction reaction of toluene led to the formation of benzyl radicals. Subsequently, the recombination reaction of benzyl radicals, along with their reaction with other resonantly stabilized radicals, facilitates the enhanced formation of PAHs such as indene, naphthalene (10), phenanthrene, or higher polyaromatic hydrocarbons. For instance, indenyl radicals are generated through the hydrogen abstraction reaction of indene, and their reaction with cyclopentadienyl and A2 CH\({}_{2}\) radicals leads to the formation of phenanthrene (11, 12), and the recombination reaction of indenyl radicals leads to the formation of pyrene (13). This discussion emphasizes that the high production rate and slow dissociation of resonantly stabilized radicals contribute to increased concentrations, which are crucial for the formation of aromatic and PAHs [78; 79; 73; 80]. \[\text{A2 CH}_{2}+\text{C}_{3}\text{H}_{3}=\text{A3}+\text{2H} \tag{8}\]  
\[\text{2C}_{5}\text{H}_{5}=\text{A2}+\text{2H} \tag{9}\]  
\[\text{A1 CH}_{2}+\text{C}_{3}\text{H}_{3}=\text{A2}+\text{H}_{2} \tag{10}\]  
\[\text{C}_{9}\text{H}_{7}+\text{C}_{5}\text{H}_{5}=\text{A3}+\text{2H} \tag{11}\]  
\[\text{A2 CH}_{2}+\text{C}_{9}\text{H}_{7}=\text{A4} \tag{12}\]  
\[\text{2C}_{9}\text{H}_{7}=\text{A4}+\text{H}_{2} \tag{13}\]  
[PERSON] et al. [29] investigated toluene pyrolysis and analyzed various PAH species with a mass difference of 76 amu. Based on this difference, they proposed a phenyl addition/cyclization (PAC) mechanism followed by hydrogen abstraction. With the addition of phenyl radical, the mass increased to 77 amu, followed by hydrogen abstraction, resulting in a decrease of 1 amu.