## 2 Results and Discussion
### Distribution and Origin of the Heteroaromatic Compounds
Production of C\({}_{4}\)H\({}_{4}\)S by reaction of light alkenes, such as C\({}_{2}\)H\({}_{4}\), with FeS\({}_{2}\) and H\({}_{2}\)S was invoked to explain their presence in the volcanic gases emitted from Mt. Etna [52]. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline  & & & **2-** & & **3-** & & **3-** & & & \\
**n\({}^{\circ}\)** & **sample** & **C.H.O** & **C.H.O** & **C.H.O** & **C.H.O** & **C.H.S** & **C.H.S** & **2.4-C.H.S** & **C.H.S** & **C.H.S** \\ \hline  
rrialba volcano 5 & 4.4 & 0.6 & 1.7 & 0.8 & 5.7 & 0.1 & \(<\)0.1 & 9.1 & 0.3 \\
26 & Turrialba volcano 6 & 4.4 & 0.8 & 2.3 & 0.7 & 6.2 & 0.3 & \(<\)0.1 & 8.8 & 0.1 \\ \hline \end{tabular}
\end{table} 
Table 3: Composition of heteroaromatics, C\({}_{6}\)H\({}_{6}\) and C\({}_{7}\)H\({}_{8}\). Concentrations are in ppbv. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline  & & & **2-** & & **3-** & & **3-** & & **3-** & \\
**n\({}^{\circ}\)** & **sample** & **C\({}_{\text{{\small{H}}}}\)O** & **C\({}_{\text{{\small{H}}}}\)O** & **C\({}_{\text{{\small{H}}}}\)O** & **C\({}_{\text{{\small{H}}}}\)O** & **C\({}_{\text{{\small{H}}}}\)S** & **C\({}_{\text{{\small{H}}}}\)S** & **C\({}_{\text{{\small{H}}}}\)S** & **2.4-C\({}_{\text{{\small{H}}}}\)S** & **C\({}_{\text{{\small{H}}}}\)** & **C\({}_{\text{{\small{H}}}}\)** \\ \hline  
On an industrial scale thiophene is synthesized through the following catalytic processes: 1) reaction of C\({}_{4}\)+ alcohols or carbonyls with CS\({}_{2}\) over alkali-promoted alumina; 2) reaction of unsaturated aldehydes with H\({}_{2}\)S over an alkali-promoted alumina; 3) reaction of C\({}_{4}\)+ alkyl hydrocarbons or olefins with CS\({}_{2}\), S, and H\({}_{2}\)S over alkali-promoted alumina; 4) catalytic dehydrogenation of tetrahydrothiophene; 5) synthesis from furan and H\({}_{2}\)S over alumina [53, 54, 55, 56]. The thiophene Paal-Knorr synthesis involves the reaction of 1,4-diketones with H\({}_{2}\)S as sulphurising agent [57]. Generally speaking, in gases from natural fluid discharges, the most reliable genetic mechanism for the formation of thiophene is through the addition of H\({}_{2}\)S to dienes in the presence of H+ and metal catalysts (Scheme 1). **Scheme 1. ** Synthesis of C\({}_{4}\)H\({}_{4}\)S from butadiene. \begin{table}
\begin{tabular}{l c c c c c c c c c} \hline  & & & **2-** & & **3-** & & **3-** & & \\ \(\mathbf{n}^{\circ}\) & **sample** & **C\(\mathbf{\_}\)H\(\mathbf{\_}\)O** & **C\(\mathbf{\_}\)H\(\mathbf{\_}\)O** & **C\(\mathbf{\_}\)H\(\mathbf{\_}\)O** & **C\(\mathbf{\_}\)H\(\mathbf{\_}\)S** & **C\(\mathbf{\_}\)H\(\mathbf{\_}\)S** & **2\(\mathbf{\_}\)C-H\(\mathbf{\_}\)S** & **C\(\mathbf{\_}\)H\(\mathbf{\_}\)** & **C\(\mathbf{\_}\)H\(\mathbf{\_}\)** \\ \hline  
\hline \end{tabular}
\end{table} 
Table 3: Cont. In the \(M\) gases heteroaromatics and inorganic sulphur-bearing gases, the latter being constituted by SO\({}_{2}\) and H\({}_{2}\)S at comparable concentrations (Table 1), are apparently showing an inverse correlation (Figure 2). This suggests that thiophenes, which are less reactive than other five-membered heteroaromatics, including furans, serving as dienes during Diels-Alder reactions [58], tend to be destroyed when fluid reservoirs are affected by conspicuous contribution from magmatic degassing. **Figure 2.**(H\({}_{2}\)S + SO\({}_{2}\)) _vs._ (C\({}_{4}\)H\({}_{4}\)S + 3C\({}_{5}\)H\({}_{6}\)S + 2,4C\({}_{6}\)H\({}_{8}\)S) binary diagram. Green triangle:  
The C\({}_{4}\)H\({}_{4}\)S concentrations and those of 3-C\({}_{5}\)H\({}_{6}\)S (Figure 3a) and C\({}_{6}\)H\({}_{6}\) (Figure 3b) show a positive correlation in both \(H\) and \(M\) gases. This supports the following hypotheses: 1) at hydrothermal conditions mono-aromatics and thiophenes are efficiently produced by similar genetic processes; 2) all these compounds have a similar behaviour in response to thermodynamic conditions caused by presence of oxidizing and high temperature (\(>\)400 \({}^{\circ}\)C) magmatic fluids.