# Evaluation of the SOA Formation in the Reaction of Furfural with Atmospheric Oxidants
## 3 Results and Discussion  
### Gas-Phase Products  
In order to facilitate the identification of the generated products, a general mechanism has been proposed for hydrogen abstraction and double bond addition. The presence of the -CHO group in C2 leads to the deactivation of the C3 and C5 positions of the aromatic ring due to its negative inductive effect. However, according to the number of resonance forms generated by the attack of the oxidant X (Cl, OH or NO\({}_{3}\); Scheme S1, Supplementary Materials), the most favored positions seem to be C2 and C5. This assumption was confirmed by theoretical calculations [9; 10]. Of these two positions, C2 and C5, the first is hindered due to the presence of the carbonyl group in the same position. As a consequence, it is expected that the C5 attack will be preferred. Similar double bond addition mechanisms have been proposed for the reactivity of alkylfurans [1; 17]. Moreover, the furfural can react by aldehydic hydrogen abstraction but, according to the bibliographic data, the abstraction of hydrogen from the aromatic ring is less likely [18]. This behavior has also been described in previous theoretical works regarding the reactivity of furfural with OH [10] and NO\({}_{3}\)[9]. According to these works, a similar reaction mechanism for the reactions with chlorine atoms has been proposed (see Scheme 1). The formation of maleic anhydride (P2) and 5-chloro-2-(5H)-furanone (P1) as reaction products was evidenced by GC-TOFMS. The presence of maleic anhydride was confirmed by comparing the retention time and mass spectrum with those of the commercial compound. Furthermore, 5-chloro-2-(5H)-furanone is not commercially available so its formation was confirmed by comparison with the EI mass spectrum obtained in previous works [17]. The formation of this compound was also observed in the experiments performed by FTIR spectroscopy. An example of the IR bands due to the reaction products generated in the reaction of furfural with Cl atoms is shown in Figure 1.  
The bands centered at 2890, 2351 and 2100 cm\({}^{-1}\) (Figure 1b) are assigned to HCl, CO\({}_{2}\) and CO, respectively. CO may be generated by the decarboxylation of the CHO group and HCl is a coproduct of the hydrogen abstraction process. However, the concentrations of these compounds must be treated with caution since they are also present in the chamber when air and Cl\({}_{2}\) are exposed to visible light. The absorption band at 1829 cm\({}^{-1}\) is assigned to 5-chloro-2-(5H)-furanone, which has been proposed as a product from the C5 addition. This type of furanone has also been detected in previous works as a product of the reaction of alkylfurans with Cl atoms [17]. There is a very good agreement between the residual product spectrum and the bibliographic spectra of 5-chloro-2(5\(H\))-furanone [17] (Figure 1d), which strongly supports the finding that the residual FTIR features are mainly attributable to 5-chloro-2(5\(H\))-furanone. In the residual product spectrum, the formation of maleic anhydride was not observed, possibly due to overlap with the chloroformanone bands (Supplementary Materials Figure S2). However, the formation of a broad band at around 1797-1760 cm\({}^{-1}\) suggests the presence of one or more compounds with a lactone-like structure. This fact, along with the band observed at 3750 cm\({}^{-1}\) (\(\
u\)O-H), could indicate the formation of 5-hydroperoxy-2(5H)-furanone (P3), which could be generated from the chlorine addition in C2--as proposed in the literature for NO\({}_{3}\)[9] and OH [10] radicals (blue route in Scheme 1). In the latter case, ring-opening products have also been theoretically proposed in the literature, although this route is only expected to be minor. Moreover, maleic anhydride, which was detected in our experiments, can also be formed in the H abstraction route (Scheme 2), albeit to a lesser extent. Product formation for the reaction of furfural with chlorine atoms in the presence of NO was also studied.