## 4 Discussion  
### Comparison of the Cl Reactivity with Unsaturated Aldehydes  
As far as we know, the present research is the first kinetic study of the reaction Cl + T2M2B. The rate coefficient that has been determined in this study is within the same order of magnitude of the reported for other aldehydes as summarized in Table 4. The rate coefficients for the other aldehydes were determined by using a relative method in all cases, similarly to this research, with the techniques shown in Table 4.  
Considering the available kinetic data for the Cl reaction with other unsaturated aldehydes, a structure-activity trend of the Cl reactivity can be derived. As shown in Table 4 and also reported by [PERSON] et al. [9], the rate coefficient seems to slightly increase with the hydrocarbon chain for the linear unsaturated aldehydes, _E_-CH\({}_{3}\)(CH\({}_{2}\))\({}_{\rm{\tiny{\circ}}}\)CH=CHC(O)H, when x = 5-7. This would indicate that the H-abstraction from the hydrocarbon chain is playing a role. However, the rate coefficient determined for a shorter unsaturated aldehyde, _E_-CH\({}_{3}\)CH=CHC(O)H, reported by [PERSON] et al. [10], indicates that this trend may not be as clear and further studies would be needed. For branched unsaturated aldehydes with a central double bond, such as _E_-CH\({}_{3}\)CH=C(CH\({}_{3}\))C(O)H and (CH\({}_{3}\))\({}_{2}\)C=CHC(O)H, the Cl reactivity seems to increase, according to what has been reported by [PERSON] et al. [10], \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{2}{c}{**Unsaturated Aldehyde**} & \multicolumn{2}{c}{\(k\)**/10\({}^{-10}\) cm\({}^{3}\)**} & \multirow{2}{*}{**Technique**} & \multirow{2}{*}{**Reference**} \\ \cline{3-3} \multicolumn{1}{c}{Linear} & _E_-CH\({}_{3}\)CH=CHC(O)H & 2.60 \(\pm\) 0.04 & GC-FID & [10] \\  & _E_-CH\({}_{3}\)CH\({}_{2}\)CH=CHC(O)H & 1.31 \(\pm\) 0.19 & GC-MS & [9] \\  & _E_-CH\({}_{3}\)(CH\({}_{2}\))\({}_{\rm{\tiny{\circ}}}\)CH=CHC(O)H & 1.92 \(\pm\) 0.22 & GC-MS & [9] \\  & _E_-CH\({}_{3}\)(CH\({}_{2}\))\({}_{\rm{\tiny{\circ}}}\)CH=CHC(O)H & 2.40 \(\pm\) 0.29 & GC-MS & [9] \\  & _E_-CH\({}_{3}\)CH=C(CH\({}_{3}\))C(O)H & 2.45 \(\pm\) 0.32 & FTIR & This research \\  & (CH\({}_{3}\))\({}_{2}\)=CHC(O)H & 2.48 \(\pm\) 0.71 & FTIR & [12] \\  & CH\({}_{2}\)=C(CH\({}_{3}\))C(O)H & 2.9 \(\pm\) 0.8 & FTIR and GC-FID & [11] \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Summary of the Cl-rate coefficients of selected unsaturated aldehydes at 298 K.  
Figure 7: SOA yield curve determined in this research for the _trans_-2-methyl-2-butenal reaction with Cl. with respect to that of the linear unsaturated aldehyde \(E\)-CH\({}_{3}\)CH\({}_{2}\)CH=CHC(O)H, that has the same number of C and H atoms. This trend could indicate that the branching of the main chain by the electron donor methyl groups accelerates the reaction. However, the rate coefficient for \(E\)-CH\({}_{3}\)CH=CHC(O)H, reported by [PERSON] et al. [10], is similar to the reported in this research for \(E\)-CH\({}_{3}\)CH=C(CH\({}_{3}\))C(O)H, which would indicate that the presence of the methyl group has no effect on the reactivity. As stated above, further studies on the reactivity of the linear unsaturated aldehydes are needed to better establish the reactivity trend as a function of the structure. When looking at the length of the chain of branched unsaturated aldehydes, it seems to slightly reduce the Cl reactivity as observed when comparing the rate coefficients for T2M2B and methacrolein (CH\({}_{2}\)=C(CH\({}_{3}\))C(O)H) [11]. It is also possible to compare the effect of the position of the methyl group in the reactivity by looking at the rate coefficients of 3-methyl-2-butenal, (CH\({}_{3}\))\({}_{2}\)C=CHC(O)H, [12], and T2M2B. Both compounds have almost the same rate coefficient (considering the errors), indicating that the position of the methyl group does not affect the Cl reactivity in this case. Finally, the Cl-reactivity of saturated aldehydes was reported to be faster than that for unsaturated aldehydes [9].