Experimental Method

All experiments were performed in HIRAC at 296 ± 3 K and at 1 bar. The same relative rate method was used in order to be able reproduce previous experiments carried out in HIRAC (Hemavibool 2009; Malkin 2010), thus validating the method and the experimental conditions, and to expand the range of esters studied in HIRAC to propionates. The chemicals obtained from commercial sources all had purities of

≥98%: methyl acetate (99%, Alfa Aesar), acetone (≥99%, Fisher), and chlorine (99.5%, Sigma Aldrich), methyl propionate (99+%, Fischer Scientific), ethyl propionate (99+%, Fischer Scientific), propyl propionate (99%, Sigma Aldrich), isobutyl propionate (98%, Acros Organics), n-butyl propionate (99%, Alfa Aesar), chlorine (99.5+%, Aldrich), propane (98%, Sigma Aldrich), n-butane (99%, Sigma Aldrich) and nitrogen (>99.995%, Dominick Hunter N2 generator, MAX116).

Cl + CH3C(O)OCH3 → products (R4.17)

Cl + C2H5C(O)OCH3 → products (R4.18)

Cl + C2H5C(O)OC2H5 → products (R4.19)

Cl + C2H5C(O)OC3H7 → products (R4.20)

Cl + C2H5C(O)OCH(CH3)2 → products (R4.21)

Cl + C2H5C(O)OC(CH3)3 → products (R4.22)

Cl + C3H8 (reference) → products (R4.23)

Cl + C4H10 (reference) → products (R4.24)

With the exception of the gases used, all chemicals were freeze-pump-thawed several times prior to use. This minimised the impurities introduced and gave a better accuracy of the concentrations of the ester that were being introduced into the chamber.

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4.7.3 Results

Previous measurement of the rate of Cl + acetates reactions had been carried out by Malkin and Hemavibool using both GC-FID and FTIR instruments for detection of reactants and references (Malkin, 2010, Hemavibool, 2009). In this work the FTIR was unavailable so only GC-FID measurements are reported, and methyl acetate was the only ester to be studied, with acetone being used as the reference compound. Figure 4.7 demonstrates the high accuracy in the GC relative rate results obtained for methyl acetate in HIRAC, which are within error of Hemavibool and Malkin’s previous work. A comparison of the results obtained with other reported data are shown in Table 4.7. Results for both GC-FID and FTIR measurements are in excellent agreement with the previously reported results from HIRAC and with previous literature, particularly Christensen, Ball and Wallington’s relative rate results, indicating satisfactory conditions for carrying out relative rate experiments, and a good methodology.

Table 4.7: Measured rate coefficients for methyl acetate + Cl from relative rate ratios obtained in HIRAC compared to previous work from HIRAC and other laboratories. RR: relative rate method, PLP-RF: pulsed laser photolysis-resonance

fluorescence technique.

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Figure 4.7: Relative rate data for reaction of Cl atoms with methylacetate at 298 K and 1000 mbar in U.H.P. nitrogen measured using GC-FID.

The relative rate plots for methyl propionate, ethyl propionate, propyl propionate, n-butyl propionate and iso-butyl propionate are shown in Figure 4.8 and 4.9. All experiments were conducted at least twice each for each propionate investigated. As can be noted for the propyl and butyl propionate plots, less data points were obtained. This was due to the operation of longer cycles of 6 minutes to avoid overlap of the unidentified product peaks with the propionate and reference. The decision to use fewer data points stems from the logarithmic increase in uncertainty associated with this method.

The results obtained for the propionates show good agreement (within 5%) with previous relative rate data for methyl propionate, but the rate coefficients obtained for the ethyl, propyl and n-butyl propionates all appear to be slightly higher than previously reported literature data, however are all within the experimental error.

No previous literature data has to our knowledge been reported on iso-butyl propionate. As can be seen in Figure 4.8, fewer data points have been obtained for propyl propionate in comparison with methyl and ethyl propionates. This is due to longer sampling rates on the GC set for this compound (same for butyl analogues) to ensure adequate resolution of all peaks in the gas chromatogram.

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Figure 4.8: Relative rate plots for reaction of Cl atoms with methyl, ethyl and propyl propionates at 298 K and 1000 mbar in U.H.P. nitrogen measured using GC-FID.

Figure 4.9: Relative rate plots for reaction of Cl atoms with n-butyl and iso-butyl of propionates at 298 K and 1000 mbar in U.H.P. nitrogen measured using GC-FID.

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A comparison of the rate coefficients obtained for Cl atoms with propionates is shown in Table 4.8.

kmethyl propionate<<kethyl propionate <kn-propyl propionate<ki-butyl propionate<kn-butyl propionate (Eq 4.11)

Propionate kpropionate/kreference kpropionate (10^-11cm³ molecule^-1 s^-1) Source

Table 4.8: Comparison of rate constants and relative rate ratios of reactions of Cl atoms with a series of propionates using ethane (for methyl, ethyl and propyl) and

propane (for butyl) as a reference at 1000 mbar and 293 K (*relative to C2H5Cl;

**relative to propane).

There is a clear trend as for the acetates reported by Malkin (2010) for the propionates with the rate of these reactions rapidly increasing in rate coefficient with size. The obtained values show a much lower reactivity for methyl propionate compared with the other propionates. Furthermore, one can note that the rate constant increases with the number of CHx groups in the ester molecule. Excellent agreement is observed for the rate coefficient obtained for methyl and ethyl propionates with recent work by Schütze et al. (2010) and Cometto et al. (2009) respectively. Some discrepancies with the literature were noted for the rate coefficient obtained for propyl propionate which could have been improved by rerunning the experiment another time to obtain more data points on the relative rate plot presented in Figure 4.8. The same may apply for n-butyl propionate, which is however within error of the only other literature value by Liang et al. (2010). Iso-butyl propionate has to our best knowledge

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never been reported in the literature and the value obtained seems to fit in well with the trend expected from the SARs of these group of compounds, which will be discussed further in the following section.