A thermodynamic investigation of the kinetics of the reaction of bromine atoms with ethers
LE3 .A278 2006
Roscoe, John M.
Bachelor of Science
A reaction chamber for studying atmospheric photochemistry was used to obtain the rate constants for the reaction of diethyl ether with atomic bromine as a function of temperature. Analysis was performed with gas chromatography and rate constants were calculated from these results using the relative rate method. 2,2,4- Trimethylpentane was used as the reference reactant. The Arrhenius parameters were evaluated from the temperature dependence of the rate constant for this reaction. It was found that the reaction of diethyl ether with atomic bromine had a pre-exponential factor of 1.96 (Â± 0.21) Ã— 10-10 cm3 molecule-1 s-1 and an activation energy of 19.8 (Â± 2.6) kJ mol-1. The reaction mechanism was determined to be hydrogen abstraction. A thermodynamic analysis of the Arrhenius parameters was performed and comparisons were made between reactions of ethers with atomic bromine and atomic oxygen. A mechanistic approach was used to explain the differences in reactivity between diethyl ether and dimethyl ether. It was found that the enthalpy of activation for the reactions of diethyl ether and dimethyl ether with atomic bromine agreed within experimental error; however, the entropy of activation of the reaction with diethyl ether was 40 J K-1 mol-1 larger than that for the reaction with dimethyl ether. Comparison of the corresponding reactions of atomic oxygen also showed that there is a larger value of the entropy of activation for the reaction with diethyl ether than for the reaction with dimethyl ether. This difference was interpreted to indicate a looser transition state for the diethyl ether reactions than for those of dimethyl ether. The more substantial difference in entropy of activation of the six bromine reactions was attributed to the decreased ability of bromine atoms to polarize the C-H bond as compared to atomic oxygen. This decreased polarization of the C-H bond led to a looser transition state and, in turn, a larger value of the entropy of activation. A comparison was made between the rate constant for the reaction of diethyl ether with atomic bromine and that for the bromine atom catalysed destruction of ozone. It was found that in order for the two reactions to compete, the atmospheric concentration of diethyl ether would have to be approximately 18 times larger than the concentration of ozone.
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