Absolute Rate Measurements of O-Atom Reactions With Ethylene and With Butane

Abstract

The absolute rate constants for the primary reactions of 0-atoms with ethylene and with butane have been determined in a fast-flow system. The 0-atoms were produced by electrodeless discharge of 0 2 and allowed to react with the hydrocarbon introduced downstream. Addition of a small amount of NO to the gas stream enabled a photometric monitoring of the 0-atoms in the reaction zone. The initial atom concentration was determined by NO2 titration. The reaction could be stopped a t any point in the reaction zone by means of a movable s~~r f a c e of cobalt oxide. The hydrocarbon concentration a t any point coulcl then be determined by condensation downstream and analysis of the proclucts.Rate measurements were carried O L I~ over the te~nperature range -50' to 200' C. T h e rates were found to be consistent with a two-body mechanism. For 0-atoms with ethylene, k = 1.8(&0.6) X10-l1 e-l6O01RT cm3 molecule-' sec-I and for 0-atoms with butane, INTRODUCTIONCvetanoviC has recently measured relative rate constants for the reactions of oxygen atoms with a series of olefins (I). This study enabled him to make interesting deductions about the effect of structure on the rates and mechanisms of these reactions. Absolute rate constants Lvere estimated by comparison with the value for the reaction 0 + NO, reported by Ford and Endow (2), which was, in turn, based on the rate constant for the reaction 0 + O2 + iVI, obtained by Benson and Axworthy (3) from their mechanism for the thermal decomposition of ozone.Kotwithstanding the high caliber of the \\.orl< of these authors, it seemed desirable to measure the absolute value of the rate constant directly for one of these olefin reactions. I t also seemed important to determine its activation energy and steric factor, since such data are virtually non-existent.The present paper reports the measurement of the absolute rate constant and its temperature depeildence for the reaction of 0-atoms with ethylene. For comparison purposes, similar measure~nents mere also obtained for the reaction of 0-atoms with n-butane. Principle of the MethodThe rate constant of interest is that of the initial attack of an 0-atom on an olefin molecule. The subsequent reactions are co~llplicated and lead to a variety of products which depend on the ratio of atomic to molecular oxygen. CvetanoviC has found, however, that the presence of molecular oxygen does not affect the rate of consunlption of the olefin (4), which indicates that the initial 0-atom attack is rate-determining. Consequently, no attempt was made in this worl< a t a detailed product analysis. Can.

The kinetics of the reactions of O(3P) atoms with dimethyl ether and methanol

“…[ l ] employed the value of 0 + n-butane of 3.01 X 1013 exp (-4200/RT) cm3 mole-' sec-' as obtained by Elias and Schiff [12]. However, this value is approximately a factor of 1.5 times larger than the more recent result of Herron and …”

Temperature dependence of the rate constants of the reactions of oxygen atoms with 1‐pentene, 1‐hexene, cis‐2‐pentene, and trans‐2‐pentene

“…These investigations resulted in a number of relative rate constants. In the 60's, discharge flow systems [5] us-ing various detection methods were applied to obtain first absolute rate constants. The following decades then brought a variety of direct absolute methods, the results of which are collected in a data base [6] and several review articles [7 -9].…”

Discharge flow-chemiluminescence study of the rate coefficients for O(3P) + CF2?CCl2 and O(3P) + CF3CF?CF2 reactions at 298 K

“…This effect can be attributed to the strong electron-withdrawing capacity of the F atom and the CF 3 radical which reduces the charge density on the carbon atom next to the double bond as well as the polarizability of the electrons, leading to a decrease of the rate constant value consistent with the electrophilic character of the O( 3 P) atom [14]. Thus, Ͼ k CH ϭCH…”