The Temperature Dependence of the Ratio k_d/k_c for Ethyl Radicals

Abstract

The ratio of the rate constants for disproportionation and combination for ethyl radicals, k d / k kd 2C2Hs + C2H4 + C2Hs has been measured over the temperature range 298-173 K in the gas phase. Ethyl radicals were produced by direct photolysis of ethylenefollowed by addition of hydrogen atoms to ethylene. At low temperatures the only important reactions of the radicals were combination and disproportionation. The ratio kd/kc was obtained from measurements of the rates of formation of ethane, butane, and butene. No change in the ratio kd/k, was observed over the temperature range studied, leading to the conclusion that Ed -E, = 0, within the experimental error. The significance of this result is discussed in relation to other measurements in both gas and liquid phase and to the nature of the transition state for this reaction.

The kinetics of the bimolecular self‐reaction of t‐butyl radicals in solution. II. Disproportionation/combination ratios

“…For ethyl radicals, Pd/Pc is well known [3] [22] [23] it is equal to the temperature independent gas phase value of ca. 0.15 [51], and it increases by somewhat less than a factor of two if determined in viscous ethylene glycol [22], or if an external pressure of up to 600 MPa is applied to the solvent [23]. However, for -181" in liquid methane, Gillis [52] reported Pd/Pc= 1 for C2HD4 radicals.…”

Relative rates of radical–radical processes in the pyrolysis of ethane

“…The small difference in zero point energies leads to the slight increase in the ratio at low temperatures. This increase is not clearly established experimentally (24). The illustrated model is not intended to be exact, but to indicate that most of the data were compatible with a single, reasonable explanation.…”

The photolysis of ethylene at 193 nm

“…The primary products of the photolysis can be accounted for by the three primary photodissociation reactions, [l], [2], and [3] followed by a sequence of reactions of hydrogen atoms and…”

“…If reactions [4] to [ l l ] are assumed to account for all the primary products (i.e., neglecting C2H6 formation by reaction [13]), the quantum yields for reactions [ 1 1, [2], and [3] can be estimated from the following expressions:…”

“…It is seen that @(I) is given by the total yield of C2H2 minus that formed in reaction [2]; @(2) is equal to the total yield of C2H5 radicals (which in turn is a measure of the H a t~m~~i e l d ) , minus the H atoms formed in [3]; @(3) is given by the yield of C2H3 radicals found in the products. Values of these primary photodissociation quantum yields calculated in this way, using initial extrapolated product yields, are shown in Table 3.…”