Gas-Phase Reaction of Alkanes with Oxygen

Thermal reactions of light alkanes with oxygen in the combustion process have been studied extensively (6, 7). These studies were typically conducted at high temperatures—flame temperatures. The elementary reactions of the hydrocarbon species often involve reactions with atomic (H, O) or free radical species (OH, alkyl, etc.). The initiation step is the homolytic cleavage of C—C single bonds to form alkyl radicals. The C—C bonds are the weakest bonds in an alkane molecule (Table I). The chain-propagation step 

Activation Energies and Rate Constants of Some Gas-Phase Reactions of Ethane, Ethene, Propane, and Propene (Derived from Ref. 7) 

From the activation energies and the preexponential factors, the rate constants at 873 K can be calculated. They are listed in Table II. They show that for the gas-phase homogeneous reactions, the reactions of O atoms and OH radicals with ethene are very rapid and somewhat faster than their reactions with ethane. This fact would limit the maximum yield of ethene. It is well known that if the reactions of an alkane and an alkene are both first order in the hydrocarbon, then the maximum yield for the alkene of about 35% would be obtained when the rate constants, kA and kn, for the two reactions have equal values  

If the rate constant kr. is larger than kA, that is, the alkene reacts more rapidly than the alkane, the maximum yield would be lower. 

The same consideration applies to the reaction of propane. The reactions of O atoms and OH radicals are more rapid with propene than with propane. In fact, the differences in the rate constants are larger for the reactions of Cj s than C2 s. This would result in a lower maximum yield for propene from propane than for ethene from ethane. 

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