The non-isothermal oxidation of 2 -methylpentane. - III. The reaction at high pressure

The non-isothermal oxidation of 2-methylpentane has been studied at pressures of 1-4 MN m -2 and temperatures of 440 to 660 °C in an arrested-piston rapid-compression machine. The variations with pressure and temperature of the induction periods leading to cool-flame reaction and hot ignition have been determined, and the products of the reaction have been analysed by gas-liquid chromatography. At high temperatures and pressures the cool-flame reaction occurs by a free-radical chain process in which homogeneous isomerization and subsequent decomposition of alkylperoxy radicals propagate the chain. The resulting propa­gation cycle is substantially the same as that which has been established at lower tempera­tures and subatmospheric pressures. At high temperatures and pressures the reaction is, however, even more unselective, and oxidation of β -hydroperoxyalkyl radicals competes more successfully with their unimolecular decomposition, thus leading to the formation of β -ketoaldehydes. These compounds, together with the conjugated unsaturated carbonyl compounds, account quantitatively for the absorption of ultraviolet light by reacting 2-methylpentane/air mixtures. The mechanism of chain branching in the cool-flame reaction probably involves the pyrolysis of hydroperoxides. In the second stage of two-stage ignition, the propagation cycle is the same as that occurring in the cool flame but the important difference is that the cool flame has formed substantial concentrations of compounds with labile hydrogen atoms; these react readily with alkylperoxy radicals to form hydroperoxides, the pyrolysis of which again branches the chain.