Absolute rate constants for hydrocarbon autoxidation. XIII. Aldehydes: photo-oxidation, co-oxidation, and inhibition

Abstract

The oxidations of acetaldehyde, heptanal, octanal, cyclohexanecarboxaldehyde, pivaldehyde, and benzaldehyde in chlorobenzene at 0 "C have been studied. These aldehydes oxidize at similar rates under similar conditions because there are compensating changes in the rate constants for chain propagation (k,) and chain termination (2k,). The termination rate constants increase from -7 x lo6 M-I S-I f or pivaldehyde and cyclohexanecarboxaldehyde to -2 x lo9 M-' s-' f or benzaldehyde. The propagation rate constants increase from -1 x lo3 M-'s-' for pivaldehyde to -1 x lo4 M-l s-' f ox benzaldehyde.The rate of oxidation of the aldehydes was decreased by the addition of 1,4-cyclohexadiene, tetralin, tetralin hydroperoxide, cumene, cumene hydroperoxide, t-butyl hydroperoxide, and 2,6-di-t-butyl-4-methylphenol. As a result of product analysis and absolute rate constant measurements, it is concluded that the peroxy radicals derived from aldehydes are considerably more reactive in hydrogen atom abstraction from hydrocarbons than are the peroxy radicals derived from the hydrocarbons. In the abstraction from cyclohexadiene, the acylperoxy radicals appear to be from 15 to 70 times as reactive, and the benzoylperoxy radicals about 8W900 times as reactive, as the hydroperoxy radical. The differences in reactivity are very much less pronounced in the abstraction from 2,6-di-t-butyl-4-methylphenol.The great ease of oxidation of all aldehydes, and particularly benzaldehyde, is due at least in part to the high reactivity of the peroxy radicals formed in these reactions.

Absolute Rate Constants for Hydrocarbon Autoxidation. XXI. Activation Energies for Propagation and the Correlation of Propagation Rate Constants with Carbon–Hydrogen Bond Strengths

“…The high reactivities of benzyl chloride, benzyl acetate, benzyl benzoate and, particularly, benzaldehyde towards their own peroxy radicals are due to the exceptional reactivities of these radicals which arise from the presence of the electron withdrawing a-substituents (51,52). Towards the t-butylperoxy radical these substrates exhibit a normal pattern of reactivity which implies that polar effects in the hydrogen donor are of less importance in determining k, than polar effects in the peroxy radical.…”

Absolute rate constants for hydrocarbon autoxidation. XIX. Oxidation of some α-substituted toluenes

“…The termination activation energy was taken to be 2 kcall mole. The difference in reactivity of benzaldehyde towards the benzoylperoxy radical and towards the t-butylperoxy radical, kp/k,br , -40 000, is 40-100 times higher than was previously estimated for the difference in reactivity of benzoylperoxy radicals and tetralylperoxy radicals (7). Tertiary peroxy radicals are generally about 5 times less reactive than secondary peroxy radicals towards a hydrocarbon (19).…”

“…The photochemically initiated oxidation of benzyl alcohol is also autocatalytic because of initiation from photolysis of one of the major reaction products, benza!dehyde (7).…”

“…Walling and McElhill(6) have reported that p-substitution apparently in--fluences the reactivity of benzoylperoxy radicals. Benzoylperoxy radicals have been found to be 500-900 times more reactive than tetralylperoxy radicals (7). This difference in reactivity must be because acylperoxy radicals are more electrophilic than alkylperoxy radicals.…”

“…Increasing the electron donating capacity of R increases the rate constant for hydrogen abstraction by a peroxy radical (6,7). Polar effects may play an important role in the reactivity of a peroxy radical as well as in the reactivity of a substrate.…”