THE PHOTOCHEMICAL OXIDATION OF ALDEHYDES IN THE GASEOUS PHASE: PART III. THE ABSOLUTE VALUES OF THE VELOCITY CONSTANTS FOR THE PROPAGATING AND TERMINATING STEPS IN THE PHOTOCHEMICAL OXIDATION OF ACETALDEHYDE AND PROPIONALDEHYDE

Abstract

It has been established (Parts I and II) that the photochemical oxidation of acetaldehyde and propionaldehyde obeys the kinetic expression[Formula: see text]where RCHO represents the aldehyde and RCO3H the corresponding peracid. k3 is the velocity constant for the propagating reaction, and k6 that for the terminating reaction involving the mutual interaction of two RCO3 radicals. [Formula: see text] represents the rate of initiation of the photooxidation.Alcohols have been found to retard the photooxidation and it has been shown that the retarded reaction obeys the kinetic expression:[Formula: see text]It has thus been possible to determine the rates of initiation [Formula: see text] for each of the photooxidations. Application of the rotating sector technique has enabled the lifetime of the oxidation chains to be measured. These data together with the information given in Parts I and II have been used to calculate the absolute values for the velocity constants for the propagating and terminating reactions, k3 and k6:[Formula: see text]in the gas-phase photooxidations of acetaldehyde and propionaldehyde.The values of the respective velocity constants are: acetaldehyde at 20°, k3 = 8.05( ±2.04) × 103 1. mole−1 sec−1, k6 = 8.93( ±4.20) × 1010 1. mole−1 sec−1; propionaldehyde at 22°, k3 = 4.35( ±0.9) × 104 1. mole−1 sec−1, k6 = 2.69( ±1.35) × 1010 1 mole−1 sec−1.These values for the velocity constants for the "recombination" reactions of the peracetic and perpropionic radicals indicate that for these radicals reaction [6] is very efficient.