The sensitisation by Rose Bengal of the photo‐oxidation of 2‐dimethylamino‐5,6‐ dimethylpyrimidin‐4‐01 (I) has been studied in aqueous solution by monitoring the rate of oxygen consumption in the system. The results are in accord with a mechanism whereby I is oxidised via a reaction with singlet oxygen. Values for the rate constant for the reaction of I with singlet oxygen have been determined over the temperature range 279‐296 K, and the energy of activation and entropy of activation for the reaction have been evaluated as 71.9 kJ mol−1 and 150.5 J K−1 mol−1, respectively. The positive value for the entropy of activation suggests that the reaction intermediate could have a zwitterionic form. The effect of pH on the reaction rate has been investigated, and the increased rate at high pH has been attributed to an increase in singlet oxygen production as the pH is raised.

The kinetic and analytical features of the oxidation of ethane and ethylene at 362" are described.In the early stages of the oxidation of ethane, 80 % of the hydrocarbon consumed gives ethylene.In the intermediate stages the ethylene is itself oxidized. In the initial stages of the oxidation of pure ethylene over 80 % of the hydrocarbon consumed gives formaldehyde, and in the intermediate stages the formaldehyde is oxidized to carbon monoxide. The main molecular stages in the oxidation are therefore ethane, ethylene, formaldehyde, carbon monoxide, with water also being formed at the appropriate stages.The kinetics of the ethane oxidation are similar to those of the propane oxidation and the autocatalysis of the reaction is probably due to the oxidation of formaldehyde. The slow start of the acceleration is in part due to the fact that the branching intermediate is not an initial product of the reaction.The general nature of hydrocarbon oxidation reactions is discussed. It is concluded that the main reaction of alkyl radicals with oxygen at 300-400" is usually an abstraction reaction However, there is strong evidence that the H02 radical is unable to abstract H from ethane or most other hydrocarbons, and it is proposed that a key reaction in alkane oxidation is the conversion of HO2 to OH. The reaction 2H02 = 20H+02 is suggested.CnH2n+1+ 0 2 = CnH2n+ HO2.

The ability of soil samples of pH 4·2, 5·5 and 7·2 to generate gaseous singlet oxygen was investigated using a separated‐surface‐sensitised reactor. It was found that the soil samples can act as sensitisers for the production of singlet oxygen in a similar fashion to the well known sensitisers chlorophyll and Rose Bengal. The reaction of singlet oxygen so produced with the pyrethroid insecticide, bioresmethrin, has been investigated.

From detailed analysis of the initial stages of the oxidation of ethane, water, ethylene and formaldehyde are the only significant initial products at 362" and acetaldehyde is an additional initial product at 31 8". Methanol, ethanol, ethylene oxide, carbon monoxide and carbon dioxide are secondary products. As oxygen has no effect on the relative yields of initial products they are produced by reactions of the same kinetics. Between 362 and 318" there is a profound change in the distribution of initial products. It is proposed that the equilibrium (1) is established(1) and that ethylene is formed by direct H atom abstraction from ethyl while formaldehyde is formed from the ethyl peroxy radical.The initial products from the oxidation of ethylene are ethylene oxide, formaldehyde and acetaldehyde. The secondary products are the same as from ethane. Various possible mechanisms are discussed for their formation. It is concluded that HO2 is the main radical attacking ethylene. The end products of the oxidations are methane, ethane (from ethylene) and ethanol.

Rate constants have been measured for the reaction in chloroform solution of singlet oxygen with the fungicides ethirimol (5‐butyl‐2‐ethylamino‐6‐methylpyrimidin‐4‐ol) and dimethirimol (5‐butyl‐2‐dimethylamino‐6‐methylpyrimidin‐4‐ol), and with the compounds 2‐dimethylamino‐5,6‐dimethylpyrimidin‐4‐ol, 2‐dimethylamino‐6‐methylpyrimidin‐4‐ol, 4‐benzyloxy‐2‐dimethylamino‐5,6‐dimethylpyrimidine and 2‐diethylamino‐6‐methylpyrimidin‐4‐ol. The values obtained show that singlet oxygen reacts readily with these compounds; the differences between the rate constant values have been rationalised in terms of the different structural features of the compounds studied. The possibility that singlet oxygen may react with agricultural chemicals under natural conditions is considered.