The colourful world of radical reactions: The kinetics and thermochemistryof the reactions

In this lecture, we survey the kinetic characteristics of various types of free radical reactions: among the unimolecular processes we deal with the d ecomposition and isomerization reactions, from the bimolecular processes we discuss the hydrogen atom abstraction (or metathesis) reactions, the addit ions, the combinations and disproportionations. Using the results of our ow n investigations, we outline the main features of the kinetics and thermoch emistry of free radical reactions. Direct kinetic methods, such as fast flow and laser flash photolysis techni ques were used; the experimental studies were supplemented by ab initio cal culations. The results presented are related to four important areas of che mical kinetics: 1. Structure-Reactivity Relationships of Hydrogen Atom Abstractions Reactio ns: Factors Determining the Rate of Reactions. The rate of atom transfer reactions may be significantly influenced by two factors, which art the enthalpy change of the reaction and the polarity of the transition state. Among the energy-controlled elementary reactions we d ealt with the hydrogen atom abstractions from cycloalkanes by hydroxyl radi cals. Deviations were observed, in the temperature dependence of the rate c oefficients, from the well-known Arrhenius low. The hydroxyl radical and br omine atom reactions with aliphatic aldehydes were studied. These reactions were characterized by polar transitions states and the reaction rates were found to be determined by the polar effect. 2. Relationship between the Kinetics and Thermochemistry: Determination of Heat of Formation for Free Radicals by Kinetics Methods. In order to determine the heat of formation for the hydroxymethyl radical ( (C) over dot H2OH), which plays an important role in combustion and flames, we studied the hydrogen abstraction reaction of bromine atom with methanol in both directions. From the temperature dependence of the rate constants, the equilibrium constant of the reaction and the reaction enthalpy were ob tained. From the latter, the heat of formation of the hydroxymethyl radical was derived. 3. The Temperature Dependence of the Rate Coefficient: The Arrhenius Equati on. Curved Arrhenius plot and negative activation energy were obtained for the bromine atom reaction with methanol. It was shown that the negative tempera ture dependence is the result of a mechanism in which a short-lived complex participates. From the analysis of the mechanism, the conditions leading t o negative activation energy was formulated. 4. The Kinetics of Radical-Radical Reactions. The radical-radical reactions are of importance in combustion and flames. T heir kinetic investigation is difficult since they are often multi-channel processes. Our investigations, within the methanol project, dealt with reac tions which occurred via vibrationally excited methanol molecules. In case of reactions H + CH3(O) over dot and H + (C) over dot H2OH, two dominant pr ocesses were found to occur. These are the disproportionation of the reacta nts which leads to stable endproducts and the combination which gives vibra tionally excited methanol molecule. The major decomposition path of the lat ter is the reaction yielding (C) over dot H-3 + (O) over dot H products for med by C-O bond rupture. The reaction (C) over dot H-3 + (O) over dot H was studied in a wide pressu re range. The experimental technique allowed us to monitor in time simultan eously the concentration of two short-lived free radicals. We found that, a t 1.33 mbar pressure and 298 K temperature, the branching ratio for the (CH 2)-C-1 + H2O product channel is 0.89 +/- 0.09. This high branching ratio ma y modify our currently accepted view on the mechanism of combustion of orga nic compounds.