Is H Atom Abstraction Important in the Reaction of Cl with 1-Alkenes?

The relative yields of products of the reaction of Cl atoms with 1-alkenes (C4-C9) were determined to see whether H atom abstraction is an important channel and if it is to identify the preferred position of abstraction. The presence of all the possible positional isomers of long chain alkenones and alkenols among the products, along with chloroketones and chloroalcohols, confirms the occurrence of H atom abstraction. A consistent pattern of distribution of abstraction products is observed with oxidation at C4 (next to allyl) being the lowest and that at CH2 groups away from the double bond being the highest. This contradicts with the higher stability of allyl (C3) radical. For a better understanding of the relative reactivity, ab initio calculations at MP2/6-311+G (d,p) level of theory are carried out in the case of 1-heptene. The total rate coefficient, calculated using conventional transition state theory, was found to be in good agreement with the experimental value at room temperature. The preferred position of Cl atom addition is predicted to be the terminal carbon atom, which matches with the experimental observation, whereas the rate coefficients calculated for individual channels of H atom abstraction do not explain the observed pattern of products. The distribution of abstraction products except at C4 is found to be better explained by reported structure activity relationship, developed from experimental rate coefficient data. This implies the reactions to be kinetically dictated and emphasizes the importance of secondary reactions.

Reactivity of Cl atom with triple-bonded molecules. An experimental and theoretical study with alcohols.

The reactivities of the Cl atom with triple-bonded molecules were examined by determining the rate coefficients of reactions of four triple-bonded alcohols (TA), namely, 2-propyn-1-ol, 3-butyn-1-ol, 3-butyn-2-ol, and 2-methyl-3-butyn-2-ol, using the relative rate method, at 298 K. The rate coefficients (k) of reaction of the four alcohols with Cl vary in the range (3.5-4.3) × 10(-10) cm(3) molecule(-1) s(-1). These values imply significant contribution of the Cl reaction in the tropospheric degradation of TAs in the conditions of the marine boundary layer. A striking difference is observed in the reactivity trend of Cl from that of OH/O3. Although the reactivity of OH/O3 is lower with triple-bonded molecules, as compared to the double-bonded analogues, the reactivity of the Cl atom is similar for both. For a deeper insight, the reactions of Cl and OH with the simplest TA, 2-propyn-1-ol, are investigated theoretically. Conventional transition state theory is applied to compute the values of k, using the calculated energies at QCISD and QCISD(T) levels of theory of the optimized geometries of the reactants, transition states (TS), and the product radicals of all the possible reaction pathways at the MP2/6-311++G(d,p) level. The k values calculated at the QCISD level for Cl and the QCISD(T) level for OH reactions are found to be very close to the experimental values at 298 K. In the case of the Cl reaction, the abstraction of α-H atoms as well as the addition at the terminal and middle carbon atoms have submerged TS and the contribution of the abstraction reaction is found to be significant at room temperature, at all levels of calculations. Addition at the terminal carbon atom is prominent compared to that at the middle carbon. In contrast to the Cl reaction, only addition at the middle carbon is associated with such low lying TS in the case of OH. The individual rate coefficients of addition and abstraction of OH are lower than that of Cl. The negative temperature dependence of the computed rate coefficients in the temperature range 200-400 K shows that the difference in the TS energy of Cl and OH affects the pre-exponential factor more than the activation energy.