2 research outputs found
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 CH<sub>2</sub> 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 (<i>k</i>) of reaction of
the four alcohols with Cl vary in the range (3.5–4.3) ×
10<sup>–10</sup> cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup>. 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/O<sub>3</sub>. Although
the reactivity of OH/O<sub>3</sub> 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 <i>k</i>, using the calculated energies at QCISD and
QCISDÂ(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++GÂ(d,p) level. The <i>k</i> values calculated at the QCISD level for Cl and the QCISDÂ(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