Effects of ring strain on gas‐phase rate constants. I. Ozone reactions with cycloalkenes

Abstract

Rate constants for the gas-phase reactions of 0 3 with a series of cycloalkenes and with cis-2-butene have been determined at 297 f 1 K. ]-2-octene, 0.728 f 0.090. These data for cis-2-butene, cyclopentene, and cyclohexene are compared with previous literature data, and the effects of ring strain on the rate constants are discussed.

Rate constants for the gas‐phase reactions of O3with a series of cycloalkenes at 296 ± 2 K

“…ously been explored [13,21,22]. Consistent with previous observations [13,21], the room temperature rate constants per C C double bond for cyclopentene, cyclohexene, 1,4-cyclohexadiene, cycloheptene, ciscyclooctene, 1,5-cyclooctadiene, and cis-2-butene (the representative acyclic alkene) increase with increasing ring strain (Fig. 8).…”

Effects of ring strain on gas‐phase rate constants. 2. OH radical reactions with cycloalkenes

“…In particular, in contrast to the case of 0 3 reactions [8], the two nonconjugated cycloalkenes for which the most ring strain is expected to be released on addition of an OH radical, bicyclo[2.2.1]-2,5-heptadiene and bicyclo[2.2.1]-2-heptene, show no increase over the rate contants calculated solely on the number of double bonds. In addition the rate constant for 1,3-cyclohexadiene is only 13% higher than that for the acyclic analog 2,4-hexadiene [17].…”

“…A recent study of the reactions of OH radicals with a series of cyclic and polycyclic alkanes, where the reactions proceed via H-atom abstraction, showed [7] that ring strain energies in excess of -5 kcal mol-I lead to a decrease in the rate constants relative to those expected for the strain-free molecules. In contrast, the room temperature rate constants for the addition reactions of 0 3 with a series of cycloalkenes appear to correlate with the difference in ring strain energy between the cycloalkene and the corresponding cycloalkene or cycloalkane with one less double bond [8], with, for example, the rate constant for bicyclo[2.2.1]-2-heptene being a factor of -16 higher than that for cis-2-butene (which has the same number and configuration of substituents). However, the limited data available for OH radical addition reactions all concern six-membered low-strain rings [6], and no conclusions can yet be drawn as to the effect of ring strain on these reactions.…”

“…In order to avoid interferences in the gas chromatographic analyses, four pairs of cycloalkenes concentrations at the end of the irradiations were always below the detection limit of 2 ppb (i.e., <5 X 1O1O molecule cm-3), and, totally analogous to the previous determination of OH radical rate constants for a series of acyclic alkenes [9], data were taken before the INO,J/[NOJ ratios (taking into account unreacted CH30NO which was monitored as NO, by the chemiluminescence analyzer used) exceeded 2. 2,3-Dimethyl-2-butene was included in these mixtures since, while its OH radical rate constant is similar to that for isoprene [9], its 0 3 reaction rate constant is higher by a factor of -100 than that of isoprene [lo-121, being of a similar magnitude to those for the more reactive cycloalkenes studied here [8]. Thus since the reactivities of isoprene and 2,3-dimethyl-2-butene toward O3 essentially span those of the cycloalkenes studied, the relative rate constant data obtained for isoprene and 2,3-dimethyl-2-butene served to check that reaction with 0 3 was not contributing to any significant extent to the observed alkene and cycloalkene disappearance rates.…”

“…From this study and our previous study on the reactions of 0 3 with cycloalkenes [8] it is clear that, at least for certain cycloalkenes, the OH radical and 0 3 rate constants do not correlate well, though both of these reactive species are regarded as being electrophilic in character. Figure 4 shows a linear free energy plot for the alkenes and cycloalkenes, and methyl vinyl ether and methyl vinyl ketone, for which reliable rate constant data are available.…”