R. J. Cvetanović

Relative Rates of Reactions of Oxygen Atoms with Olefins

Relative rates of addition of oxygen atoms in their ground electronic state to a number of olefins in the gas phase have been determined at room temperature. These reactions exhibit the following regularities: (1) systematic variation of the rates with the structure of the olefins; (2) specificity with respect to the position of addition of oxygen atoms; and (3) partial molecular rearrangement of the addition products according to definite rules. The observed structural effects and some close analogies with certain olefin addition reactions in solution are discussed in the light of electronic theories of organic chemical reactions. A comparison is made of the absolute values of the rate constants of the reactions of oxygen atoms with olefins, derived from various sources.

Relative Rates of Addition of Hydrogen Atoms to Olefines

Hydrogen atoms were produced by the mercury‐photosensitized decomposition of n‐butane at 435±5 mm pressure and at 23.5±1°C. By introducing known amounts of olefine, the rate of addition of hydrogen atoms to the olefine could be measured relative to the rate of abstraction from the n‐butane by observing the fall in the rate of production of hydrogen. In this way, relative rates of addition to eight olefines were obtained, and the logarithms of the relative rate constants were found to correlate reasonably well with the atom localization energies of the olefines, but not with their maximum free valence.

Reaction of Oxygen Atoms with Ethylene

Reaction of O‐atoms, produced by mercury photosensitized decomposition of nitrous oxide, with ethylene has been studied at room temperature and 123°C. The primary step of the process appears to be a direct addition of the atom to the double bond to form an energy‐rich intermediate which undergoes further reactions. The following products are formed: aldehydes (CH3CHO, C2H5CHO, and C3H7CHO), CO, CH4, C2H6, C3H8, C4H10, H2, very small amounts of some other substances and traces of HCHO. At 24°C O‐atoms react with ethylene 22±5 times as readily as with n‐butane. Assuming comparable pre‐exponential factors for the two processes ΔE= — 1.9 kcal/mole, so that the activation energy of addition of oxygen atoms to the double bond in ethylene is estimated at close to 3 kcal/mole. In the presence of molecular oxygen the character of the reaction is profoundly changed.

Isotopic Effects and Collisional Deactivation in the Mercury Photosensitized Decomposition of Ethylene

Mercury photosensitized decomposition of C2H4, C2D4, cis‐C2D2H2, and C2H4 with excess xenon added, has been investigated at room temperature. An isotopic effect in the intramolecular decomposition of ethylene has been established, as well as an extensive isotopic isomerization in the course of collisional deactivation of the energy rich molecules. The cis‐dideuteroethylene isomerizes not only to the trans‐ but also to the anti‐form. The experimental results are discussed in terms of the intramolecular decomposition and collisional deactivation of the excited molecules formed in the primary process.

Temperature Dependence of the Rates of Addition of Oxygen Atoms to Olefins

Relative rates of addition of oxygen atoms to several representative olefins have been measured at 25° and 125°C, and the ratios of the pre‐exponential factors and differences in activation energies have been calculated from the mean values of the relative rate constants at the two temperatures. The results obtained show that the differences in reactivities are primarily due to differences in the activation energies while the pre‐exponential factors remain approximately constant.

Unimolecular Decomposition of Chemically Activated Propyl Radicals. Normal Intermolecular Secondary Kinetic Isotope Effect

The vibrationally excited radical pairs n‐propyl‐d0, n‐propyl‐d6 and isopropyl‐d0, isopropyl‐d6 were produced at 25°C by the addition of H atoms to propylene‐d0 and propylene‐d6. Decomposition of n‐propyl is by C–C rupture, whereas C–H rupture appears to be the predominant path for isopropyl decomposition, possibly to the exclusion of C–C rupture. The average rates of decomposition ka were determined relative to collisional stabilization by complete product analysis: for n‐propyl‐d0 C–C rupture, kan≃108 sec—1; for isopropyl‐d0 C–H rupture, kai(—1)≃10—2 kan. For vibrationally excited propane, formed by the addition of H atoms to isopropyl radicals, kapr≃6.6×105 sec—1. The secondary kinetic isotope effect for n‐propyl unimolecular decomposition in this nonequilibrium system was found to be ∼5.0 at the low‐pressure limit, and ∼4.5 at the high‐pressure limit; that found for isopropyl decomposition is ∼6.The Marcus—Rice specific rate expression was used with a quantum statistical harmonic oscillator model to c...

Mercury Photosensitized Decomposition of Nitrous Oxide

The reaction of nitrous oxide with Hg(3P1) atoms at 23°C has been investigated. The evidence obtained indicates that the sole primary process is a splitting of the nitrous oxide molecule into a nitrogen molecule and an oxygen atom. The possible secondary reactions are discussed.

Relative Efficiencies of Quenching of Mercury Resonance Radiation and Their Temperature Independence. Mechanism of Mercury Photosensitized Decomposition of Saturated Hydrocarbons

Relative over‐all efficiencies of quenching of mercury resonance radiation have been determined for N2O, n‐C4H10, C2H6, and C2H4 by studying the binary mixtures of each of the hydrocarbons with nitrous oxide at 23° and 120°C. For N2O and n‐C4H10 the relative efficiency of quenching to the ground state as well as the fractions of quenching to the ground and to the metastable 6(3P0) state of mercury for each of the two compounds have also been determined. Temperature independence of the primary quenching processes has been demonstrated.A proposed mechanism of quenching by saturated hydrocarbons, based on additional experimental information obtained with n‐butane alone and its mixtures with nitrous oxide, interprets fully the experimental facts and in particular accounts for the repeatedly observed low quantum yields of hydrogen formation.

Molecular Rearrangement in the Mercury‐Photosensitized Reaction of Butene‐1

Mercury‐photosensitized reaction of butene‐1 has been studied at room temperature in the pressure range of 14 to 230 mm. Methyl cyclopropane is found to be formed by molecular rearrangement and its yield goes through a maximum at a pressure of about 65 mm. The dependence of the yield of this product on pressure is explained by postulating participation in the process of two excited states of equal lifetimes. These are, perhaps, the vibrationally excited triplets of butene‐1 and of methyl cyclopropane, the latter being formed from the former by an internal 1,2‐ or approximately 1,4‐ migration of an H atom. Molecular rearrangement to butene‐2 does not occur.

Mechanism of the Mercury Photosensitized Decomposition of n‐Butane

The results of further studies of the mercury photosensitized reactions at room temperature of n‐butane and of its mixtures with nitrous oxide are reported. The importance of a self‐scavenging process responsible for low quantum yields of hydrogen at finite conversions has been demonstrated by carrying out experiments at low conversions closely approaching initial conditions. The initial quantum yield of hydrogen has been determined relative to that of nitrogen from the mercury photosensitized decomposition of nitrous oxide. The mechanism of the self‐scavenging process is discussed.