Molecular elimination

Two of the many products of ethylene radiolysis—methane and propane—show no or only negligible variation with field strength. Methane is produced by a molecular elimination process, as evidenced by the inability of oxygen or nitric oxide to quench its formation even when these additives are present in 65 mole % concentration (34). Propane is completely eliminated by trace amounts of the above scavengers, suggesting methyl and ethyl radicals as precursors ... 

Like other metal reactions studied previously in our laboratory, H2 elimination is initiated by insertion into one of the C-H bonds forming HMC3H5. The reaction rate constant for Y + cyclopropane was found to be very small at room temperature, 0.7 x 10 12 cm3 s 1, and it was suggested that the reaction most likely involved termolecular stabilization of C-H or C-C insertion complexes, rather than molecular elimination.22 By analogy with other systems studied, the dynamically most favorable route to H2 loss in this case is likely via H atom migration to the Y-H moiety, with concerted... 

Experimental activation energies presented in Table XI indeed are consistent with this observation. Furthermore, we can postulate that molecular eliminations that proceed with the formation of five-membered or larger cyclic transition states should exhibit activation energies that are much closer to the heats of reaction (O Neil and Benson, 1967). 

Since polarizabilities of groups or atoms are also manifested in bond strengths, some form of Hirschfelder s rule can also be developed to treat these types of reactions (Laidler, 1987). For example, the activation energies for many molecular elimination reactions seem to be correlated with one-third of the sum of the dissociation energies of the bonds that are being broken. This empirical relationship seem to hold well for a large variety of HX elimination reactions (see Table XI). 

The reverse reactions to these additions involve the molecular elimination of H-X or H-R from a stable molecule to leave behind an unsaturated species. These are picturesquely called snap-out reactions. 

In the same category are the direct, molecular eliminations of molecules from aldehydes ... 

Although the yield of hydrogen chloride is reduced, it is not completely suppressed by the presence of iodine which would act as an efficient third body in the recombination of chlorine atoms. The yield of hydrogen chloride is not suppressed by the presence of nitric oxide, probably because the combination of nitric oxide and chlorine involves third-body restriction. There may be an alternative primary process involving the molecular elimination of hydrogen chloride. 

Experimental evidence for the distortion of a kinetic growth curve due to a closed ion source was reported by Martinez et al. [42]. In an FPTRMS investigation of the infrared laser multiphoton dissociation of CF2HC1, these authors found that the risetime of the HC1 molecular elimination product, which was expected to be formed on a microsecond time scale, was approximately 2 ms. This they attributed to holdup in the ion source, which was a partially enclosed box. Gas entered the ion source chamber through a hole at one end and exited through several holes at either end or at its sides. 

G. Atomic versus Molecular Elimination in Halogenated Hydrocarbons... 

Other more complex molecular elimination reactions are known to occur. One of the most interesting is the elimination of methane from acetaldehyde74 75. Several primary steps are possible... 

One of the most interesting molecular elimination reactions was first discovered by Norrish and Appleyard65 in 1934 and studied further by Bamford and Nor-rish66-69 in papers appearing in 1935 and 1938. These authors found that, on photolysis, aliphatic ketones with hydrogen atoms on carbons in the gamma position to the carbonyl yielded olefins and a methyl ketone. An early example was found in 2-hexanone, viz. 

Ultrafast molecular elimination of iodine from IF2C-CF2I has been studied using the velocity map ion imaging technique in combination with femtosecond pump-probe laser excitation.51 By varying the femtosecond delay between pump and probe pulse, it has been found that elimination of molecular iodine is a concerted process, although the two carbon-iodine bonds are not broken synchronously. 

By comparison, the photolysis of Me4Ge with an argon fluoride laser at 193 nm yields trimethylgermane, ethane, methane, ethylene and layers of Ge as the end products209. These results have been interpreted as indicative of electronically excited Me4Ge undergoing preferential molecular elimination (equations 37a and 37b) over the dissociation shown in equation 36. 

It is a known fact that the gas-phase pyrolysis kinetics of alkyl bromides have not been extensively investigated due to the experimental difficulties as well as to the complexity of concurrent unimolecular and radical chain mechanisms. However, when these organic bromides are pyrolyzed under maximum inhibition, the reaction in the presence of a free radical suppressor is a molecular elimination. Sometimes, these organic bromides are pyrolyzed under maximum catalysis with HBr gas, and the process may proceed by an autocatalytic molecular mechanism. 

Ethyl bromide, in a static system, was studied at 724.5-755.1 K103. The pressure dependence for the HBr elimination was observed in its fall-off region. Evaluation of the rate coefficients was performed by using the RRKM theory and the values were compared with the experimental observation. The work reported an activation energy of 216.3 kJ moT1 and an Arrhenius A factor of 1012 5. The low-frequency factor was rationalized in terms of the formation of a tight activated complex and a molecular elimination as a prevalent reaction mode. 

In summary, it appears that the primary process in water photolysis is mainly production of H + OH. The molecular elimination of H2 assumes some importance in the 1240 A. band. The question of whether molecular elimination of H2 occurs in the first and second continua remains unresolved. 

Thus, molecular elimination of H2 occurs. The only alternative to (106) would be the production of PH(A3n) and two H atoms. This process is energetically impossible.111 An additional primary process giving PH2 and H is also observed. 

There is no definite proof that H2 is not formed by molecular elimination from excited B2H6. 

Methane, a. Molecular Elimination of Hydrogen. Because of analytical difficulties, the early work116117 on methane photolysis was of questionable significance with regard to the attempts to understand the mechanism of the photolysis. Recently, the primary process in methane photolysis has been elucidated by examining the isotopic... 

---