Bond strength energy decomposition

The hydroxyl radical will be the predominant entity which attacks the alkane to regenerate an alkyl radical (Reaction 10) under conditions where isomerization and decomposition are the usual fate of alkylperoxy radicals. The activation energy for attack on an alkane molecule by OH, although difficult to determine accurately (30), is low (I, 3) (1-2 kcal. per mole). This has an important consequence. The reaction will be unselective, being insensitive to C—H bond strength. Each and every alkyl radical derived from the alkane skeleton will therefore be formed. To describe the chain-propagation steps under conditions where isomerization is a frequent fate of alkylperoxy radicals it is necessary, then, to consider each and every alkylperoxy radical derived from the alkane and not just the tertiary radicals. 

Evidence that the cleavage of the RX bond does not take place in the rate-determiningstep could be derived from the absence of a solvent effect on the rate of e +EX reactions (Anbar and Hart, 1965a) and from the fact that the activation energy of the (e q + RX) reaction is the same for compounds with quite different R—Cl bond strengths (Anbar and Hart, 1967). The decomposition of RX-, which is probably a very fast reaction, is induced by the solvation of X- and may be regarded as a typical SN1 process. 

The atomic mechanism responsible for monomolecular reactions, including thermal decompositions, was first discussed by Polanyi and Wigner.26 Their model assumes that decomposition occurs when, due to energy fluctuations in the bonds of the molecule, the bond strength is exceeded or more precisely, the bond energy resides in harmonic vibrations and that decomposition occurs when their amplitude is exceeded. The resulting expression for the first-order Polanyi-Wigner rate constant is... 

LiCC04 and NaCC04 melt before anion breakdown. The induction periods to the decompositions of K, Rb and CsCC04 are attributed [4] to slow anion breakdown within the solid, followed by a more rapid reaction in the melt formed as a eutectic between reactant and products. Because the activation energies for these reactions are about 270 kJ mol , it was concluded from consideration of bond strengths that the rate-limiting step is the rupture of a CC-0 bond. A sequence of fiirther comparable dismption or dissociation reactions yields oxygen and the metal chloride as the final residual product. 

Boldyrev et al. [46], from quantum mechanical calculations of bond strengths in the oxalate anion, and from observations [38] of the decomposition of this species in potassium bromide matrices, concluded that the most probable controlling step in the breakdown of the oxalate ion is rupture of the C-C bond. This model is (again) based on the observation that the magnitudes of the activation energies for decompositions of many metal salts of oxalic acid are comparable. This model was successfiilly applied [46,68] to the decompositions of many oxalates, with the possible exception of silver oxalate where the strengths of the C-C and Ag-0 bonds are similar. 

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