Substituent effects unimolecular elimination

MP2 study of substituent effects of 2-substituted alkyl ethyl methylcarbamates in homogeneous, unimolecular gas phase elimination reaction ... 

In contrast to the tertiary alcohols, primary (e.g., ethyl alcohol) and secondary alcohols e.g., isopropyl alcohol) decompose to products at temperatures above 800 °K via complex free radical chain processes . This mechanistic inversion is not surprising. Based on the magnitude of substituent effects in four-center elimination reactions, particularly the variations found in the series r-BuCI, i-PrCl, EtCl - , one would estimate that the isopropyl alcohol unimolecular elimination of water should have an activation energy about 6 kcal.mole higher than that for r-butyl alcohol. The. 4-factor can be estimated by transition state methods, and one obtains for the unimolecular decomposition... 

Nevertheless, several kinetic studies have shown that certain nucleophilic substitution reactions of aryl diazonium ions are first order and independent of the concentration of the nucleophilic species. Solvent effects, isotope effects, and substituent effects are also in agreement with a rate-determining unimolecular decomposition of the aryl diazonium ion. In other reactions, an adduct of the nucleophile and diazonium ion is a distinct intermediate. Substitution results when nitrogen is eliminated from the adduct. Finally, substitution can occur via radical... 

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