Abstract: MINDO/ 3 SCF calculations have been carried out to study the thermal elimination of N-substituted ethyl pyrazole;H₃C₃N₂CH₂CH₂X→H₄C₃N₂+H₂C=CHX (I,X=H;Ⅰ, X=CH₃;Ⅲ,X=F)Aconcerted reaction involving a five membered ring transition state(TS) was found. The results obtained show that there is a weak influence on the reactivity by the substituents on ft carbon of the ethyl group. The hyperconjugation of the methyl group, and the strong electronegativity and the weak conjugation of the fluorine atom stabilize the corresponding TSrespectively, the activation barriers of these two reactions are lower than that of the non-substituted reactent.

Key words: Thermal elimination, Transition state, Intrinsic reaction coordinate