Steric factors in the kinetically controlled direction of elimination of secondary and tertiary esters in the gas phase. The pyrolysis of 4,4‐dimethylpent‐2‐yl acetate

Abstract

AbstractThe pyrolysis kinetics of 4,4‐dimethylpent‐2‐yl acetate, in a static system and in a vessel seasoned with allyl bromide, have been studied in the temperature range of 300–350°C and the pressure range of 48–211 torr. The olefin products were 4,4‐dimethylpent‐1‐ene, cis‐4,4‐dimethylpent‐2‐ene, and trans‐4,4‐dimethylpent‐2‐ene. The rate coefficient for the homogeneous unimolecular elimination of this ester is given by the following Arrhenius equation: log k(sec−1) = (12.87 ± 0.31) − (181.2 ± 3.4)kJ/mol/2.303RT. The direction of elimination of this acetate has been found to proceed to the formation of the corresponding olefin by kinetic control. The present data, together with other pyrolysis work subject to kinetic control, imply that the direction of elimination of bulky alkyl esters is determined by steric hindrance in the eclipsed cis conformation. However, further analyses reveal that if a series of esters are compared, in the case of a gradual increase of alkyl branching when adjacent to a hydrogen atom (alkyl–H interactions), the rate was determined by steric acceleration, owing to the crowding effect at the highly substituted carbon atom. Otherwise if this gradual alkyl increase in size happened to be adjacent to another alkyl substituent (alkyl–alkyl interactions), the rate was affected by steric hindrance of the eclipsed cis conformation.