Influence of Polar α Substituents in the Gas-Phase Pyrolysis Kinetics of Tertiary Chlorides. Correlation of Alkyl and Polar Groups

Article abstract of DOI:10.1021/j100265a042

The kinetics of the gas-phase pyrolysis of several polar α-substituted tertiary chlorides were determined in a static system over the temperature range of 339.5-420.1 deg C and the pressure range of 46-221 torr.The reactions in seasoned vessels and in the presence of a free-radical suppressor, are homogeneous and unimolecular and follow a first-order rate low.The variation of the rate coefficients with temperature is given by the following Arrhenius equations: for 1,2-dichloro-2-methylpropane, log k₁ (s^-1) = (14.29 +/- 0.53) - (207.6 +/- 6.4) kJ mol^-1 (2.303RT)^-1; for 2,2-dichloropropane, log k₁ (s^-1) = (12.88 +/- 0.53) - (199.0 +/- 6.5) kJ mol^-1 (2.303RT)^-1; for methyl 2-chloro-2-methylpropionate, log k₁ (s^-1) = (13.81 +/- 0.26) - (215.2 +/- 3.3) kJ mol^-1 (2.303RT)^-1.The log k_rel of polar α substituents against the polar substituent constant ?^* values give an approximate straight line with ρ^* = -0.73, correlation coefficient r = 0.912, and intercept = -0.194 at 300 deg C.This slope inflects at the point where ?^*(CH3) = 0.00 with another reported very good straight line derived by plotting log k/k₀ vs. ?^* values for 2-chloro-2-alkylpropane pyrolyses (ρ^* = -4.75, r = 0.993, and intercept = 0.047 at 300 deg C).The present results has been rationalized, as before, in terms of a slight alteration in the polarity of the transition state due to changes in electronic transmission at the carbon reaction center.