Gas-Phase Elimination Kinetics of (Dimethylamino)alkyl Acetates. The Ion-Pair Mechanism through Neighboring Group Participation.

Article abstract of DOI:10.1021/jo00196a011

The gas-phase elimination kinetics of some amino esters and a keto acetate have been studied in the temperature region of 260.0-411.5 deg C and in the pressure range of 21.5-170.0 torr.These eliminations, in vessels seasoned with allyl bromide, are predominantly unimolecular and homogenous and obey a first order rate law.The rate coefficients for the reactions are expressible by the following Arrhenius equations: for 3-(dimethylamino)-1-propyl acetate (1), log k₁ (s^-1) = (12.97 +/- 0.20) - (202.1 +/- 2.5) kJ mol^-1 (2.303RT)^-1; for 4-(dimethylamino)-1-butyl acetate (4), log k₁ (s^-1) = (11.91 +/- 0.43) - (163.5 +/- 4.8) kJ mol^-1 (2.303 RT)^-1; for 4-oxo-1-pentyl acetate (7), log k₁ (s^-1) = (12.77 +/- 0.36) - (202.8 +/- 4.6) kJ mol^-1 (2.303RT)^-1.The presence of the (CH3)2N group in these acetates appears to provide anchimeric assistance in the elimination; methyl acetate and the corresponding heterocyclic products arise from an intimate ion-pair mechanism.The CH3CO substituent is believed to influence the pyrolysis rate of 5-acetoxy-2-pentanone by a weak steric acceleration.