Gas-phase pyrolytic reactions. Rate data for pyrolysis of N-t-butylthioacetamide and N-acetylthioacetamide: role of polarity of transition state and γ-carbonyl group protophilicity

Abstract

In the gas phase, both N -t -butylthioacetamide and N-acetylthioacetamide undergo unimolecular firstorder elimination reactions for which log A = 11.58 s-' and 10.64 s-', and €, = 16.45 kJ mol-' and 117.1 5 kJ mol-', respectively. The results are in accord with a reaction pathway involving a cyclic six-membered transition state, and show each compound to be more reactive than its oxygencontaining analogue. At 600 K, the statistically corrected reactivity ratios: t-butyl thioacetate (1 ) / t-butyl acetate (2); N-acetylthioacetamide (3)/diacetamide (4); and N -t -butylthioacetamide (5)/N-t-butylacetamide (6) are 83, 173, and 1 404, respectively. The above rate factors are consistent with the tenet that as C,-X bond fission becomes less rate-contributing in these electrocyclic reactions, so attack by the C=Y bond upon the P-hydrogen atoms becomes more important. Thus, whereas t-butyl acetate at 600 K is some 68 700 times more reactive than N-t-butylacetamide, t-butyl thioacetate is only 4 060 times more reactive than N -t -butylthioacetamide.The gas-phase thermal elimination of esters, amides, and anhydrides containing P-hydrogen atoms are thought to involve pathways in which the transition states approximate to cyclic six-membered structures (Scheme 1).

Kinetic and mechanistic study on the thermal reactivity of stabilized phosphorus ylides, part 3: [(Acetyl)(arylcarbamoyl)methylene]triphenylphosphoranes and [(alkoxycarbonyl)(arylcarbamoyl)methylene]triphenylphosphoranes and their thiocarbamoyl analogues

“…A platinum resistance thermocouple is connected from the other end to a Comark microprocessor thermometer to read the actual pyrolysis temperature. Reactions were ascertained to be homogeneous and free from surface effects and radical pathways in auxiliary experiments in which the tubes were packed with quartz helices so as to increase the surface area by approximately sixfold [6,7]. The rates at 600 K are calculated using the kinetic equation for first-order reactions:…”

Synthesis, thermal reactivity, and kinetics of stabilized phosphorus ylides, part 2: [(Arylcarbamoyl)(cyano)methylene]triphenylphosphoranes and their thiocarbamoyl analogues

“…A platinum resistance thermocouple is connected from the other end to a comark microprocessor thermometer to read the actual pyrolysis temperature. Reactions were ascertained to be homogeneous and free from surface effects and radical pathways in auxiliary experiments in which the tubes were packed with quartz helices so as to increase the surface area by approximately six-fold [32,33]. The rates at 600 K are calculated using the kinetic equation for first-order reactions: log k = log A − E a 4.575T Treatment of the kinetic data has been detailed elsewhere [34][35][36].…”

Pyrolysis of ?-hydroxyketones and ?-ketoesters: Gas-phase elimination kinetics of 3-hydroxy-3-methyl-2-butanone and methyl benzoylformate

“…The pyrolysates are swept into the GC-MS system by the carrier gas. The conditions of the GC and the MS were adjusted to affect a good separation of the pyrolysates and to allow proper identification and (ii) Using flow technique: This technique has been described in an earlier communication [5].…”