Kinetics and mechanism of gas-phase pyrolysis of N-aryl-3-oxobutanamide ketoanilides, their 2-arylhydrazono derivatives, and related compounds

Rates and products of reaction and Arrhenius activation parameters were determined for the gas-phase thermolysis of 14 substrates of the title compounds using sealed pyrex reactor tubes and HPLC/UV-VIS to monitor substrate pyrolysis. The 14 compounds under study are N-phenyl-3-oxo- (1), N-(p-chlorophenyl)-3-oxo- (2), N-(p-methylphenyl)-3-oxo- (3), and N-(p-methoxyphenyl)-3-oxobutanamide (4), in addition to (i) four substrates (5-8) obtained by the replacement of the pairs of methylene hydrogens at the 2-position of compounds (1-4), each pair by a phenylhydrazono group; (ii) three arylhydrazono derivatives (9-11) in which Cl, CH3, or OCH3 groups are substituted at the para position of the phenylhydrazono moiety of compound 5, (iii) 3-oxobutanamide (acetoacetamide, 12), N-phenyl-3-oxo-3-phenylpropanamide (13), and N,N-diphenylpropanediamide (14). The reactions were conducted over 374-546 K temperature range, and the values of the Arrhenius log A(s(-1)) and E-a(kJ mol(-1)) of these reactions were, respectively, 12.0 +/- 2.0 and 119.2 +/- 17.0 for the ketoanilides (1-4, 12-14), and 13.0 +/- 0.7 and 157.5 +/- 8.6 for the arylhyrazono compounds (5-11). Kinetically, the arylhydrazono derivatives were found to be ca. 1.4 x 10(3) to 5.7 x 10(3) times less reactive than the parent ketoanilides. A mechanism is proposed to account for reaction products and to rationalize molecular reactivities. (c) 2006 Wiley Periodicals, Inc.