Thermal stability and degradation of aromatic polyamides. Part 2. Structure-reactivity relationships in the pyrolysis and hydrolysis of benzamides

Abstract

An experimental study to determine the effect of ring and nitrogen substitution on the rate and product distribution of the pyrolytic and hydrolytic degradation of polyamides was carried out. Two families of benzamides were chosen as models of the amide functionality obtained from the reaction of an aromatic acid and an aliphatic amine. The first family included benzamide and other primary amides with ring substituents. The second family comprised secondary or tertiary amides. The benzamides were reacted in an inert argon atmosphere and in the presence of added water at 350°C. The influence of nitrogen substitution was dramatic; the rate of disappearance of the primary amides was an order of magnitude faster than that for secondary or tertiary amides. The major products formed from primary amides were the corresponding benzonitrile and benzoic acid, which was a secondary product of pyrolysis and a primary product of hydrolysis. Pyrolysis of secondary and tertiary benzamides led to the corresponding lesser-substituted benzamide. A combination of a free radical and a concerted mechanism was proposed that accounted for the conversion and product selectivities for both families of benzamides. A nonspecific free radical mechanism with short chain length combined with a [1,3] sigmatropic shift and subsequent fission described the reaction of primary benzamides and methyl substituted benzamides that lacked a β CH. The higher selectivity to benzamide and benzonitrile for benzamides witha β CH suggested β-carbon directed chemistry. Appliction of a free radical mechanism with nonzero chain length afforded benzamide and benzonitrile, but failed to account for the observed product selectivities. A concerted reaction mechanism with a six-membered cyclic transition state led to high selectivity of benzamide and benzonitrile.