Pyrolysis mechanisms benzoic acid

Figure 15.3 Thermal cracking mechanism for polyethylene terephthalate. Note under mild conditions terephthalic acid predominates, but under more severe pyrolysis conditions the terephthalic acid decomposes to benzoic acid and benzene...

It is interesting to note that the decarboxylation mechanism of aromatic acids is probably an electrophilic substitution. This reaction is not uncommon during pyrolysis. For example, the decarboxylation of benzoic acid takes place as an aromatic electrophilic substitution ... 

The similar pyrolysis mechanisms of benzoic acid and benzaidehyde are proposed in Figures 8.6 and 8.7 [19] , the optimized structures and their atom numbers of reactants, intermediates, transition states, and products are shown in Figures 8.8 and 8.9 and the energy profiles of the stationary points for benzoic acid and benzaidehyde pyrolysis reactions are shown in Figures 8.10 and 8.11. According to the transition state theory [64], activation enthalpy activation entropy and activation energy can be obtained from Eqs (8.1 )- 8.3), respectively. The rate constant k can be expressed as shown in Eq. (8.4). These kinetic parameters are listed in Table 8.1. 

L. X. Ling, L. J. Zhao, R. G. Zhang, andB. J. Wang, Pyrolysis mechanisms of benzoic acid and benzaldehyde based on quantum chemistry. Chin. J. Chem. Eng. 60(5), 1224-1230 (2009). 

Poly(ethylene terephthalate) decomposes upon heating through a series of different reactions. These run either concurrently or consecutively. The result is a complex mixture of volatile and nonvolatile products. It was found that when poly(ethylene terephthalate) is maintained in molten condition under an inert atmosphere at 282-323°C, it slowly converts to a mixture of gaseous low molecular weight fragments [581]. The major products from pyrolysis of poly(ethylene terephthalate) are carbon dioxide, acetaldehyde and terephthalic acid. In addition, there can be detected trace amounts of anhydrides, benzoic acid, p-acetylbenzoic acid, acetophenone, vinyl benzoate, water, methane, ethylene, acetylene, and some ketones [505]. The following mechanism of degradation was postulated [505] ... 

R = D) has now been shown to be formed. Hence, the carbene mechanism is ruled out, and a proposed alternative involving two sigmatropic rearrangements followed by elimination of benzoic acid (Scheme 30) finds support in the formation of 2,5-bismethylene-2,5-dihydrofuran on pyrolysis of (329). ... 

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