In the present study, the most favourable reaction mechanism in the flash vacuum pyrolysis of 2-[(2E)-2-benzylidenehydrazinyl]pyridine and analogous was theoretically studied using B3LYP-D3/6-311G(d,p) level. The influence of different substituents on the phenyl ring in the para position is evaluated at 500 K. A deep analysis of the mechanisms reveals the influence of the substituents in the activation energy, where the reactivity order OCH3 > CH3 > H > Cl > NO2 is found, following Hammet parameters (σp). To reach the transition state, geometric changes contribute more than 70% of the energy in agreement with the reaction force analysis. A recent independent gradient model (IGM) is also applied to establish an influence between the substituent and the strength of the bonds involved in the mechanism, through an intrinsic bond strength index (IBSI). Furthermore, the influence of the substituent was also evaluated in terms of interatomic distances and evolution percentages, finding high correlations with Hammet σp values.