Benzocaine: A comprehensive thermochemical study

Abstract In the present work, a detailed thermochemical, experimental and theoretical, study of benzocaine is presented. The enthalpy of formation in crystalline state at T = 298.15 K was obtained from combustion calorimetry experiments [ Δ f H m ° c r = - 415.2 ± 1.7 k J ∙ m o l - 1 ], within an oxygen atmosphere, using a static bomb calorimeter. The phase transition enthalpies (fusion, vaporization, and sublimation) were obtained by different techniques, namely differential scanning calorimetry, Calvet microcalorimetry, thermogravimetry, and the Knudsen effusion method. The results obtained by the different techniques are as follows: Δ cr l H m ° 298.15 K = 21.4 ± 0.1  kJ⋅mol−1; Δ l g H m ° 298.15 K = 84.9 ± 1.0  kJ⋅mol−1; Δ cr g H m ° 298.15 K = 106.8 ± 0.4  kJ⋅mol−1. From the experimental results, the enthalpy of formation of the aforesaid compound, in the gas phase, was calculated at T = 298.15 K as: Δ f H m ° g = - 308.4 ± 1.8 k J ∙ m o l - 1 . Theoretical enthalpies were computed using the Gaussian G4 composite method, atomization reactions, and the weighted Boltzmann average method. For the latter, the conformational diversity of the molecular structure of the compound was considered. Using the above data and using a similar approach, the theoretical entropy of benzocaine was computed as well. The experimental and theoretical values obtained were compared and an excellent accordance was found. Using the experimental and theoretical results, Gibbs energy of formation in crystalline and gaseous states of benzocaine, at T = 298.15 K were calculated as: Δ f G m ° c r = - 164.4 k J ∙ m o l - 1 and Δ f G m ° g = - 123.9 k J ∙ m o l - 1 , respectively. Finally, the results obtained from the enthalpies of phase change are compared with those previously reported in the literature, in order to propose an exact value for these properties.