Kinetics and Mechanism of the Cleavage of Phthalic Anhydride in Glacial Acetic Acid Solvent Containing Aniline

Apparent second-order rate constants (k_n^app) for the nucleophilic reaction of aniline (Ani) with phthalic anhydride (PAn) vary from 6.30 to 7.56 M⁻¹ s⁻¹ with the increase of temperature from 30 to 50 °C in pure glacial acetic acid (AcOH). However, the values of pseudo-first-order rate constants (k_s) for the acetolysis of PAn in pure AcOH increase from 16.5 × 10⁻⁴ to 10.7 × 10⁻³ s⁻¹ with the increase of temperature from 30 to 50 °C. The values of k_n^app and k_s vary from 5.84 to 7.56 M⁻¹ s⁻¹ and from 35.1 × 10⁻⁴ to 12.4 × 10⁻⁴ s⁻¹, respectively, with the increase of CH₃CN content from 1% to 80% v/v in mixed AcOH solvents at 35 °C. The plot of k_s versus CH₃CN content shows a minimum (with 10⁴ k_s = 4.40 s⁻¹) at 50% v/v CH₃CN. Similarly, the variations of k_n^app and k_s with the increasing content of tetrahydrofuran (THF) in mixed AcOH solvent reveal respective a maximum (with k_n^app = 17.5−15.6 M⁻¹ s⁻¹) at 40−60% v/v THF and a minimum (with k_s = ∼ 0−1.2 × 10⁻⁴ s ⁻¹) at 60−70% v/v THF. The respective values of ΔH* and Δ S * are 15.3 ± 1.2 kcal mol⁻¹ and −20.1 ± 3.8 cal K⁻¹ mol⁻¹ for k_s and 1.1 ± 0.5 kcal mol⁻¹ and −51.2 ± 1.7 cal K⁻¹ mol⁻¹ for k_n^app, while the values of k_n (= k_n^app/f_b with f_b representing the fraction of free aniline base) are almost independent of temperature within the range 30−50 °C. A spectrophotometric approach has been described to determine f_b in AcOH as well as mixed AcOH−CH₃CN and AcOH−THF solvents. Thus, the observed data, obtained under different reaction conditions, have been explained quantitatively. An optimum reaction condition, within the domain of present reaction conditions, has been suggested for the maximum yield of the desired product, N-phenylphthalamic acid.