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

Apparent second-order rate constants (knapp) 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 (ks) 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 knapp and ks 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 ks versus CH3CN content shows a minimum (with 104 ks = 4.40 s−1) at 50% v/v CH3CN. Similarly, the variations of knapp and ks with the increasing content of tetrahydrofuran (THF) in mixed AcOH solvent reveal respective a maximum (with knapp = 17.5−15.6 M−1 s−1) at 40−60% v/v THF and a minimum (with ks = ∼ 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 ks and 1.1 ± 0.5 kcal mol−1 and −51.2 ± 1.7 cal K−1 mol−1 for knapp, while the values of kn (= knapp/fb with fb 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 fb 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.