Kinetics and mechanism of the acid–base reaction of the diacid salts of porphyrin with pyridine in chloroform

Abstract

The acid–base reaction of the diacid salts of 5,10,15,20-tetraphenylporphyrin [(H₄tpp)²⁺(X^–)₂; X^– = Cl^– and Br^–] with pyridine is so slow in chloroform that the reaction has been followed using a conventional stopped-flow apparatus. The reaction rate is first-order with respect to the concentrations of porphyrin and pyridine. The reaction rate is enhanced by water and suppressed by 1,4-dioxane. The rate equation is given by the following equation:

-d[(H₄tpp)²⁺(X^–)₂]/dt = k₁′[(H₄tpp)²⁺(X^–)₂][H₂O][py] + k₂′[(H₄tpp)²⁺(X^–)₂][H₂O]²[py] =

      k₁[(H₄tpp)²⁺(X^–)₂(H₂O)][py] + k₂[(H₄tpp)²⁺(X^–)₂(H₂O)₂][py] 

The values of k₁′ and k₂′ are (4.5 ± 0.2)  10⁴ dm⁶ mol^–2 s^–1 and (4.3 ± 0.3)  10⁵ dm⁹ mol^–3 s^–1 for Br^–, and (1.95 ± 0.07)  10⁴ dm⁶ mol^–2 s^–1 and (1.11 ± 0.14)  10⁵ dm⁹ mol^–3 s^–1 for Cl^– at 25 °C, respectively. The reaction mechanism is discussed from the viewpoint of hydration of the diacid porphyrin, pyridine and 1,4-dioxane.