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DOI: 10.1039/A803507E (Paper) Reference Section for: J. Chem. Soc., Perkin Trans. 2, 1998, 1539-1544

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

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Jun Nishimoto, Masaaki Tabata, Tomoko Eguchi and Junpei Takauchi

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.

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