Kinetics and equilibrium in the ammonolysis of substituted phthalimides

Publisher Website
Google Scholar
Abstract
Kinetic studies are reported for the base hydrolysis to phthalamic acid anions (H) and ammonolysis to phthalamides (A) for seven phthalimides (P): 1, unsubstituted; 2, 4-NO2; 3, 4-Cl; 4, 4-t Bu; 5, 3-NO2; 6, 3-Me; 7, 3-Me3Si. The hydrolysis kinetics require two mechanisms, one which is first order in neutral imide and first order in hydroxide ion, and a second, which is important only in quite concentrated NaOH, which is first order in neutral phthalimide and second order in hydroxide ion. Ammonolysis kinetics for 1–5 revealed the rate law: Rate = kN [Unionized phthalimide] [NH3][OH−]. A mechanism is proposed with rate-determining breakdown of the anionic form of the tetrahedral intermediate derived by addition of NH3 to the phthalimide. The ammonolysis is reversible. The phthalamide hydrolyzes to the phthalamic acid via cyclization to an intermediate phthalimide, which is detected in concentrated base where its formation from phthalamide is more rapid than its subsequent hydrolysis. Rate constants for the cyclization follow the rate law: Rate = kcyc [Phthalamide][OH−]. This reaction is the microscopic reverse of the ammonolysis, and the ratio kN/kcyc provides the equilibrium constant Keq for the reaction P + NH3 = A. Values for 1–5 lie in the range 2 × 102 – 4 × 103. With 3-methylphthalimide, kinetics in aqueous ammonia do not obey a first-order relationship, but they could be analyzed by a scheme whereby the phthalimide is converted reversibly to the phthalamide and simultaneously undergoes an irreversible hydrolysis. The value of Keq in the system is 1.8. With 3-trimethylsilylphthalimide the value of Keq is further reduced to 0.01. The ammonolysis reaction does occur more quickly than hydrolysis but the equilibrium is so unfavorable that even in concentrated ammonia only a small amount of the phthalamide is ever formed.

This publication has 0 references indexed in Scilit: