84567-61-3Relevant articles and documents
Direct Spectrophotometric Observation of an O-Acylisourea Intermediate: Concerted General Acid Catalysis in the Reaction of Acetate Ion with a Water-soluble Carbodi-imide
Ibrahim, Ibrahim T.,Williams, Andrew
, p. 1459 - 1466 (2007/10/02)
The rate constants for formation and decay of O-acylureas from carbodi-imide and acids have been measured using aqueous media.The O-acetylisourea from acetate and N-ethyl-N'-(3-trimethylammoniopropyl)carbodi-imide (ETC) possesses an acidic group of pK 6.8 and decomposes (k2) in its acid form as the dication by reaction with acetate ion or water.The reaction of the carboxylate anion with ETC is general acid catalysed (k1 = ΣkHA-> and the deuterium oxide solvent isotope effect indicates a rate-limiting proton transfer except for the oxonium ion acting as acid.The Broensted α value for variation of the structure of HA (0.67) is consistent with a proton transfer concerted with nucleophilic attack by the acetate anion.A concerted mechanism is consistent with the weak basicity of the carbodi-imide and the weak acidity of the isourea adduct.The third-order term involving acetic acid, acetate ion, and carbodi-imide carries ca. 60 percent of the total reaction flux at pH 6.80 and 1 M total acetic acid buffer concentration.At this pH ca. 40 percent of the reaction flux goes through the stepwise 'Khorana' mechanism with specific acid catalysis.Intramolecular general acid catalysis is demonstrated to occur in the reaction of 2,2-diethylmalonic acid monoanion with ETC and the effective molarity compared with intermolecular catalysis is 15 M.Attack of carboxylate anions on ETC with N-chloroethylmorpholinium ion as the general acid has a Broensted type βN of 0.46.