Kinetic studies of the azo coupling reaction between 1-(4-hydroxy-6-methylpyrimidin-2-yl)-3-methylpyrazolin-5-one and substituted benzenediazonium salts

Autor:	Ioan Panea, Adina Ghirisan, Ioan Baldea
Rok vydání:	1992
Předmět:	chemistry.chemical_classification Diazonium Compounds Deprotonation Reaction rate constant Chemistry Electronic effect Organic chemistry Azo coupling Rate-determining step Medicinal chemistry Coupling reaction Acid dissociation constant
Zdroj:	Journal of the Chemical Society, Perkin Transactions 2. :1715
ISSN:	1364-5471 0300-9580
Popis:	Kinetic studies have been carried out on the azo coupling reactions between 1-(4-hydroxy-6-methylpyrimidin-2-yl)-3-methylpyrazolin-5-one and seven substituted benzenediazonium salts. The effects of acidity and temperature upon the rate have been investigated and activation parameters determined, and the involvement of deprotonated substrate in the rate determining step demonstrated. Acid dissociation equilibrium of the substrate is characterized by pKaHA= 5.07 ± 0.16 at 293 K and µ= 0.2. Close activation enthalpies, between 48 and 59 kJ mol–1 and negative entropies between –22 and –53 J mol–1 K–1 have been found, depending on the electronic effects of substituents on the benzene nucleus of the diazonium salts. A Hammett relationship log10k2= 1.0 ± 0.1 +(3.3 ± 0.3)σ holds for coupling reactions. The greater the electron-withdrawing effect, the faster is the coupling rate. 1-(2-Pyrimidinyl)pyrazolin-5-one behaves similarly to 1-arylpyrazolin-5-one and phenols in reactions with arenediazonium cations.
Databáze:	OpenAIRE
Externí odkaz:	https://explore.openaire.eu/search/publication?articleId=doi_________::ac781e35ebfd2e1f5ab9fb54e135386a https://doi.org/10.1039/p29920001715 Zobrazit plný text záznamu