Kinetics and mechanism of covalent addition of S(IV) species to the acridinium cation

Abstract

Abstract:The reaction of acridine with S(1V) species (SO,.H,O, HSO;, and SO,' -) to form the adduct acridine-S (IV) has been studied spectrophotometrically throughout the pH range 2.6-8 in aqueous solutions. The observed pseudo-first-order rate constants, k,,,, were determined at 25°C and ionic strength I = 0.11 M, and the pH profile of the rate reached a maximum at pH = 6.1. At constant pH the kobs values were a linear function of the total S(1V) concentration with slopes that increased significantly with pH. These data are consistent with the rate-determining attack of S0,H-and SO,' -upon the C-9 of the acridinium cation. A nonlinear least-squares fitting of the experimental values to the model equation, within the overall pH region studied, yields the pH-independent rate constants k, = 3.7 2 0.1 and k, = (6.24 C 0.04) x lo4 M-' s-' for the attack of these two species, respectively. The experimental results agree very well with the kinetic model. Due to the experimental conditions used we did not detect any possible pseudobase formation in the pH range studied. The reactivity of the S(IV) species with acridine follows the order: SO: ->> HSO; >> S02,H,0. The value obtained for the ratio k,lk, is similar to the results given for other addition reactions of S(1V) species to the double bond of carbonyl compounds such as benzaldehyde and formaldehyde.Key words: covalent addition, acridine, acridine -S(1V) adducts, kinetics and mechanism.RCsumC : L'acridine rCagit avec le S(1V) sous la forme de SO,.HZO, HS0,-et ~0 , ' -~o u r conduire j. des adduits que l'on a CtudiC d'une faqon spectrophotomCtrique j. tous les pH allant de 2,6 j. 8, en solutions aqueuses. On a dtterminC les constantes de vitesse du pseudopremier ordre, kobs, j. 25°C et j. une force ionique I = 0,I I M; les profiles des vitesses en fonction du pH atteignent un maximum j. pH = 6,l. A des pH constants, les valeurs de k,,, sont des fonctions lineaires de la concentration totale en S(1V) avec des pentes qui augmentent d'une f a~o n importante avec le pH. Ces donnees sont en accord avec une attaque qui dtterminerait la vitesse de la reaction du HS0,-et du SO:-en position C-9 du cation acridinium. Un ajustement non linCaire par la mCthode des moindres carrCs des valeurs experimentales obtenues dans la region globale de pH CtudiCs avec 1'Cquation modkle permet de tirer des constantes de vitesses independantes du pH, k, = 3,7 t 0,l et k? = (6,24 2 0.04) x 10' M-' s-I pour les attaques respectives de ces deux espkces. Les resultats experimentaux sont en bon accord avec le modkle cinktique. A cause des conditions experimentales utilisees, on n'a pas pu dCtecter la formation de pseudobases dans la plage de pH CtudiCs. La rCactivit6 des espkces S(1V) avec I'acridine est dans l'ordre suivant : SO,' ->> HSO; >> SO,.H,O. La valeur obtenue pour le rapport k,lk, est semblable j. celle tiree de resultats rapportes pour d'autres reactions d'addition d'espkces S(1V) 21 des doubles liaisons de composts carbonyles, tels que le benzaldkhyde et le formaldehyde.

Electrochemiluminescence Determination of 2‘,6‘-Difluorophenyl 10-Methylacridan-9-carboxylate

“…The CL quantum yield is typically between 1 and 10% . Nucleophilic attack of peroxide anions on acridinium esters is extremely rapid, but in the absence of peroxide, other nucleophiles can form an adduct with the 9-position of the acridinium nucleus . When hydroxide is the addend, the product is known as a pseudobase or carbinol (Scheme ). , Although this reacts with hydrogen peroxide to give N -methylacridone, structural constraints preclude the formation of a dioxetanone intermediate, and therefore, no light is emitted.…”

Studies on toluidine blue reaction with sulfite in aqueous solution and role of Cu(II) as promoter

“…Literature shows contradicting reports on the reducing efficiency of S(IV) as a function pH. Increased efficiency at low pH was reported with HSO 3 Ϫ as reactive species [5,6] and increasing pH was reported as optimum reducing condition with SO 3 2Ϫ as the reactive species [7,8]. Thus, understanding the solution chemistry of sulfite and simple procedures for monitoring the SO 2 levels are of environmental and applied interest.…”

Chemiluminescence of Electrochemically Oxidized 9‐(Phenylthiophosphoryloxymethylidene)‐10‐methylacridan Derivatives

“…However, these compounds are subject to nucleophilic attack leading to pseudobase formation and thus inactivation due to non-luminescent carboxylic acid formation with prolonged storage in aqueous solutions and especially at an alkaline pH. Nucleophillic attack of hydroperoxide anions on acridinium esters is extremely rapid, but in the absence of peroxide other nucleophiles can form an adduct with the acridinium nucleus [7,8]. When hydroxide is the addend the product is known as a pseudobase or carbinol, which does not favor CL emission [2,7,8].…”

“…Nucleophillic attack of hydroperoxide anions on acridinium esters is extremely rapid, but in the absence of peroxide other nucleophiles can form an adduct with the acridinium nucleus [7,8]. When hydroxide is the addend the product is known as a pseudobase or carbinol, which does not favor CL emission [2,7,8]. It is thus advantageous to form acridinium esters in situ at the time of experiment to avoid decomposition and/or loss of activity.…”