Use of metal ion catalysis in detection and determination of microamounts of complexing agents. Autoxidation of L-ascorbic acid as an "indicator" reaction

1968 ◽  
Vol 40 (8) ◽  
pp. 1263-1266 ◽  
Author(s):  
Horacio A. Mottola ◽  
Martha S. Haro ◽  
Henry. Freiser
Keyword(s):  
Ascorbic Acid ◽  
Metal Ion ◽  
Complexing Agents ◽  
Indicator Reaction ◽  
Metal Ion Catalysis

Metal ion catalysis in nucleophilic displacement reactions at carbon, phosphorus, and sulfur centers. I. Catalysis by metal ions in the reaction of p-nitrophenyl diphenylphosphinate with ethoxide

1989 ◽  
Vol 67 (9) ◽  
pp. 1440-1448 ◽  
Author(s):  
Edward J. Dunn ◽  
Erwin Buncel
Keyword(s):  
Crown Ether ◽  
Alkali Metal ◽  
Transition State ◽  
Metal Ions ◽  
Ground State ◽  
Metal Ion ◽  
Complexing Agents ◽  
Transition State Stabilization ◽  
Nucleophilic Displacement ◽  
Metal Ion Catalysis

The effect of macrocyclic crown ether and cryptand complexing agents on the rate of the nucleophilic displacement reaction of p-nitrophenyl diphenylphosphinate by alkali metal ethoxides in ethanol at 25 °C has been studied by spectrophotometric techniques. For the reactions of potassium ethoxide, sodium ethoxide, and lithium ethoxide, the observed rate constant increased in the order KOEt < NaOEt < LiOEt. Crown ether and cryptand cation-complexing agents have a retarding effect on the rate. Increasing the ratio of complexing agent to base results in a decrease in kobs to a minimum value corresponding to the rate of reaction of free ethoxide ion. In complementary experiments, alkali metal ions were added to these reaction systems in the form of unreactive salts, causing an increase in reaction rate. The kinetic data were analysed in terms of ion-pairing treatments, which allowed evaluation of rate coefficients due to free ethoxide ions and metal ion – ethoxide ion pairs. Possible roles of the metal cations are discussed in terms of ground state and transition state stabilization. Evaluation of the equilibrium constants for association of the metal ion with ground state (Ka) and the transition state (K′a) shows that catalysis occurs as a result of enhanced association between the metal ion and the transition state, with (K′a) values increasing in the order K+ < Na+ < Li+. A model is proposed in which transition state stabilization arises largely from chelation of the solvated metal ion to two charged oxygen centers. This appears to be the first reported instance of catalysis by alkali metal cations in nucleophilic displacement at phosphoryl centers. Keywords: nucleophilic displacement at phosphorus, alkali-metal-ion catalysis.

scholarly journals Metal ion catalysis in nucleophilic displacement reactions at carbon, phosphorus, and sulfur centers. II. Metal ion catalysis in the reaction of p-nitrophenyl diphenylphosphinate with alkali metal phenoxides in ethanol

1990 ◽  
Vol 68 (10) ◽  
pp. 1837-1845 ◽  
Author(s):  
Edward J. Dunn ◽  
Robert Y. Moir ◽  
Erwin Buncel ◽  
J. Garfield Purdon ◽  
Robert A. B. Bannard
Keyword(s):  
Alkali Metal ◽  
Transition State ◽  
Metal Ions ◽  
Metal Ion ◽  
Ion Pairs ◽  
Rate Coefficients ◽  
Complexing Agents ◽  
Specific Rate ◽  
Nucleophilic Displacement ◽  
Metal Ion Catalysis

The reactions of p-nitrophenyl diphenylphosphinate (1) with lithium, sodium, potassium, and benzyltrimethylammonium phenoxides (BTMAOPh) have been studied by spectrophotometric techniques in anhydrous ethanol at 25 °C. The reactivity (kobs) of the alkali metal phenoxides increases in the order BTMAOPh < KOPh < NaOPh < LiOPh. The rate of reaction of 1 with LiOPh is enhanced when lithium salts (LiSCN, LiNO3, LiClO4, LiOAc) are added to the reaction media. The addition of the alkali metal complexing agents dicyclohexyl-18-crown-6 ether or [2.2.2]cryptand for Na+, and [2.1.1]cryptand for Li+, to each of the alkali metal phenoxide reactions resulted in a decrease in rate, indicating catalysis by the alkali metal ions. The kinetic data are analyzed to obtain specific rate coefficients of reactions of phenoxide and ethoxide as the dissociated ions and as alkali metal – phenoxide ion pairs. Reactivities follow the order [Formula: see text]; [Formula: see text]; [Formula: see text]; [Formula: see text]. A mechanism is proposed in which the ion-paired metal phenoxide is more reactive towards the substrate than is the dissociated phenoxide. Analysis of the data in terms of initial state and transition state interactions with metal ions indicates that the increased reactivity of the ion-paired species results from greater stabilization of the negatively charged transition state relative to stabilization of the ion-paired nucleophile. Keywords: nucleophilic displacement at phosphorus by phenoxide, alkali-metal-ion catalysis.

Fluorometric determination of ascorbic acid in the absence of the oxidant in juices of common citrus

Planta Medica ◽  
2010 ◽  
Vol 76 (12) ◽  
Author(s):  
A Copra-Janicijevic ◽  
E Sofic ◽  
L Klepo ◽  
A Topcagic ◽  
I Tahirovic ◽  
...  
Keyword(s):  
Ascorbic Acid ◽  
Fluorometric Determination
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