The Mutarotation of Glucose in Dimethylsulfoxide and Water Mixtures

1973 ◽  
Vol 51 (4) ◽  
pp. 556-564 ◽  
Author(s):  
N. M. Ballash ◽  
E. B. Robertson
Keyword(s):  
Transition State ◽  
Activation Parameters ◽  
Solvent Composition ◽  
Water Molecules ◽  
Solvent Mixtures ◽  
Cyclic Transition ◽  
Proton Transfers ◽  
Poor Region ◽  
Cyclic Transition State ◽  
Acid Catalyzed

The mutarotation of glucose is a general acid – base-catalyzed reaction which involves two proton transfers. Whether these proton transfers are carried out in a stepwise or concerted manner, the latter alternative possibly incorporating several water molecules in a cyclic transition state, is not known with certainty.This study was undertaken in an attempt to clarify the mechanism by determining directly the number of water molecules participating in the transition state. The method chosen was to determine rates for the acid-catalyzed mutarotation of glucose as a function of solvent composition in dimethylsulfoxide and water mixtures, particularly in the water poor region. The results show that for the acid-catalyzed reaction the order with respect to water is zero. For water catalysis, the results are less reliable but suggest that about three water molecules may be involved. These facts are interpreted to mean that a stepwise mechanism is operative in the proton-catalyzed reaction, but that the solvent-catalyzed process may involve a cyclic concerted mechanism.Rate measurements were also carried out as a function of temperature for various solvent mixtures. The calculated activation parameters remain practically constant over the solvent composition region and attest to the constancy of the mechanism over this range. Thus the results which pertain strictly to solutions of high dimethylsulfoxide content should apply to aqueous solutions as well.

Cyclic transition state in the acid catalyzed intramolecular allylstannane-aldehyde condensation

1993 ◽  
Vol 34 (8) ◽  
pp. 1313-1316 ◽  
Author(s):  
Vladimir Gevorgyan ◽  
Isao Kadota ◽  
Yoshinori Yamamoto
Keyword(s):  
Transition State ◽  
Cyclic Transition ◽  
Cyclic Transition State ◽  
Acid Catalyzed

Kinetic Studies of [4n+2]π-Thermal Cyclodimerization of 1-(3-Pyridazinyl)-3-oxidopyridinium Betaines

1992 ◽  
Vol 57 (9) ◽  
pp. 1951-1959 ◽  
Author(s):  
Madlene L. Iskander ◽  
Samia A. El-Abbady ◽  
Alyaa A. Shalaby ◽  
Ahmed H. Moustafa
Keyword(s):  
Energy Gap ◽  
Activation Parameters ◽  
Kinetic Studies ◽  
Cyclic Transition ◽  
Concerted Mechanism ◽  
First Order ◽  
Thermodynamic Activation Parameters ◽  
Cyclic Transition State ◽  
Complete Dissociation ◽  
Homo Lumo

The reactivity of the base induced cyclodimerization of 1-(6-arylpyridazin-3-yl)-3-oxidopyridinium chlorides in a pericyclic process have been investigated kinetically at λ 380 nm. The reaction was found to be second order with respect to the liberated betaine and zero order with respect to the base. On the other hand dedimerization (monomer formation) was found to be first order. It was shown that dimerization is favoured at low temperature, whereas dedimerization process is favoured at relatively high temperature (ca 70 °C). Solvent effects on the reaction rate have been found to follow the order ethanol > chloroform ≈ 1,2-dichloroethane. Complete dissociation was accomplished only in 1,2-dichloroethane at ca 70 °C. The thermodynamic activation parameters have been calculated by a standard method. Thus, ∆G# has been found to be independent on substituents and solvents. The high negative values of ∆S# supports the cyclic transition state which is in favour with the concerted mechanism. MO calculations using SCF-PPP approximation method indicated low HOMO-LUMO energy gap of the investigated betaines.

Solvolysis of isopropyl bromide in ethanol–water mixtures. Dissection into initial state and transition state contributions

1976 ◽  
Vol 54 (24) ◽  
pp. 3944-3948 ◽  
Author(s):  
Wiendelt Drenth ◽  
Michael Cocivera
Keyword(s):  
Vapor Pressure ◽  
Transition State ◽  
Thermodynamic Parameters ◽  
Activation Parameters ◽  
Solvent Composition ◽  
Initial State ◽  
Partial Vapor Pressure ◽  
State Variation

Rates were determined for the solvolysis of isopropyl bromide in ethanol–water mixtures (20 to 80% by volume of ethanol) at 50 and 75 °C and the corresponding activation parameters calculated. From the partial vapor pressure of isopropyl bromide over the various solutions at 50 and 75 °C, the variations in its initial state thermodynamic parameters were calculated. Thus, the variation in the activation parameters with solvent composition could be analyzed in terms of initial and transition state contributions. The initial state variation dominates according to a unimolecular as well as to a bimolecular treatment of data.

Divergent Reactivity of Stannane and Silane in the Trifluoromethylation of PdII: Cyclic Transition State versus Difluorocarbene Release

2018 ◽  
Vol 130 (46) ◽  
pp. 15301-15305 ◽  
Author(s):  
Maoping Pu ◽  
Italo A. Sanhueza ◽  
Erdem Senol ◽  
Franziska Schoenebeck
Keyword(s):  
Transition State ◽  
Cyclic Transition ◽  
Cyclic Transition State

Photoreaction of Nitroso Compounds in Solution. Part XXXI. Mechanisms of Intramolecular Acid Catalyzed Cleavage Reactions and a Hydride Initiated Rearrangement of C-Nitroso Alkanes

1975 ◽  
Vol 53 (20) ◽  
pp. 3014-3021 ◽  
Author(s):  
K. Somosekharen Pillay ◽  
Shi C. Chen ◽  
Thomas Mojelsky ◽  
Yuan L. Chow
Keyword(s):  
Trans Isomer ◽  
Intramolecular Proton Transfer ◽  
Nitroso Compounds ◽  
Cyclic Transition ◽  
Compound A ◽  
Ring Enlargement ◽  
Hydride Reduction ◽  
Cyclic Transition State ◽  
Acid Catalyzed ◽  
Cleavage Reactions

Two novel rearrangements of nitroso alkanes, one intramolecular acid catalyzed cleavage reaction and the other ring enlargement during hydride reduction, are described. Aminium radical initiated addition to norbornene under nitrogen gave cis-exo- and trans-2-nitroso-3-ammonium norbornanes in a good yield. Whereas the trans isomer underwent the usual nitroso group reactions, i.e., tautomerization, dimerization, and HNO addition, the cis-exo isomer and its hydroxy analog cleaved extremely readily to give 1,3-bisformylcyclopentane derivatives. The stereoelectronic driving force in this cleavage is interpreted as the intramolecular proton transfer and the electron reorganization via a cyclic transition state. Metal hydride reduction of the trans isomer was accompanied by ejection of the neighboring amino group and an expansion of the ring to form an azabicyclic compound. A sequence involving an intramolecular displacement to form aziridine followed by the reductive cleavage is proposed.

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