The 4,4'-dimethoxytrityl carbenium ion by ionization of 4,4'-dimethoxytrityl alcohol in acetonitrile - aqueous perchloric and nitric acids containing electrolytes: kinetics, equilibria, and ion-pair formation

1999 ◽  
Vol 77 (5-6) ◽  
pp. 530-536 ◽  
Author(s):  
Juan Crugeiras ◽  
Howard Maskill
Keyword(s):  
Rate Constant ◽  
Perchloric Acid ◽  
Reaction Rate ◽  
Reaction Rate Constant ◽  
Ion Pair ◽  
Ion Concentration ◽  
Forward Rate ◽  
First Order ◽  
Hydronium Ion ◽  
Carbenium Ion

We have studied the equilibration shown in eq. [3] of 4,4prime-dimethoxytrityl alcohol in aqueous perchloric and nitric acids containing low proportions of acetonitrile using stopped-flow kinetics techniques. The rate constants for the overall progress to equilibrium, kobs, have been resolved into forward and reverse components using the equilibrium UV absorbance and a value for the molar absorptivity of the 4,4prime-dimethoxytrityl carbenium ion determined in concentrated aqueous perchloric acid. The forward reaction (rate constant kf) is first order in both the alcohol and the acid concentrations; the reverse reaction (rate constant kr) is pseudo first order with respect to the carbocation. At constant hydronium ion concentration, the forward rate constant increases linearly with the concentration of electrolyte, whereas the reverse rate constant decreases. These effects depend upon the nature of the anion, but not the cation, and are not ionic strength effects. At constant anion concentrations, kf in both acids, and kr in perchloric acid, are independent of hydronium ion concentration; however, kr decreases with increasing hydronium ion concentration at constant nitrate concentration. At nonconstant ionic strength, changes in kf and kr observed in increasing concentrations of perchloric acid are attributable wholly to changes in perchlorate concentration. A mechanism is proposed which involves pre-equilibrium protonation of the alcohol, heterolysis of the protonated alcohol to give a 4,4prime-dimethoxytrityl carbenium ion - water ion-molecule pair, then conversion of this into a dissociated carbenium ion in equilibrium with ion pairs. To account for the strong effects of perchlorate and nitrate upon the forward rate constants, it is proposed that these anions provide additional reaction channels from the ion-molecule pair. However, we find no evidence of acid catalysis in the reaction of the ion-molecule pair (in contrast to our finding for the reaction of the corresponding ion-molecule pair formed from dimethoxytritylamine in acidic media). Some of the elementary rate and equilibrium constants of the proposed mechanism have been evaluated.Key words: trityl, carbenium ion, stopped-flow, ion pair, ion-molecule pair.

scholarly journals Determination of exothermic batch reactor specific model parameters

2019 ◽  
Vol 292 ◽  
pp. 01063
Author(s):  
Lubomír Macků
Keyword(s):  
Rate Constant ◽  
Reaction Rate ◽  
Batch Reactor ◽  
Specific Model ◽  
Reaction Rate Constant ◽  
Order Reaction ◽  
First Order Reaction ◽  
Model Parameters ◽  
Reactor Model ◽  
First Order

An alternative method of determining exothermic reactor model parameters which include first order reaction rate constant is described in this paper. The method is based on known in reactor temperature development and is suitable for processes with changing quality of input substances. This method allows us to evaluate the reaction substances composition change and is also capable of the reaction rate constant (parameters of the Arrhenius equation) determination. Method can be used in exothermic batch or semi- batch reactors running processes based on the first order reaction. An example of such process is given here and the problem is shown on its mathematical model with the help of simulations.

Bisulfite Degrades Aflatoxin: Effect of Temperature and Concentration of Bisulfite

1978 ◽  
Vol 41 (10) ◽  
pp. 774-780 ◽  
Author(s):  
M. P. DOYLE ◽  
E. H. MARTH
Keyword(s):  
Rate Constant ◽  
Aflatoxin B1 ◽  
Reaction Rate ◽  
Reaction Rates ◽  
Reaction Rate Constant ◽  
Effect Of Temperature ◽  
Kcal Mole ◽  
First Order ◽  
Potassium Acid Phthalate ◽  
Acid Phthalate

Bisulfite reacted with aflatoxin B1 and G1 resulting in their loss of fluorescence. The reaction was first order with rate depending on bisulfite (or the bisulfite and sulfite) concentration(s). Aflatoxin G1 reacted more rapidly with bisulfite than did aflatoxin B1. In the presence of 0.035 M potassium acid phthalate-NaOH buffer (pH 5.5) plus 1.3% (vol/vol) methanol at 25 C, the reaction rate constant for degradation of aflatoxin G1 was 2.23 × 10−2h− and that for aflatoxin B1 was 1.87 × 10−2h− when 50 ml of reaction mixture contained 1.60 g of K2SO3. Besides bisulfite concentrations, temperature influenced reaction rates. The Q10 for the bisulfite-aflatoxin reaction was approximately 2 while activation energies for degrading aflatoxin B1 and aflatoxin G1 were 13.1 and 12.6 kcal/mole, respectively. Data suggest that treating foods with 50 to 500 ppm SO2 probably would not effectively degrade appreciable amounts of aflatoxin. Treating foods with 2000 ppm SO2 or more and increasing the temperature might reduce aflatoxin to an acceptable level.

Pyrolysis Modeling in a Woodstove

2008 ◽  
Author(s):  
Rajesh Gupta
Keyword(s):  
Rate Constant ◽  
Empirical Model ◽  
Reaction Rate ◽  
Packed Bed ◽  
Reaction Rate Constant ◽  
Order Reaction ◽  
Pseudo First Order Reaction ◽  
First Order ◽  
Pseudo First Order ◽  
Simple Empirical Model

A simple empirical model for predicting the pyrolysis rate of fuel packed bed of a woodstove has been presented. The thermolytic behavior of the fuel bed has been approximated by a pseudo-first order reaction. The reaction rate constant has been determined as function of temperature. The effect of orientation of twigs in the fuel bed arrangement and twig diameter on the reaction rate constant has been analyzed. It has been concluded that the effect of twig orientation is insignificant while the peak magnitude of reaction rate constant increased with increasing twig diameter.

Influence of Reduction Agent of SSI on its Cadmium Removal from Wastewater

2012 ◽  
Vol 251 ◽  
pp. 406-410
Author(s):  
Jun Guo Li ◽  
Yan Shi ◽  
Na Bi
Keyword(s):  
Rate Constant ◽  
Reaction Rate ◽  
Reaction Order ◽  
Direct Reduction ◽  
Reaction Rate Constant ◽  
Cadmium Removal ◽  
Pseudo First Order Reaction ◽  
First Order ◽  
Apparent Reaction Rate Constant ◽  
Apparent Reaction Rate

Spherical sponge iron (SSI) with high activity and intension could be prepared through direct reduction by charcoal or hydrogen. The capability of cadmium removal by SSI was investigated in. It was suggested that the reaction of SSI reduced by hydrogen was higher than that reduced by charcoal, and the increasing rate of pH and cadmium removal in solution by SSI reduced by hydrogen was higher than that reduced by charcoal. Moreover, cadmium removal percentage by SSI reduced with hydrogen was much higher than that reduced by charcoal. When the original concentration of cadmium was 50mg/L, cadmium removal by SSI appeared to be the pseudo-first-order reaction because the reaction order was from 0.861 to 0.984. The apparent reaction rate constant of cadmium removal by SSI reduced with charcoal was 0.586 h-1. While hydrogen was utilized as reduction agent, the apparent reaction rate constant of cadmium removal was increased by 7.3 and 13.7 times.

A pressure- and flow-insensitive reference electrode liquid junction

1976 ◽  
Vol 41 (1) ◽  
pp. 125-128 ◽  
Author(s):  
E. D. Crandall ◽  
J. DeLong
Keyword(s):  
Response Time ◽  
Rate Constant ◽  
Reaction Rate ◽  
Reference Electrode ◽  
Reaction Rate Constant ◽  
Ion Concentration ◽  
Measuring System ◽  
Dissociation Reaction ◽  
Liquid Junction ◽  
Measuring Systems

The design and construction of a pressure- and flow-insensitive reference liquid junction for use in ion concentration electrode measuring systems is described. The junction is inexpensive, is very easily and rapidly constructed, is rugged, and is adaptable to various applications. When used in apH-measuring system, drift, pressure artifacts, and flow artifacts are negligible. The response time of the system appears to be less than 10 ms. Using the pH electrode device as described, the dissociation reaction rate constant of H2CO3 at 24 degrees C was determined to be 22 s-1.

Effective reaction rate on a heterogeneous surface

2017 ◽  
Vol 830 ◽  
pp. 350-368 ◽  
Author(s):  
Ashok S. Sangani
Keyword(s):  
Boundary Condition ◽  
Integral Equation ◽  
Rate Constant ◽  
Current Density ◽  
Reaction Rate ◽  
Effective Rate Constant ◽  
Effective Rate ◽  
Reaction Rate Constant ◽  
Heterogeneous Surface ◽  
First Order

We examine the problem of prescribing the macroscale boundary condition to the solute convective–diffusive mass transport equation at a heterogeneous surface consisting of reactive circular disks distributed uniformly on a non-reactive surface. The reaction rate at the disks is characterized by a first-order kinetics. This problem was examined by Shah & Shaqfeh (J. Fluid Mech., vol. 782, 2015, pp. 260–299) who obtained the boundary condition in terms of an effective first-order rate constant, which they determined as a function of the Péclet number $Pe=\dot{\unicode[STIX]{x1D6FE}}a^{2}/D$, the fraction $\unicode[STIX]{x1D719}$ of the surface area occupied by the reactive disks and the non-dimensional reaction rate constant $K=ka/D$. Here, $a$ is the radius of the disks, $D$ is the solute diffusivity, $\dot{\unicode[STIX]{x1D6FE}}$ is the wall shear rate and $k$ is the first-order surface-reaction rate constant. Their analysis assumed that $Pe$ and $K$ are $O(1)$ while the ratio of the microscale $a$ to the macroscale $H$ is small. The macroscale transport process is convection–diffusion dominated under these conditions. We examine here the case when the non-dimensional numbers based on the macroscale $H$ are $O(1)$. In this limit the microscale transport problem is reaction rate dominated. We find that the boundary condition can be expressed in terms of an effective rate constant only up to $O(\unicode[STIX]{x1D716})$, where $\unicode[STIX]{x1D716}=a/H$. Higher-order expressions for the mass flux involve both the macroscopic concentration and its surface gradient. The $O(\unicode[STIX]{x1D716})$ microscale problem is relatively easy to solve as the convective effects are unimportant and it is possible to obtain analytical expressions for the effective rate constant as a function of $\unicode[STIX]{x1D719}$ for both periodic and random arrangement of the disks without having to solve the boundary integral equation as was done by Shah and Shaqfeh. The results thus obtained are shown to be in good agreement with those obtained numerically by Shah and Shaqfeh for $Pe=0$. In a separate study, Shah et al. (J. Fluid Mech., vol. 811, 2017, pp. 372–399) examined the inverse-geometry problem in which the disks are inert and the rest of the surface surrounding them is reactive. We show that the two problems are related when $Pe=0$ and $kH/D=O(1)$. Finally, a related problem of determining the current density at a surface consisting of an array of microelectrodes is also examined and the analytical results obtained for the current density are found to agree well with the computed values obtained by solving the integral equation numerically by Lucas et al. (SIAM J. Appl. Maths, vol. 57(6), 1997, pp. 1615–1638) over a wide range of parameters characterizing this problem.

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