• Bulletin of the Korean Chemical Society
• /
• v.6 no.5
• /
• pp.280-284
• /
• 1985

Kinetic measurements of the 5-nitrofurfural-hydrazine reaction in various pH ranges of aquous solution were carried out by ultraviolet spectrophotometry at $25^{\circ},\;35^{\circ}\;and\;45^{\circ}C$. The observed rate of the reaction varies with the change in pH, which gives characteristic "bell-type" rate acidity profile. The maximum rate is shown in the vicinity of pH 4. This reaction procceds with rate-determining attack of hydrazine on the 5-nitrofurfural at low pH and undergoes a change in rate-determining step to dehydration of the addition intermediate as pH increases. The reaction has a "reactant-like transition state" which precedes intermediate in low pH and "product-like transition state" which follows it in neutral pH. The geometry of 5-nitrofurfural-hydrazine intermediate was estimated with PCILO method associated with CNDO/2 scheme.

• Bulletin of the Korean Chemical Society
• /
• v.31 no.3
• /
• pp.689-693
• /
• 2010

Second-order rate constants ($k_{CN^-}$) have been measured for nucleophilic substitution reactions of Y-substituted phenyl benzoates (1a-r) with $CN^-$ ion in 80 mol % $H_2O$/20 mol % DMSO at $25.0{\pm}0.1^{\circ}C$. The Br${\o}$nsted-type plot is linear with ${\beta}_{1g}$ = -0.49, a typical ${\beta}_{1g}$ value for reactions reported to proceed through a concerted mechanism. Hammett plots correlated with ${\sigma}^{\circ}$ and ${\sigma}^-$ constants exhibit many scattered points. In contrast, the Yukawa-Tsuno plot for the same reaction exhibits excellent linearity with ${\rho}_Y$ = 1.37 and r = 0.34, indicating that a negative charge develops partially on the oxygen atom of the leaving aryloxide in the rate-determining step (RDS). Although two different mechanisms are plausible (i.e., a concerted mechanism and a stepwise pathway in which expulsion of the leaving group occurs at the RDS), the reaction has been concluded to proceed through a concerted mechanism on the basis of the magnitude of ${\beta}_{1g}$ and ${\rho}_Y$ values.

• Bulletin of the Korean Chemical Society
• /
• v.19 no.7
• /
• pp.776-779
• /
• 1998

Second-order rate constants have been measured spectrophotometrically for the addition reaction of a series of alicyclic amines to 3-butyn-2-one to yield their respective enamines at 25.0 'C. The reactivity of the amines increases with increasing the basicity of the amines. However, the Bronsted-type plot obtained exhibits a downward curvature as the basicity of the amines increases, i.e. βnuc decreases from 0.3 for low basic amines (pKa < 9) and to 0.1 for highly basic amines (pKa > 9). Such a curvature in the Bronsted-type plot is clearly indicative of a change in the reaction mechanism or transition state structure. From the corresponding reactions run in D2O, the magnitude of kinetic isotope effect (KIE) has been calculated to be about 0.8 for highly basic amines and 1.21 for weakly basic amines. The difference in the magnitude of KIE also supports a change in the reaction mechanism or transition state structure upon changing the basicity of the amines. Furthermore, the small KIE clearly suggests that H+ transfer is not involved in the rate-determining step, i.e. the addition reaction is considered to proceed via a stepwise mechanism in which the attack of the amines to the acetylene is the rate-determining step. The curvature in the Bronsted-type plot has been attributed to a change in the degree of bond formation between the amine and the acetylene.

• Bulletin of the Korean Chemical Society
• /
• v.24 no.9
• /
• pp.1245-1250
• /
• 2003

Pseudo-first-order rate constants ($k_{obs}$) have been measured spectrophotometrically for the reactions of 4-nitrophenyl benzoate with a series of pyridines in $H_2O$ containing 20 mol % DMSO. The plots of kobs vs pyridine concentration are linear up to ca. 0.1 M pyridines, indicating that the effect of self-association of pyridines with their conjugate acids are insignificant in this concentration range. The Bronsted-type plot has been obtained to be linear with a ${\beta}_{nuc}$ value 1.11, suggesting that the pyridinolyses proceed through a rate-determining breakdown of the zwiterionic addition intermediate. The pyridines studied have shown higher reactivity than isobasic primary and secondary amines toward the substrate. 1-Benzoyl-4-dimethylaminopyridinium ion, a possible intermediate, has not been detected since the rate of its hydrolysis in the reaction condition is comparable with or even faster than its formation.

• Bulletin of the Korean Chemical Society
• /
• v.18 no.8
• /
• pp.880-886
• /
• 1997

Ab initio calculations were carried out on the gas phase acid-catalyzed hydrolysis reactions of sulfinamide using the 3-21G* basis sets. Single point calculations were also performed at the MP2/6-31G* level. The first step in the acid-catalyzed hydrolysis of N-methylmethanesulfinamide, Ⅰ, involves protonation. The most favorable form is the O-protonated one, Ⅱ, which is then transformed into a sulfurane intermediate, Ⅲ, by addition of a water molecule. The reaction proceeds further by an intramolecular proton transfer from O to N (TS2), which is followed by N-S bond cleavage (TS3) leading to the final products. The rate determining step is the N-S bond cleavage (TS3) at the RHF/3-21G* level, whereas it becomes indeterminable at the MP2/6-31G*//3-21G* level of theory. However, the substituent effect studies with N-protonated N-arylmethanesulfinamide, ⅩⅢ, at the MP2/6-31G*//3-21G* level support the N-S bond breaking step as rate limiting.

• Bulletin of the Korean Chemical Society
• /
• v.29 no.12
• /
• pp.2477-2481
• /
• 2008

• Bulletin of the Korean Chemical Society
• /
• v.32 no.4
• /
• pp.1268-1272
• /
• 2011

The specific rates of solvolysis of phenyl fluorothionoformate (PhOCSF, 1) have been determined in 22 pure and binary solvents at $10.0^{\circ}C$. The extended Grunwald-Winstein equation has been applied to the specific rates of solvolysis of 1 over the full range of solvents. The sensitivities (l = $1.32{\pm}0.13$ and m = $0.39{\pm}0.08$) toward the changes in solvent nucleophilicity and solvent ionizing power, and the $k_F/k_{Cl}$ values are similar to those previously observed for solvolyses of acyl haloformate esters, consistent with the addition step of an additionelimination pathway being rate-determining. The large negative values for the entropies of activation are consistent with the bimolecular nature of the proposed rate-determining step. The results are compared with those reported earlier for phenyl chloroformate and chlorothionoformate esters and mechanistic conclusions are drawn.

• Journal of the Korean Ceramic Society
• /
• v.36 no.9
• /
• pp.982-990
• /
• 1999

YSZ/LSM composite cathode was fabricated by dip-coating of YSZ sol on the internal pore surface of a LSM cathode followed by sintering at low temperature (800-100$0^{\circ}C$) The YSZ coating significantly increased the TPB(Triple Phase Boundary) where the gas the electrode and the electrolyte were in contact with each other. Sinter the formation of resistive materials such as La2Zr2O7 or SrZrO3 was prevented due to the low processing temperature and TPB was increased due to the YSZ film coating the electrode resistance (Rel) was reduced about 100 times compared to non-modified cathode. From the analysis of a.c impedance it was shown that microstructural change of the cathode caused by YSZ film coating affected the oxygen reduction reaction. In the case of non-modified cathode the RDS (rate determining step) was electrode reactions rather than mass transfer or the oxygen gas diffusion in the experimental conditions employed in this study ($600^{\circ}C$-100$0^{\circ}C$ and 0,01-1 atm of Po2) for the YSZ film coated cathode however the RDS involved the oxygen diffusion through micropores of YSZ film at high temperature of 950-100$0^{\circ}C$ and low oxygen partial pressure of 0.01-0.03 atm.

• Bulletin of the Korean Chemical Society
• /
• v.29 no.4
• /
• pp.767-771
• /
• 2008

Second-order rate constants have been measured spectrophotometrically for the Michael-type reaction of 1-(Xsubstituted phenyl)-2-propyn-1-ones (2a-f) with amines in $H_2O$ at 25.0 ${\pm}$ 0.1 ${^{\circ}C}$. A linear Brønsted-type plot is obtained with ${\beta}_{nuc}$ = 0.25 ${\pm}$ 0.02, a typical $\beta_{nuc}$ value for reactions which proceed through a stepwise mechanism with attack of amine on the electrophilic center being the rate-determining step. Secondary alicyclic amines are found to be more reactive than isobasic primary amines. The Hammett plot for the reactions of 2a-f with morpholine is not linear, i.e., the substrate with a strong electron-donating group (e.g., 4-MeO) exhibits a negative deviation from the Hammett plot. However, the Yukawa-Tsuno plot for the same reactions exhibits an excellent linear correlation with ρ = 0.62 and r = 0.82. Thus, it has been proposed that the nonlinear Hammett plot is not due to a change in the ra te-determining step but due to ground-state stabilization through resonance interactions.

• Bulletin of the Korean Chemical Society
• /
• v.28 no.4
• /
• pp.657-661
• /
• 2007

The specific rates of solvolysis of isopropyl fluoroformate are well correlated using the extended Grunwald-Winstein equation, with a sensitivity (l ) to changes in solvent nucleophilicity (NT) and a sensitivity (m) to changes in solvent ionizing power (YCl). The sensitivities (l = 1.59 ± 0.16 and m = 0.80 ± 0.06) toward changes in solvent nucleophilicity and solvent ionizing power, and the kF/kCl values are very similar to those for solvolyses of n-octyl fluoroformate, suggesting that the addition step of an addition-elimination mechanism is rate-determining. For methanolysis, a solvent deuterium isotope effect of 2.53 is compatible with the incorporation of general-base catalysis into the substitution process. The large negative values for the entropies of activation are consistent with the bimolecular nature of the proposed rate-determining step. These observations are also compared with those previously reported for the corresponding chloroformate and fluoroformate esters.