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엄익환,Kalsoom Akhtar 대한화학회 2008 Bulletin of the Korean Chemical Society Vol.29 No.4
Second-order rate constants (kN) have been measured spectrophotometrically for reactions of 3,4-dintrophenyl 2-furoate (2) with a series of secondary alicyclic amines in 80 mol % H₂O/20 mol % dimethyl sulfoxide (DMSO) at 25.0 ℃. The Brnsted-type plot exhibits a downward curvature for the aminolysis of 2, which is similar to that reported for the corresponding reactions of 2,4-dintrophenyl 2-furoate (1). Substrate 2 is less reactive than 1 toward all the amines studied but the reactivity difference becomes smaller as the amine basicity increases. Dissection of the second-order rate constants into the microscopic rate constants has revealed that the reaction of 2 results in a smaller k2/k1 ratio but slightly larger k1 value than that of 1. Steric hindrance has been suggested to be responsible for the smaller k1 value found for the reactions of 1, since the ortho-substituent of 1 would inhibit the attack of amines (i.e., the k1 process).
엄익환,Kalsoom Akhtar,Youn-Min Park,Sher Bahadar Khan 대한화학회 2007 Bulletin of the Korean Chemical Society Vol.28 No.8
Second-order rate constants (kN) have been determined spectrophotometrically for reactions of Y-substituted phenyl 2-furoates (1a-h) with piperidine and morpholine in 80 mol % H2O/20 mol % DMSO at 25.0 0.1oC. The Brnsted-type plot exhibits a downward curvature for the reactions with strongly basic piperidine but is linear for the reactions with weakly basic morpholine. The slope of the curved Brnsted-type plot changes from 1.25 to 0.28 as the pKa of the conjugate acid of the leaving aryloxides decreases. The pKa at the center of the Brnsted curvature, defined as pKao, was determined to be 6.4. The aminolysis of 1a-h has been concluded to proceed through a stepwise mechanism on the basis of the curved Brnsted-type plot. The reactions of Y-substituted phenyl cinnamates (2a-g) with piperidine resulted in a curved Brnsted-type plot with a pKao values of 6.4. However, the curved Brnsted-type plot has been suggested to be not due to a change in the RDS but due to a normal Hammond effect of a concerted mechanism, since the Brnsted-type plot for the corresponding reactions with morpholine results in also a curved Brnsted-type plot with a pKao values of 6.1. The furoates with a basic leaving group (i.e., 1b-g) are less reactive than the corresponding cinnamates (i.e., 2b-g). The k2/k1 ratios for the reactions of 1b-h are much smaller than unity, which has been suggested to be responsible for their low reactivity.
엄익환,Sun-Mee Chun,Kalsoom Akhtar 대한화학회 2007 Bulletin of the Korean Chemical Society Vol.28 No.2
Second-order rate constants (kN) have been determined spectrophotometrically for reactions of 2,4-dintrophenyl 2-furoate (2) with a series of alicyclic secondary amines in 80 mol % H2O/20 mol % dimethyl sulfoxide (DMSO) at 25.0 oC. The furoate 2 is more reactive than 2,4-dintrophenyl benzoate (1) toward all the amines studied. The higher acidity of 2-furoic acid (pKa = 3.16) compared with benzoic acid (pKa = 4.20) has been suggested to be responsible for the reactivity order, at least in part. The Brnsted-type plots for the reactions of 1 and 2 are curved downwardly, indicating that the aminolyses of both 1 and 2 proceed through a zwitterionic tetrahedral intermediate (T?) with a change in the rate-determining step on changing the amine basicity. Dissection of the kN values into their microscopic rate constants has revealed that the pKao and k2/k1 ratios for the reactions of 1 and 2 are identical, indicating that the nature of the nonleaving group (i.e., benzoyl and 2-furoyl) does not affect the reaction mechanism. The k1 values have been found to be larger for the reactions of 2 than for those of 1, which is fully responsible for the fact that the former is more reactive than the latter.
Um, Ik-Hwan,Akhtar, Kalsoom Korean Chemical Society 2008 Bulletin of the Korean Chemical Society Vol.29 No.4
Second-order rate constants ($k_N$) have been measured spectrophotometrically for reactions of 3,4-dintrophenyl 2-furoate (2) with a series of secondary alicyclic amines in 80 mol % $H_2O$/20 mol % dimethyl sulfoxide (DMSO) at 25.0 ${^{\circ}C}$. The Bronsted-type plot exhibits a downward curvature for the aminolysis of 2, which is similar to that reported for the corresponding reactions of 2,4-dintrophenyl 2-furoate (1). Substrate 2 is less reactive than 1 toward all the amines studied but the reactivity difference becomes smaller as the amine basicity increases. Dissection of the second-order rate constants into the microscopic rate constants has revealed that the reaction of 2 results in a smaller $k_2/k_{-1}$ ratio but slightly larger $k_1$ value than that of 1. Steric hindrance has been suggested to be responsible for the smaller $k_1$ value found for the reactions of 1, since the ortho-substituent of 1 would inhibit the attack of amines (i.e., the $k_1$ process).
Um, Ik-Hwan,Akhtar, Kalsoom,Park, Youn-Min,Khan, Sher Bahadar Korean Chemical Society 2007 Bulletin of the Korean Chemical Society Vol.28 No.8
Second-order rate constants (kN) have been determined spectrophotometrically for reactions of Y-substituted phenyl 2-furoates (1a-h) with piperidine and morpholine in 80 mol % H2O/20 mol % DMSO at 25.0 ± 0.1 oC. The Brønsted-type plot exhibits a downward curvature for the reactions with strongly basic piperidine but is linear for the reactions with weakly basic morpholine. The slope of the curved Brønsted-type plot changes from -1.25 to ?0.28 as the pKa of the conjugate acid of the leaving aryloxides decreases. The pKa at the center of the Brønsted curvature, defined as pKa°, was determined to be 6.4. The aminolysis of 1a-h has been concluded to proceed through a stepwise mechanism on the basis of the curved Brønsted-type plot. The reactions of Ysubstituted phenyl cinnamates (2a-g) with piperidine resulted in a curved Brønsted-type plot with a pKa° values of 6.4. However, the curved Brønsted-type plot has been suggested to be not due to a change in the RDS but due to a normal Hammond effect of a concerted mechanism, since the Brønsted-type plot for the corresponding reactions with morpholine results in also a curved Brønsted-type plot with a pKa° values of 6.1. The furoates with a basic leaving group (i.e., 1b-g) are less reactive than the corresponding cinnamates (i.e., 2b-g). The k2/ k-1 ratios for the reactions of 1b-h are much smaller than unity, which has been suggested to be responsible for their low reactivity.
Sher Bahadar Khan,한학수,서종철,장의성,Kalsoom Akhtar,김광인 한국고분자학회 2011 Macromolecular Research Vol.19 No.9
Organic-inorganic hybrids (H1-H5) based on environmentally friendly and biodegradable polymer, poly (propylene carbonate) (PPC) and tetraethoxysilane (TEOS) were synthesized using the sol-gel technique. The synthesized hybrids were characterized structurally and morphologically by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM). H1-H5 were examined in term of detailed thermal, mechanical, and anti-water sorption properties using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) nanoindentation, and thin film diffusion analysis, which revealed that H4 has the highest thermal, mechanical, and anti-water sorption properties. H4 greatly increased the thermal decomposition temperature (Td10%) and glass transition temperature, which are 45 and 11 ℃ higher than that of pure PPC. Accordingly, H4 showed a high storage modulus (2.54 × 109 Pa), elastic modulus (2.601± 0.110 GPa), hardness (0.175 ± 0.013 GPa), and lowest water absorption. This improvement in the thermal, mechanical,and anti-water absorption properties of PPC shows that PPC can be used as a packaging and bio-material.
Um, Ik-Hwan,Chun, Sun-Mee,Akhtar, Kalsoom Korean Chemical Society 2007 Bulletin of the Korean Chemical Society Vol.28 No.2
Second-order rate constants (kN) have been determined spectrophotometrically for reactions of 2,4-dintrophenyl 2-furoate (2) with a series of alicyclic secondary amines in 80 mol % H2O/20 mol % dimethyl sulfoxide (DMSO) at 25.0 oC. The furoate 2 is more reactive than 2,4-dintrophenyl benzoate (1) toward all the amines studied. The higher acidity of 2-furoic acid (pKa = 3.16) compared with benzoic acid (pKa = 4.20) has been suggested to be responsible for the reactivity order, at least in part. The Brønsted-type plots for the reactions of 1 and 2 are curved downwardly, indicating that the aminolyses of both 1 and 2 proceed through a zwitterionic tetrahedral intermediate (T±) with a change in the rate-determining step on changing the amine basicity. Dissection of the kN values into their microscopic rate constants has revealed that the pKao and k2/k-1 ratios for the reactions of 1 and 2 are identical, indicating that the nature of the nonleaving group (i.e., benzoyl and 2-furoyl) does not affect the reaction mechanism. The k1 values have been found to be larger for the reactions of 2 than for those of 1, which is fully responsible for the fact that the former is more reactive than the latter.