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엄익환,Se-Won Min 대한화학회 2008 Bulletin of the Korean Chemical Society Vol.29 No.3
Second-order rate constants (kN) have been measured for reactions of Y-substituted phenyl 2-thiophenecarboxylates (6a-h) with morpholine and piperidine in 80 mol % H2O/20 mol % DMSO at 25.0 0.1 oC. The Brnsted-type plot for the reactions of 6a-h with morpholine is linear with b lg = 1.29, indicating that the reactions proceed through a tetrahedral zwitterionic intermediate (T?). On the other hand, the Brnsted-type plot for the reactions of 6a-h with piperidine exhibits a downward curvature, implying that a change in the rate-determining step occurs on changing the substituent Y in the leaving group. Dissection of kN into microscopic rate constants (i.e., k1 and k2/k1 ratio) has revealed that k1 is smaller for the reactions of 6a-h than for those of Y-substituted phenyl 2-furoates (5a-h), while the k2/k1 ratio is almost the same for the reactions of 5a-h and 6a-h. It is also reported that modification of the nonleaving group from the furoyl (5a-h) to the thiophenecarbonyl (6a-h) does not influence pKao (defined as the pKa at the center of the Brnsted curvature) as well as the k2/k1 ratio.
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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.
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A Mechanistic Study on Alkaline Hydrolysis of Y-Substituted Phenyl Benzenesulfonates
엄익환,Li-Ra Im,Youn-Min Park 대한화학회 2008 Bulletin of the Korean Chemical Society Vol.29 No.12
Second-order rate constants (kOH−) have been measured spectrophotometrically for reactions of Y-substituted phenyl benzenesulfonates (1a-h) with OH– in H2O containing 20 mol % DMSO at 25.0 ± 0.1 oC. The Brønstedtype plot is linear with βlg = –0.55 including the points for the reactions of 2,4-dinitrophenyl benzenesulfonate (1a) and 4-chloro-2-nitrophenyl benzenesulfonate (1c), indicating that the ortho-nitro group on the leaving aryloxide does not exert steric hindrance in the current reactions. The Hammett plot correlated with σo constants exhibits highly scattered points, while the Hammett correlation with σ– constants results in a slightly better correlation but still many points deviate from the linearity. In contrast, the Yukawa-Tsuno plot shows an excellent linear correlation with r = 0.52, implying that leaving-group departure occurs at the RDS either in a stepwise mechanism or in a concerted pathway. However, the stepwise mechanism in which the leaving group departs in the RDS is excluded since the incoming OH– is much more basic and a poorer nucleofuge than the leaving aryloxide. Thus, the alkaline hydrolysis of 1a-h has been concluded to proceed through a concerted mechanism.
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Kinetics and Mechanism of Azidolysis of Y-Substituted Phenyl Benzoates
엄익환,김은희,Hyun-Joo Hana 대한화학회 2008 Bulletin of the Korean Chemical Society Vol.29 No.3
Second-order rate constants (kN) have been measured spectrophotometrically for reactions of Y-substituted phenyl benzoates (1a-h) with azide ion (N3) in 80 mol % H2O/20 mol % DMSO at 25.0 0.1 oC. The Brnsted-type plot for the azidolysis exhibits a downward curvature, i.e., the slope (b lg) changes from 0.97 to 0.20 as the basicity of the leaving group decreases. The pKao (defined as the pKa at the center of the Brnsted curvature) is 4.8, which is practically identical to the pKa of the conjugate acid of N3 ion (4.73). Hammett plots correlated with s o and s constants exhibit highly scattered points for the azidolysis. On the contrary, the corresponding Yukawa-Tsuno plot results in an excellent linear correlation with r = 2.45 and r = 0.40, indicating that the leaving group departs in the rate-determining step. The curved Brnsted-type plot has been interpreted as a change in the rate-determining step in a stepwise mechanism. The microscopic rate constants (k1 and k2/k1 ratio) have been calculated for the azidolysis and found to be consistent with the proposed mechanism.
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엄익환,Um Ik-Hwan Korean Chemical Society 1992 Bulletin of the Korean Chemical Society Vol.13 No.6
Second-order rate constants have been determined spectrophotometrically for nucleophilic substitution reactions of p-nitrophenyl benzoate with various anionic nucleophiles including 6 ${\alpha}$ -effect nucleophiles. The logarithmic second-order rate constants for the aryloxides give a good Bronsted correlation with the respective basicity while the ones for p-chlorothiophenoxide and hydroxide exhibit significantly positive and negative deviations, respectivity, from the Bronsted linear line. The deviations are attributed to a solvation effect rather than a change in the reaction mechanism. The ${\alpha}$-effect nucleophiles except highly basic ones demonstrate significantly higher nucleophilicity (the ${\alpha}$ -effect) than would be predicted from the respective basicity. The effect of solvation and polarizability appears to be important for the ${\alpha}$-effect as well as for the reaction rate.
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