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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 29, Number 3
BKCSDE 29(3)
March 20, 2008 

 
Title
Aminolysis of Y-Substituted Phenyl 2-Thiophenecarboxylates and 2-Furoates: Effect of Modification of Nonleaving Group from 2-Furoyl to 2-Thiophenecarbonyl on Reactivity and Mechanism
Author
Ik-Hwan Um*, Se-Won Min
Keywords
Aminolysis, Brønsted-type plot, Intermediate, Stepwise mechanism, Rate-determining step
Abstract
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 Brønsted-type plot for the reactions of 6a-h with morpholine is linear with a lg = ?1.29, indicating that the reactions proceed through a tetrahedral zwitterionic intermediate (T±). On the other hand, the Brønsted-type plot for the reactions of 6a-h with piperidine exhibits a downward curvature, implying that a change in the ratedetermining step occurs on changing the substituent Y in the leaving group. Dissection of kN into microscopic rate constants (i.e., k1 and k2/k?1 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/k?1 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 Brønsted curvature) as well as the k2/k-1 ratio.
Page
585 - 589
Full Text
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