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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 29, Number 8
BKCSDE 29(8)
August 20, 2008 

 
Title
Aminolysis of 2,4-Dinitrophenyl and 3,4-Dinitrophenyl 2-Thiophenecarboxylates: Effect of ortho-Nitro Group on Reactivity and Mechanism
Author
Jin-A Seo, Sun-Mee Chun, Ik-Hwan Um*
Keywords
Aminolysis, Brønsted-type plot, Steric hindrance, Reaction mechanism, ortho-Effect
Abstract
Second-order rate constants (kN) have been measured spectrophotometrically for reactions of 3,4-dinitrophenyl 2-thiophenecarboxylate (2) with a series of alicyclic secondary amines in 80 mol % H2O/20 mol % dimethyl sulfoxide at 25.0 oC. The Brønsted-type plot exhibits a downward curvature for the aminolysis of 2. The curved Brønsted-type plot is similar to that reported for the corresponding reactions of 2,4-dinitrophenyl 2- thiophenecarboxylate (1). The reactions of 1 and 2 have been suggested to proceed through the same mechanism, i.e., through a zwitterionic tetrahedral intermediate (T±) with a change in the rate-determining step. Substrate 2 is less reactive than 1 toward weakly basic amines (e.g., pKa < 10.4) but becomes more reactive as the basicity of amines increases further. Dissection of kN into the microscopic rate constants has revealed that the reaction of 2 results in a smaller k2/k-1 ratio but larger k1 than the corresponding reaction of 1. Steric hindrance exerted by the ortho-nitro group has been suggested to be responsible for the smaller k1 value found for the reactions of 1.
Page
1459 - 1463
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