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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 35, Number 2
BKCSDE 35(2)
February 20, 2014 

 
Title
Kinetic Study on Nucleophilic Displacement Reactions of 2-Chloro-4-Nitrophenyl X-Substituted-Benzoates with Primary Amines: Reaction Mechanism and Origin of the α-Effect
Author
Tae-Il Um, Min-Young Kim, Tae-Eun Kim, Ik-Hwan Um*
Keywords
The α-Effect, Ground state, Transition state, Intramolecular H-bonding, Yukawa-Tsuno plot
Abstract
Second-order rate constants for aminolysis of 2-chloro-4-nitrophenyl X-substituted-benzoates (1a-h) have been measured spectrophotometrically in 80 mol % H2O/20 mol % DMSO at 25.0 oC. The Brønsted-type plot for the reactions of 2-chloro-4-nitrophenyl benzoate (1d) with a series of primary amines curves downward, which has been taken as evidence for a stepwise mechanism with a change in rate-determining step (RDS). The Hammett plots for the reactions of 1a-h with hydrazine and glycylglycine are nonlinear while the Yukawa- Tsuno plots exhibit excellent linearity with X = 1.22-1.35 and r = 0.57-0.59, indicating that the nonlinear Hammett plots are not due to a change in RDS but are caused by stabilization of substrates possessing an electron-donating group (EDG) through resonance interactions between the EDG and C=O bond of the substrates. The α-effect exhibited by hydrazine increases as the substituent X changes from a strong EDG to a strong electron-withdrawing group (EWG). It has been concluded that destabilization of hydrazine through the electronic repulsion between the adjacent nonbonding electrons is not solely responsible for the substituent dependent α-effect but stabilization of the transition state is also a plausible origin of the α-effect.
Page
436 - 440
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