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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
Kinetics and Mechanism of Azidolysis of Y-Substituted Phenyl Benzoates
Author
Ik-Hwan Um*, Eun-Hee Kim, Hyun-Joo Hana
Keywords
Acyl-transfer reaction, Azidolysis, Brønsted-type plot, Hammett plot, Yukawa-Tsuno plot
Abstract
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 Brønstedtype plot for the azidolysis exhibits a downward curvature, i.e., the slope (β1g) 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 Brønsted curvature) is 4.8, which is practically identical to the pKa of the conjugate acid of N3- ion (4.73). Hammett plots correlated with σo and σ- constants exhibit highly scattered points for the azidolysis. On the contrary, the corresponding Yukawa-Tsuno plot results in an excellent linear correlation with ρ = 2.45 and r = 0.40, indicating that the leaving group departs in the rate-determining step. The curved Brønsted-type plot has been interpreted as a change in the rate-determining step in a stepwise mechanism. The microscopic rate constants (k1 and k2/k-1 ratio) have been calculated for the azidolysis and found to be consistent with the proposed mechanism.
Page
580 - 584
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