Kinetics and Mechanism of Azidolysis of Y-Substituted Phenyl Benzoates

Bulletin of the Korean chemical society

Eun-Hee Kim¹  Hyun-Joo Hana¹  Ik-Hwan Um¹ 

关键词: Acyl-transfer reaction;    Azidolysis;    Br©ªnsted-type plot;    Hammett plot;    Yukawa-Tsuno plot;   

DOI  :  

学科分类：化学（综合）

来源: Korean Chemical Society

【 摘 要 】

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 Br┆nstedtype plot for the azidolysis exhibits a downward curvature, i.e., the slope (�?_1g) changes from ?0.97 to ?0.20 as the basicity of the leaving group decreases. The pKa^o (defined as the pKa at the center of the Br┆nsted curvature) is 4.8, which is practically identical to the pKa of the conjugate acid of N₃^- ion (4.73). Hammett plots correlated with �?^o and �?^- constants exhibit highly scattered points for the azidolysis. On the contrary, the corresponding Yukawa-Tsuno plot results in an excellent linear correlation with �? = 2.45 and r = 0.40, indicating that the leaving group departs in the rate-determining step. The curved Br┆nsted-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.

【 授权许可】

Unknown