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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 30, Number 1
BKCSDE 30(1)
January 20, 2009 

A Kinetic Study on Aminolysis of S-4-Nitrophenyl Thiobenzoate in H2O Containing 20 mol % DMSO and 44 wt % EtOH: Effect of Medium on Reactivity and Mechanism
Jung Ae Ahn, Youn Min Park, Ik Hwan Um*
Brønsted-type plot, Aminolysis, Medium effect, Polarizability.
Second-order rate constants (kN) have been measured for nucleophilic substitution reactions of S-4-nitrophenyl thiobenzoate with a series of alicyclic secondary amines in H2O containing 20 mol % DMSO at 25.0 ± 0.1 oC. The Brønsted-type plot exhibits a downward curvature, i.e., βnuc decreases from 0.94 to 0.34 as the amine basicity increases. The reactions in the aqueous DMSO have also been suggested to proceed through a zwitterionic tetrahedral intermediate (T±) with change in the RDS on the basis of the curved Brønsted-type plot. The reactions in the aqueous DMSO exhibit larger kN values than those in the aqueous EtOH. The macroscopic rate constants (kN) for the reactions in the two solvent systems have been dissected into the microscopic rate constants (k1 and k2/k?1 ratio) to investigate effect of medium on reactivity in the microscopic level. It has been found that the k2/k?1 ratios are similar for the reactions in the two solvent systems, while k1 values are larger for the reactions in 20 mol % DMSO than for those in 44 wt % EtOH, indicating that the larger k1 is mainly responsible for the larger kN. It has been suggested that the transition state is more stabilized in 20 mol % DMSO through mutual polarizability interaction than in 44 wt % EtOH through H-bonding interaction.
214 - 218
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