A Kinetic Study on Aminolysis of Benzyl 2-Pyridyl Thionocarbonate and t-Butyl 2-Pyridyl Thionocarbonate: Effects of Polarizability and Steric Hindrance on Reactivity and Reaction Mechanism 


Vol. 34,  No. 8, pp. 2325-2329, Aug.  2013
10.5012/bkcs.2013.34.8.2325


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  Abstract

Second-order rate constants kN have been measured for reactions of benzyl 2-pyridyl thionocarbonate (4b) and t-butyl 2-pyridyl thionocarbonate (5b) with a series of cyclic secondary amines in MeCN at 25.0 ± 0.1 oC. The kN values for the reactions of 4b and 5b have been compared with those reported previously for the corresponding reactions of benzyl 2-pyridyl carbonate (4a) and t-butyl 2-pyridyl carbonate (5a) to investigate the effect of changing the electrophilic center from C=O to C=S on reactivity and reaction mechanism. The thiono compound 4b is more reactive than its oxygen analogue 4a. The Brønsted-type plots for the reactions of 4a and 4b are linear with βnuc = 0.57 and 0.37, respectively. The reactions of 4a were previously reported to proceed through a concerted mechanism, while those of 4b in this study have been concluded to proceed through a stepwise mechanism with formation of an intermediate being the rate-determining step on the basis of the βnuc value of 0.37. Enhanced polarizability upon changing the C=O in 4a by C=S has been suggested to be responsible for the reactivity order and the contrasting reaction mechanisms. In contrast, the reactivity of 5a and 5b is similar, but they are much less reactive than 4a and 4b. Furthermore, the reactions of 5a and 5b have been concluded to proceed through the same mechanism (i.e., a concerted mechanism) on the basis of linear Brønsted-type plots with βnuc = 0.45 or 0.47. It has been concluded that the strong steric hindrance exerted by the t-Bu in 5a and 5b causes a decrease in their reactivity and forces the reactions to proceed through a concerted mechanism.

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  Cite this article

[IEEE Style]

M. Kim, A. R. Bae, I. Um, "A Kinetic Study on Aminolysis of Benzyl 2-Pyridyl Thionocarbonate and t-Butyl 2-Pyridyl Thionocarbonate: Effects of Polarizability and Steric Hindrance on Reactivity and Reaction Mechanism," Bulletin of the Korean Chemical Society, vol. 34, no. 8, pp. 2325-2329, 2013. DOI: 10.5012/bkcs.2013.34.8.2325.

[ACM Style]

Min-Young Kim, Ae Ri Bae, and Ik-Hwan Um. 2013. A Kinetic Study on Aminolysis of Benzyl 2-Pyridyl Thionocarbonate and t-Butyl 2-Pyridyl Thionocarbonate: Effects of Polarizability and Steric Hindrance on Reactivity and Reaction Mechanism. Bulletin of the Korean Chemical Society, 34, 8, (2013), 2325-2329. DOI: 10.5012/bkcs.2013.34.8.2325.

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ISSN(Print) 0253-2964
ISSN(Online) 1229-5949