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Choice of solvent (MeCN vs H2O) decides rate-limiting step in SNAr aminolysis of 1-fluoro-2,4-dinitrobenzene with secondary amines: Importance of Brønsted-type analysis in acetonitrile

Title
Choice of solvent (MeCN vs H2O) decides rate-limiting step in SNAr aminolysis of 1-fluoro-2,4-dinitrobenzene with secondary amines: Importance of Brønsted-type analysis in acetonitrile
Authors
Um I.-H.Min S.-W.Dust J.M.
Ewha Authors
엄익환
SCOPUS Author ID
엄익환scopusscopus
Issue Date
2007
Journal Title
Journal of Organic Chemistry
ISSN
0022-3263JCR Link
Citation
vol. 72, no. 23, pp. 8797 - 8803
Indexed
SCI; SCIE; SCOPUS WOS scopus
Abstract
(Chemical Equation Presented) A kinetic study is reported for nucleophilic substitution reactions of 2,4-dinitro-1-fluorobenzene (DNFB) with a series of secondary amines in MeCN and H2O at 25.0°C. The reaction in MeCN results in an upward curvature in the plot of kobsd vs [amine], indicating that the reaction proceeds through a rate-limiting proton transfer (RLPT) mechanism. On the contrary, the corresponding plot for the reaction in H2O is linear, implying that general base catalysis is absent. The ratios of the microscopic rate constants for the reactions in MeCN are consistent with the proposed mechanism, e.g., the facts that k 2/k-1 < 1 and k3/k2 > 10 2 suggest that formation of a Meisenheimer complex occurs before the rate-limiting step and the deprotonation by a second amine molecule becomes dominant when [amine] > 0.01 M, respectively. The Brønsted-type plots for k1k2/k-1 and k1k 3/k-1 are linear with βnuc values of 0.82 and 0.84, respectively, which supports the proposed mechanism. The Brønsted-type plot for the reactions in H2O is also linear with βnuc = 0.52 which has been interpreted to indicate that the reaction proceeds through rate-limiting formation of a Meisenheimer complex. DNFB is more reactive toward secondary amines in MeCN than in H2O. The enhanced basicity of amines as well as the increased stability of the intermediate whose charges are delocalized through resonance are responsible for the enhanced reactivity in the aprotic solvent. © 2007 American Chemical Society.
DOI
10.1021/jo701549h
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자연과학대학 > 화학·나노과학전공 > Journal papers
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