Document Detail


Choice of solvent (MeCN vs H(2)O) decides rate-limiting step in S(N)Ar aminolysis of 1-fluoro-2,4-dinitrobenzene with secondary amines: importance of Brønsted-type analysis in acetonitrile.
MedLine Citation:
PMID:  17949108     Owner:  NLM     Status:  MEDLINE    
Abstract/OtherAbstract:
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 degrees C. The reaction in MeCN results in an upward curvature in the plot of k(obsd) 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 k2/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 k1k3/k(-1) are linear with betanuc 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 betanuc = 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.
Authors:
Ik-Hwan Um; Se-Won Min; Julian M Dust
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Publication Detail:
Type:  Journal Article; Research Support, Non-U.S. Gov't     Date:  2007-10-20
Journal Detail:
Title:  The Journal of organic chemistry     Volume:  72     ISSN:  0022-3263     ISO Abbreviation:  J. Org. Chem.     Publication Date:  2007 Nov 
Date Detail:
Created Date:  2007-11-02     Completed Date:  2008-01-07     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  2985193R     Medline TA:  J Org Chem     Country:  United States    
Other Details:
Languages:  eng     Pagination:  8797-803     Citation Subset:  IM    
Affiliation:
Department of Chemistry and Division of Nano Sciences, Ewha Womans University, Seoul, Korea.
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MeSH Terms
Descriptor/Qualifier:
Acetonitriles / chemistry*
Amines / chemical synthesis*,  chemistry*
Dinitrofluorobenzene / chemistry*
Kinetics
Molecular Structure
Solvents / chemistry
Water / chemistry
Chemical
Reg. No./Substance:
0/Acetonitriles; 0/Amines; 0/Solvents; 70-34-8/Dinitrofluorobenzene; 75-05-8/acetonitrile; 7732-18-5/Water

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine


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