Role of hydrogen-bonded nucleophiles in aromatic nucleophilic substitutions in aprotic solvents. Reactions of halonitrobenzenes with ethylenediamine, 3-dimethylamino-1-propylamine and histamine in toluene
The kinetics of the reactions between 1-halogeno-2,4-dinitrobenzene (halogen = F, C1) and the amines ethylenediamine (EDA) and 3-dimethylamino-1- propylamine (DMPA) were studied in toluene at 25° ±0.2C under pseudo-first-order conditions using varying amounts of amine. Even with C1 as the nucleofugu...
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| Autores principales: | , |
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| Formato: | JOUR |
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| Acceso en línea: | http://hdl.handle.net/20.500.12110/paper_08943230_v18_n8_p880_Alvaro |
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| Sumario: | The kinetics of the reactions between 1-halogeno-2,4-dinitrobenzene (halogen = F, C1) and the amines ethylenediamine (EDA) and 3-dimethylamino-1- propylamine (DMPA) were studied in toluene at 25° ±0.2C under pseudo-first-order conditions using varying amounts of amine. Even with C1 as the nucleofugue (where usually the first step is rate-determining), a third-order-in-amine kinetic law was observed: these results can be interpreted within the 'dimer nucleophile' mechanism where the amine homo-aggregates are better nucleophiles than the amine monomers. To confirm this interpretation, the reaction of 2,4-dinitrofluorobenzene with histamine was studied in the same solvent. Because of the rigid geometry, an intramolecular hydrogen bond is easily established, which prevents the formation of self-aggregates. Consequently, the plot of kA vs. [histamine] is a straight line, as expected for a classical mechanism of base-catalysed decomposition of the zwitterionic intermediate. All these results are well explained in the frame of the 'dimer nucleophile' mechanism. Copyright © 2005 John Wiley & Sons, Ltd. |
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