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Trends in Organic Chemistry   Volumes    Volume 17 
Kinetics of the reactions of picryl fluoride with ortho alkyl substituted anilines in acetonitrile: The origins of the dichotomy of amine effects in anilines in aromatic nucleophilic substitution reactions
Chukwuemeka Isanbor, Thomas A. Emokpae, Oluwakemi A. Oloba-Whenu
Pages: 59 - 71
Number of pages: 13
Trends in Organic Chemistry
Volume 17 

Copyright © 2014 Research Trends. All rights reserved

Rate data are reported for the reactions of 1-fluoro-2,4,6-trinitrobenzene (picryl fluoride) with a series of ortho alkyl substituted anilines (Y = H, 2-CH3, 2-C2H5­, 2,4-(CH3)2, 2,6-(CH3­)2 or 2,6-(C2H5)2) in acetonitrile. The results are compared with those reported for the corresponding phenoxy derivatives. The kinetic data show that increasing substitution sterically inhibits nucleophlic attack by anilines and a ‘late’ transition is likely. In general, the reactions are base catalyzed, interpreted as rate-limiting deprotonation of the zwitterionic intermediate. The reactions with aniline, 2-methyl and 2,4-dimethyl aniline where steric effects are likely to be small proceed exclusively by the base-catalysed step, while those with the more sterically hindered anilines (Y = 2-C2H5, 2,6-(CH3)2, 2,6-(C2H5)2), involve both the uncatalysed and the base catalysed pathways. Values of k1, the rate constant for nucleophilic attack at the 1-position, are higher for picryl fluoride than for the phenoxy analogue due to lower steric requirements and higher electron withdrawal of the fluorine atom than the phenoxy group. Similarly, values of kAn/k-1 are increased relative to those of the phenoxy derivative. This is likely to reflect increases in the values of k-1 due to release of steric strain present in the intermediate coupled with decreases in the values of kAn for proton transfer from the zwitterionic intermediate to base due to increased crowding at the reaction centre in the case of the phenoxy compound. The present work sheds further light on the dichotomy regarding the differing reactivities of substituted anilines of comparable basicity.


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