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The replacement substituent constants 6 +, for some five-membered heteroaromatic rings were determined from 170 NMR substituent chemical shifts of the carbonyl group in trifluoroacetyl derivatives, and from log ks for solvolyses of heteroaryl analogs of 1-tert-butylbenzyl bromide and chloride. No agreement with literature values could be found. Preliminary accounts of such observations are given. Five-membered heteroaromatic rings could be considered to be derived from the replacement of the -CH=CH- moiety in benzene by a specific heteroatom. (1) Heteroaromaticity and heteroaromatic reactivity have long been the theme of continuous interest, as may be exhibited by the appearance of recent reviews(2,3) and communications.(4) Applications of Hammett-type correlation analyses to heterocyclic systems have been reviewed.(l,5,6) The heteroatom in the molecule might be regarded as a substituent, and the replacement substituent constants(l), or aryl values(7), for heteroaryl groups were determined and were compiled(6) nearly twenty years ago. Those values exhibit large variations, e. g., 6' = -0.13 to -1.32 for 2-fury1 group, much larger than that has been found for substituent groups in benzene derivatives.(8) The heteroatom is likely to play an important role, and to our knowledge, has not been examined in detail. Therefore, we undertook to redetermine certain 6 + constants for five-membered heteroaryl groups by using approaches that are differed from those employed by previous authors. In a previous study we observed an excellent linear correlation of oxygen-17 substituent chemical shifts (SCS) for various trifluoroacetyl derivatives of substituted benzenes with 6 + constants. (9) A linear relationship was obtained, (eqn. the 6 + value for heteroaromatic

pure and applied chemistry

Redetermination of σ+ constants for heteroaromatic rings. Oxygen-17 chemical shifts of trifluoroacetyl substituents and solvolytic reactivities of heteroaromatic analogs of 1-tert-butylbenzyl halides