The alpha-effect in SNAr substitutions - Reaction between oximate nucleophiles and 2,4-dinitrofluorobenzene in aqueous solution

The second-order rate constants (k(1)(ArO), k(1)(Ox))for SNAr substitutions of 2,4-dinitrofluorobenzene (DNFB) by a series of phenoxide and oximate nu cleophiles have been measured in aqueous solution at 25 degreesC, using bot h a potentiometric procedure involving the use of a fluoride ion selective electrode (FISE) and a classical spectrophotometric procedure. While the ra te data for the phenoxide ions conform to a linear Bronsted plot with a slo pe (beta (Nu) = 0.71) fitting the 0.5-0.7 range commonly found for SNAr rea ctions, those for the various oximates studied do not define a meaningful l inear plot. Interestingly, the observed variations in kill reveal a tendenc y of the reactivity of oximates of pK(a), > 7.5-8 to level off rapidly, a s ituation reminiscent of that encountered in other nucleophilic reactions of these species at carbonyl and phosphonyl centres. Our current finding rein forces the idea of a general oximate behaviour pattern originating from an especially strong need for partial desolvation before nucleophilic attack, i.e., asynchronicity or TS imbalance. A major consequence of the observed l evelling off is that the extra reactivity reflecting the a character of oxi mate nucleophiles decreases significantly in magnitude on going from weakly basic oximates (k(1)(Ox)/k(1)(ArO) approximate to 100) to strongly basic o nes (k(l)(Ox)/k(1)(ArO) approximate to 100). On the basis of the k(1)(Ox)/k (1)(ArO) ratio measured at low I I I pK(a), the a-effect associated with th e SNAr substitution of DNFB is of the same order as that measured for other reactions of oximates at spl-carbon centres.