The α-Effect in SNAr Substitutions − Reaction between Oximate Nucleophiles and 2,4-Dinitrofluorobenzene in Aqueous Solution

Abstract

The second-order rate constants (k1ArO, k1Ox) for SNAr substitutions of 2,4-dinitrofluorobenzene (DNFB) by a series of phenoxide and oximate nucleophiles have been measured in aqueous solution at 25 °C, using both a potentiometric procedure involving the use of a fluoride ion selective electrode (FISE) and a classical spectrophotometric procedure. While the rate data for the phenoxide ions conform to a linear Brönsted plot with a slope (βNu = 0.71) fitting the 0.5−0.7 range commonly found for SNAr reactions, those for the various oximates studied do not define a meaningful linear plot. Interestingly, the observed variations in k1Ox reveal a tendency of the reactivity of oximates of pKa > 7.5−8 to level off rapidly, a situation reminiscent of that encountered in other nucleophilic reactions of these species at carbonyl and phosphonyl centres. Our current finding reinforces 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 levelling off is that the extra reactivity reflecting the α character of oximate nucleophiles decreases significantly in magnitude on going from weakly basic oximates (k1Ox/k1ArO ≈︁ 100) to strongly basic ones (k1Ox/k1ArO ≈︁ 10). On the basis of the k1Ox/k1ArO ratio measured at low pKa, the α-effect associated with the SNAr substitution of DNFB is of the same order as that measured for other reactions of oximates at sp2-carbon centres.