Salt effects in some nucleophilic substitution reactions of ethoxide ions

Abstract

Kinetic studies (in anhydrous ethanol at 25 °C) are reported for the reactions: RX + MOEt → ROEt + MX where R = Et, X = I, M = Li, Na, or K;, R = 2,4-dinitrophenyl, X = Cl, M = Li, Na, K, or Cs;, and R = 2,4-dinitrophenyl, X = I, M = Li, Na, or K.

Measurements were made over a range of metal ethoxide concentrations extending down to 3 × 10^–3 mol l^–1, so that kinetic behaviour may be compared with conductimetric data for metal ethoxide ‘ion-pair’ association. The major salt effects observed are consistent with the postulate that the total reaction rate contains separate contributions from ‘free’ ethoxide ions and metal–ethoxide ‘ion-pairs’. Additional kinetic complexity arises from: (a) influence of the product salt on the association of metal ethoxides; (b) an effect at higher concentrations which is either a salt effect on the ‘free’ ion + substrate reaction, or which reflects an inadequacy in the assessment of metal ethoxide association from conductimetric data.

With ethyl iodide, ‘ion pairs’ are unreactive but with the aromatic substrates both the ‘free’ ethoxide ion and the ‘ion-pair’ react. The observed reactivity patterns are explicable in terms of different transition-state geometries and trends in strengths as electrolytes of metal halides and ethoxides in ethanol.