The stabilities of Meisenheimer complexes. Part IV. Methoxide adducts from 1-X-3,5-dinitrobenzenes

Abstract

The σ complexes from 1-X-3,5-dinitrobenzenes and methoxide ions have been examined by 1H n.m.r. and visible spectroscopy. Initial addition of base occurs at C-4 but rearrangement occurs with time so that at equilibrium mainly the C-2 adducts are present. However the free-energy difference between the isomeric adducts is usually small. The measurement of indicator ratios in methanol–dimethyl sulphoxide mixtures containing 0·098 M-sodium methoxide allowed the simultaneous determination of the JM acidity function for these media, which is compared with other JM functions, and the thermodynamic equilibrium constants K for complex formation. Values of K are X = CN, K= 1·0 l mol–1; SO2Me, 0·50; CF3, 1·2 × 10–2; CO2Me, 8·5 × 10–3; l, 8·9 × 10–4; CONEt2, 1·1 × 10–4; SMe, 3·9 × 10–5; H, 1 × 10–6; SO3–, ca. 10–4–10–5. The stabilities of these adducts are compared with those from similarly activated anisoles.