Anionic σ-complexes from sulfite ion attack on n-methyl-N-β-hydroxyethyl picramide and its spiro complex. Search for geometric isomerism in 1:2 complexes. An instructive example of complex kinetics

N-Methyl-N-β-hydroxyethyl picramide (PH) forms a 1:1 (PHS^2?) and a 1:2 (PHS) anionic σ-complex with sulfite ion. In alkaline solution PH is present mainly in the form of its spiro Meisenheimer complex (M^?) which also adds sulfite ion to form a complex MS^3?. Rate and equilibrium constants for the various reactions which interrelate the five species PH, PHS^2?, PHS, M^?, and MS^3? (scheme I) were determined. PHS and MS^3? can, in principle, exist in the form of two geometric isomers. Despite a careful search by nuclear magnetic resonance and kinetic techniques, no experimental manifestation of this isomerism could be found. Various limiting situations are discussed under which the absence of experimental evidence does or does not exclude the presence of two isomers. Our preferred but unproven interpretation is that in the case of PHS one isomer (trans?) is thermodynamically strongly favored over the other, whereas in the case of MS^3? both isomers are of similar thermodynamic stability and have also similar rates. This latter feature would make the isomerism kinetically undetectable. Several other features of scheme I make it an instructive example in complex chemical kinetics.