Mechanisms of elimination and substitution reactions. Spontaneous solvolysis reactions via carbanion and carbocation intermediates

Article abstract of DOI:10.1002/(SICI)1099-1395(199902)12:2<116::AID-POC102>3.0.CO;2-8

The reactions of 4-(4′-nitrophenyl)-4-X-butan-2-one (1-X; X = Cl, OTs) with added bases/nucleophiles exhibit second-order kinetics. These substitution and elimination reactions are concluded to be of concerted S_N2 type and irreversible E1cB type, respectively. The spontaneous formation of alkene from the chloride 1-CI is suggested to occur by a water-promoted E1cB reaction. The fraction of elimination product is smaller with the tosylate 1-OTs. It is plausible that the elimination and substitution reactions of 1-OTs are carbocation reactions since the Bronsted plot with substituted acetate anion bases shows a tenfold positive deviation for the 'water-catalyzed' elimination reaction, and there is a trace of substitution reaction with the acetonitrile component of the solvent yielding 1-NHCOMe. These results are consistent with a common, very short-lived, carbocation intermediate. Copyright