Linear free energy ortho-correlations in the thiophen series. Part III. The kinetics of piperidino-substitution of some 3-X-5-nitro-2-thienyl phenyl sulphones in methanol

Abstract

To obtain information on the influence of a leaving group with high steric requirements on the possibility of obtaining ortho-linear free energy correlations in S_NAr reactions of five-membered ring derivatives, the rates of piperidinosubstitution of some 3-X-5-nitro-2-thienyl phenyl sulphones (II; X = Me, H, Br, CONH₃, CO₂Me, SO₂Me, CN, and NO₂) have been measured in methanol solution. The logarithm of the rate constants gives a straight line when plotted against σ_p^– or against the logarithm of the rate constants of piperidino-substitution for the para-isomers(IV) except for X = Me, Br, and SO₂Me. The ρ_2,3 value (+4.60) and the ratio ρ_2,3 : ρ_2,5(1.15) compare well with those obtained for the corresponding piperidinodebromination reactions, thus confirming that an ortho-substituent affects the position of the rate-determining transition state along the reaction co-ordinate more than a para-substituent.