Linear free energy ortho-correlations in the thiophen series. Part I. The kinetics of piperidinodebromination of some 2-bromo-3-X-5-nitrothiophens in methanol

Abstract

The rates of piperidinodebromination of some 2-bromo-3-X-5-nitrothiophens (I; X = Me, H, Br, CONH₂, CO₂Me, SO₂Me, CN, or NO₂) have been measured in methoanol. The logarithm of the rate constants gives an excellent straight line (except for X = Br) when plotted against σ_p^– and against the logarithm of the rate constants of piperidnodebromination for the corresponding 2-bromo-3-nitro-5-X-thiophens (III). This confirms that a 3-substituent exerts an effect on the 2-position of the same kind (in this case extraconjugative) as that exerted from the 5-position. The ρ_2,3 value (+4·02) and the ratio ρ_2,3/ρ_2,5(1·18) show there is a larger transmission of electronic effects between the 2- and 3-positions than between the 2- and 5-positions of the thiophen ring.