Transmission of inductive effect and polar resonance effect through the benzene and the furan ring in PMR spectra

Assuming the independence and additivity of the inductive and polar resonance effects, experimental data for chemical shifts in proton magnetic resonance (PMR) spectra of mono and disubstituted benzene and furan derivatives are used to calculate transmission coefficients for the inductive and polar resonance effects due to any substituent for a proton in the 2, 3, or 4 position in the ring. Values of ^* and ^c are tabulated. With benzene compounds transmission of the polar resonance effect decreases on passing from the p and o to the m position. The coefficient of transmission of the inductive effect to a proton in the m or p position is negligible.With furan compounds the values of the coefficient ^c for 2, 5 and 2,4 ring positions are close to the corresponding values for benzene compounds. In the furan ring a considerable part of the polar resonance is transmitted through the heteroatom. The ^* coefficients are appreciably greater with furan than with benzene compounds, because the ring carbon atoms screen the proton less from the substituent. Most of the inductive effect from the 2 to the 5 position in the furan ring is direct (transmitted through space).