Measurement of electrophilic aromatic reactivities via pyrolysis of 1-arylethyl acetates. Part II. The 2- and 3-positions of furan and thiophen

Abstract

Rates of pyrolysis of 1-arylethyl acetates (where aryl = 3-furyl, 2-furyl, 2-thienyl, 3-thienyl, and phenyl) have been measured over a range of more than 50° for each compound between 311·3 and 379·1°. The relative rates of pyrolysis of the heterocyclic esters are found at 600°K to be: 2-furyl, 3·84; 2-thienyl, 3·31; 3-furyl, 1·88; 3-thienyl, 1·785, which coupled with the previously determined ρ-factor for the reaction leads to the corresponding σ⁺-values of –0·890, –0·795, –0·415, and –0·38. This is the first quantitative determination of the electrophilic reactivity of all four positions in one reaction and in the absence also of solvent, and shows the 3-position of furan to be slightly more reactive than that in thiophen, but the difference in reactivities between the 2- and 3-positions is greater in furan as indicated from qualitative work with reactions in solution. Comparison of the results with those obtained in electrophilic substitutions shows excellent agreement except in those reactions for which solvent effects have previously been postulated as important. Correlation of reactivity at the 3-position of thiophen is less good than that at the 2-position and indicates that the former is more susceptible to resonance and/or solvent effects.

Molecular orbital calculations show that for furan it is possible to correlate the σ⁺-values with those for pyridine (by means of either π-electron densities or localisation energies) by using reasonable parameters for the coulomb and resonance integrals, but for thiophen this approach fails completely indicating that the d-orbitals of sulphur play a significant role in determining the reactivity.