Aromatic reactivity. Part XLII. Substitutent effects of the ethynyl group

Abstract

Relative rates of cleavage of XC₆H₄·SnMe₃ compounds by a 50 : 1 (v/v) mixture of ethanol and aqueous perchloric acid have been recorded as follows: (X =) H, 1·00; p-Cl, 0·37; p-CCH, 0·38; m-CCH, 0·24; m-Br, 0·143; m-Cl, 0·142. The results provide the first measure of the effects of a nuclear ethynyl substituent on the ease of an electrophilic aromatic substitution, and show that the group deactivates more from the meta- than from the para-position, as expected for a substituent having an electron-withdrawing inductive effect and capable of an electron-releasing resonance effect.

From the rates of cleavage of p-ethynylbenzyltrimethyl-silane and -stannane, a value of 0·52(5) has been derived for the σ^–-constant of the p-CCH group. Comparison of this value with that of the σ-constant (0·233) shows that the ethynyl group can exert a strong electron-withdrawing resonance effect.