AROMATIC REACTIVITY .42. SUBSTITUENT EFFECTS OF ETHYNYL GROUP

Relative rates of cleavage of XC6H4·SnMe 3 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-C≡CH, 0.38: m-C≡CH, 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-C≡CH 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.