Electrophilic aromatic substitution. Part 21. The reactivity of o-, m-, and p-terphenyl in protiodetritiation

Abstract

Partial rate factors have been determined for protiodetritiation of o-, m-, and p-terphenyl in anhydrous trifluoroacetic acid at 70 °C as follows (positions in parentheses): p-terphenyl, 273(4), 176(2), 1.54(3), 59.1(2′); m-terphenyl 124(4), 52.2(2), 4690(4′), ca. 1060(2′); o-terphenyl, 89.5(4), 33.8(2), and the value for the ortho-position of biphenyl has been redetermined as 85. The results are in the main in good agreement with prediction based upon a combination of additivity and coplanarity effects. The 2′-position of m-terphenyl is, however, approximately two-fold less reactive than expected due to slight steric hindrance thereby confirming this as the factor responsible for the low reactivity towards exchange of the central ring of 1,3,5-triphenylbenzene; the 2-position of o-terphenyl and to a lesser extent the 2-position of m-terphenyl may also be subject to very slight steric hindrance. σ⁺-Constants have been assigned to all those positions which are virtually unaffected by steric hindrance. The overall reactivity of ortho-terphenyl is less than that of the other isomers, so the greater reactivity claimed for this isomer in bromination and acylation is not due to electronic effects; the marked difference in solubilities of the three isomers is almost certainly responsible for the anomaly.