Aromatic detritiation. Part IV. 1,3,5-Triphenylbenzene: a possible example of steric hindrance to hydrogen exchange

Abstract

Rates of protiodetritiation of 1,3,5-triphenylbenzene have been measured in anhydrous trifluoroacetic acid at temperatures between 70 and 130 °C and lead to the following partial rate factors: 72,600 (2-position); 88 (4′-position); 29 (2′-position). This quantitative determination of the electrophilic reactivity of the molecule shows that the isomer distributions obtained in all previous electrophilic substitutions of it are affected by steric hindrance to 2- and 2′-substitution, and further, by comparison with detritiation data for biphenyl, that even in hydrogen exchange the 2- and 2′-positions are probably slightly hindered; this being so, it constitutes the first example of steric hindrance to acid-catalysed hydrogen exchange. The data for nitration of triphenylbenzene by nitric acid in acetic anhydride is at variance with the prediction of the hydrogen exchange reaction indicating the nitration pattern to be anomalous as is that for nitration of biphenyl under these conditions.

Conjugation between the phenyl rings appears to be the same as in biphenyl, consequently the angles between the rings must be closely similar for both molecules in trifluoroacetic acid.