Aromatic detritiation. Part VI. Pentafluorobiphenyl: the novel effect of the pentafluorophenyl substituent

Abstract

Rates of protiodetritiation of 2′,3′,4′,5′,6′-pentafluoro[2-, 3-, and 4-³H]biphenyl, together with those of [2-, 3-, and 4-³H]biphenyl, some labelled monofluorobiphenyls, and [³H]benzene have been measured in trifluoroacetic acid containing aqueous perchloric acid at temperatures between 25 and 110°. Partial rate factors for protiodetritiation of pentafluorobiphenyl are: 0·0095 (2-), 0·0053 (3-), and 0·0158 (4-position), which yield σ⁺-values of 0·285 (meta) and 0·225 (para). The greater electron withdrawal by this substituent from the meta- than from the para-position contrasts with its effect in reactions which yield σ-values where the withdrawal is greater from the para-position. The pentafluoro-phenyl substituent therefore shares (with the ethynyl substituent) the novelty of being capable of producing greater electron withdrawal at either position depending upon the demand for resonance stabilisation of the transition state.

The effects of the five fluoro-substituents are less than predicted from the additivity principle (this same deviation having previously been found in electrophilic substitution of pentafluorobenzene), but it is unlikely that this arises from through-space interaction of the lone pairs of the ortho-fluorines with the deficient π-cloud of the ring undergoing substitution, since the effect of a single ortho-fluorine substituent appears to be normal.

The contradiction between the prediction from hydrogen exchange of substantial meta-substitution in other electrophilic reactions and the published data, is traced to the difficulty of resolving the meta- and para-derivatives. Re-examination of these reactions shows that up to 18% of meta-substitution occurs, and under suitable conditions this can be increased to 25%, the amount predicted. The nitration of pentafluorobiphenyl by nitric acid–sulphuric acid in acetic acid gives an ortho : para-ratio of ca. 0·45 and this is unaffected by the presence of nitrous acid. With nitric acid in acetic anhydride, acetoxylation is the main (though very slow) reaction.

A new route to the preparation of biphenylene is briefly described, as are the properties of some new bromofluorobiphenyls, and an unusual preparation of 2-butyl-2′-bromobiphenyl.