Review article

Carbocations from Arene Hydrates and Arene Oxides

Rory A. More O’Ferrall ; Department of Chemistry, University College Belfield, Dublin 4, Ireland
S. Nagaraja Rao ; Department of Chemistry, University College Belfield, Dublin 4, Ireland

Abstract

Arene hydrates are water adducts of aromatic molecules that readily dehydrate and aromatise in the presence of acids. The mechanism of this reaction is similar to that for the dehydration of alcohols. Protonation of a hydroxyl group is followed by C-O bond cleavage to form a benzenonium ionlike carbocation intermediate which then loses a proton (in contrast to alcohols, in a fast step) to yield the aromatic product. Rates of reaction are sensitive to the stability of the carbocation. They are also strongly decreased by benzoannelation: thus benzene hydrate is 500 times more reactive than l-hydroxy-l,2-dihydronaphthalene (naphthalene hydrate) which is 100 times more reactive than 9,10-phenanthrene hydrate. For the analogous dehydration of 3-substituted cis-benzenedihydrodiols to phenols p has a normal value of -7.2 but, unexpectedly, rate constants for +M resonance substituents MeO, EtO and Me are correlated by ap rather than a*. Comparison of benzene hydrate with benzene oxide shows, surprisingly, that the latter is the less reactive towards acids despite epoxides normally being 106-107 times more reactive than structurally related alcohols. For a series of arene oxides and hydrates oxide/hydrate rate ratios are inversely related to the resonance energy of the aromatised ring. This behaviour is tentatively ascribed to homoaromatic stabilisation of the arene oxide ring.

Keywords

Hrčak ID:

137120

URI

https://hrcak.srce.hr/137120

Publication date:

15.10.1992.

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