Anthryl radical, formation

Carbonization of Anthracene. The pyrolysis of anthracene has been investigated extensively (13-17). The initial reaction is not well-understood, and various radical intermediates have been proposed. The thermodynamics of anthracene dissociation has been discussed by Stein (8), and the formation of the anthryl radical by a disproportionation reaction such as Scheme II is expected to be very slow because of the instability of the a radical (1). A mechanism involving the direct formation of the anthryl radical by hydrogen dissociation, although not favorable, might be possible. A reaction scheme based on the formation of the radical (2) from anthracene and dihydroanthracene has also been proposed (17). 

Scheme II. Formation of the anthryl radical by a disproportionation reaction.

In a similar manner, p-bis(9-anthryl) phenylene gives a mono(anion-radical) or a mono(cation-radical) under reductive or oxidative conditions with spin delocalization around the whole molecular framework. In the case of m-bis(9-anthryl) phenylene, reduction or oxidation leads to the formation of dianion or dication-diradicals. Based on ESR experiments at cryogenic temperatures (6.5-85 K), these species contain two separated ion-radical moieties. They have parallel aligmnent of their spins (Tukada 1994). The work gives clear experimental evidence for the so-called ferromagnetic interaction between these ion-radical substituents. In some cases, release of the electron depends on temperature. See, for example, the anion-radical shown in Scheme 3.63. 

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