On the Origin of Alkali-Catalyzed Aromatization of Phenols

Abstract

To gain an insight of the chemistry in the alkali-promoted aromatization of oxygen-containing heavily aromatic polymers or biomass; thermal degradations of sodium phenolates with different substituents have been investigated. The -ONa group strongly destabilizes the phenolates. The thermal stability of phenolates is largely in parallel with bond strengths of Ar substituents. De-substituents and the removal of aromatic hydrogens are dominant reactions in the main degradation step. CO is formed only at a very late stage. This degradation pattern is completely different from that of phenol. To account for this distinctive decomposition; a mechanism involving an unprecedented formation of an aromatic carbon radical anion generated from the homolytic cleavage of Ar substituent (or Ar–H) in keto forms has been proposed. The homolytic cleavage of Ar substituent (or Ar–H) is facilitated by the strong electron-donating ability of the oxygen anion. A set of free-radical reactions involved in the alkali-catalyzed aromatization have been established.

Base Promoted Intumescence of Phenols

“…However, we recently discovered that alkali promotes the intumescence of phenols [22]. This is drastically different from the acid based intumescence.…”

“…Apparently, the peak at 1615 cm −1 arose from aromatic structures derived from the benzene ring. On the other hand, the peak at 1438 cm −1 was identified to be the absorption of Na 2 CO 3 (from neutralization of NaOH by CO 2 in air) [22]. Thus, reactions involving the benzene ring and the cleavage of the C-O bond were essentially responsible for the changes observed in the IR spectra.…”

“…Sodium phenolate has been observed to generate intumescence during a continuous heating treatment [22]. However, due to the quick mass loss, it was inconvenient to study the intumescent process of sodium phenolate.…”

“…The homolysis step is shown in Reaction (1), which produces a hydrogen radical and aryl radical. Hydrogen radical adds to the benzene ring of the second phenolate to generate allyl radical or NaO• and benzene, which works as a blowing agent [22]. Allyl radical subsequently reacts with another aryl radical to yield a dimer (III) of sodium phenolate.…”

“…In this sense, the generation of hydrogen radical and the subsequent hydroarylation are the two most important steps since they generate dimer. In addition, hydrogen radical attacks ipso-carbon to yield benzene, which works as a blowing agent [22]. Further, since PAHs and polyphenylenes have relatively low molecular weights, they are meltable [38].…”