Peracids and free radicals: A theoretical and experimental approach

Quantitative Investigation of Free Radicals in Bio‐Oil and their Potential Role in Condensed‐Phase Polymerization

“…If there are several p orbitals energetically close to the orbital containing the unpaired electron, the effect is more pronounced. [24] Conjugation with the lone pair of a heteroatom (O in this case) can also occur.…”

“…Most of these radicals would be expected to have half-lives of < 10 À3 s. [24] The presence of radicals in condensed bio-oil indicates that some radicals formed during pyrolysis are transformed into stable forms. Radicals generated from primary pyrolysis reactions can be trapped in the aromatics originating from lignin.…”

“…This is the principal reason for the existence of stable radicals in lignin bio-oil. [24] An example of benzylic conjugation, which could be responsible for radicals in lignin bio-oil, is shown in Scheme 1. This benzylic stabilization is also explained by an orbital-interaction diagram analogous to that for hyperconjugation.…”

“…The kinetic stability of a radical is generally due to steric factors. [24] When a central radical is encompassed by a high number of molecules, its reactivity decreases considerably becoming a persistent radical; triphenyl methyl and 2,4,6-tri-tert-butylphenyl radicals are examples of kinetic stability (Figure 3). Considering the lignin structure, the explanation of kinetic stability could also be plausible.…”

“…Because methyl radicals are planar, the unpaired electron in a 2p orbital is not delocalized. [24] To elucidate the origin of radicals in corn-stover bio-oil, PL was precipitated from it [26] and then subjected to EPR analysis. The concentration of radicals in PL was 2.0 10 20 spin g À1 , which was the same order of magnitude as observed for lignin bio-oil (Figure 2).…”