Resonance stabilization phenols

Tyrosine possesses a phenolic proton which is more readily deprotonated because deprotonation forms a resonance-stabilized phenolate ion. In contrast, deprotonation of the OH group of serine gives an aUcoxide ion that is not resonance-stabilized. As a result, the OH group of tyrosine is more acidic than the OH group of serine. 

Direct connection of pendant heteroatom to polystyrene aryl is a synthetically more difficult, but often still feasible (37), alternative. However, though bonds from phenyl to many common heteroatoms are relatively strong, resonance stabilization of partial positive charge developed on an arylated atom activates it to leave other substituents alkyl anilium salts (12) and anilines (38), as well as phenolic esters (39), are relatively easy to cleave. Aryl linkages,... 

The result of this resonance stabilization of the anion by delocalization of the negative charge makes the overall energy change required for ionization to be smaller for phenol than it is for ethanol. This can be shown in terms of energies as follows ... 

Still another possibility in the base-catalyzed reactions of carbonyl compounds is alkylation or similar reaction at the oxygen atom. This is the predominant reaction of phenoxide ion, of course, but for enolates with less resonance stabilization it is exceptional and requires special conditions. Even phenolates react at carbon when the reagent is carbon dioxide, but this may be due merely to the instability of the alternative carbonic half ester. The association of enolate ions with a proton is evidently not very different from the association with metallic cations. Although the equilibrium mixture is about 92 % ketone, the sodium derivative of acetoacetic ester reacts with acetic acid in cold petroleum ether to give the enol. The Perkin ring closure reaction, which depends on C-alkylation, gives the alternative O-alkylation only when it is applied to the synthesis of a four membered ring ... 

The OH can react with catechol, by hydrogen abstraction or addition to the aromatic ring, to produce the resonance-stabilized radical The latter could couple with other catechol molecules or oxygen to eventually form polymerized, highly colored materials, according to the scheme proposed for phenol by Voudrias (90) (Figure 6). 

For ethanol, the same procedure yields ER(EtO ) = -9.2 kJ mol-1 and s(EtO-H) = 449 kJ mol-1. This value is still some 48 kJ mol-1 higher than the value obtained for phenol, questioning our initial assumption that the bond strengths should be similar in both compounds. Yet we have forgotten two import issues the resonance stabilization of the phenoxyl radical and the hyperconjugation of the ethoxyl radical. 

Antioxidants are compounds that inhibit autoxidation reactions by rapidly reacting with radical intermediates to form less-reactive radicals that are unable to continue the chain reaction. The chain reaction is effectively stopped, since the damaging radical becomes bound to the antioxidant. Thus, vitamin E (a-tocopherol) is used commercially to retard rancidity in fatty materials in food manufacturing. Its antioxidant effect is likely to arise by reaction with peroxyl radicals. These remove a hydrogen atom from the phenol group, generating a resonance-stabilized radical that does not propagate the radical reaction. Instead, it mops up further peroxyl radicals. In due course, the tocopheryl peroxide is hydrolysed to a-tocopherylquinone. 

The acid condensation reaction of the aromatic and phenolic units is a typical reaction of lignin. The presence of acids results in resonance stabilized carbonium ion structures formed in the lignin macromolecule. These car-bonium ion structures react further, e.g., with unsubstituted positions in the lignin macromolecule. Thus, thermal treatment of powdered wood in acidic conditions causes condensation, the coniferyl aldehyde and coniferyl alcohol groups being especially reactive. In addition, other inter- and/or intramolecular condensations may occur. 

Another alternative (Scheme B) would be that an intermediate quinonemethide (e.g. XXV) dehydrogenates an easily oxidizable species, such as dihydroxydiphenylmethane, catechol, or />,p -dihydroxystilbene. The dehydrogenation of phenolic diphenylmethanes to highly colored quinonemethides has been reported by Harkin 20) and Rothenberg and Luner 37). The resulting ionized quinonemethide XXVII is likely to be quite stable because of the resonance stabilization. 

In search of novel natural antioxidant compounds that might posses a good brain bioavailability, our laboratory has focused attention on the phenolic compound ferulic acid ethyl ester (FAEE) (Fig. 18.1). Ferulic acid is a ubiquitous plant constituent that occurs primarily in seeds and leaves both in its free form and covalently linked to lignin and other biopolymers. Due to its phenolic nucleus and an extended side chain conjugation, it readily forms a resonance stabilized phenoxy radical that accounts for its potent antioxidant potential [Kanski et al., 2002 Kikuzaki et al., 2002], Ferulic acid has been shown to be protective against oxidative stress in vitro it is absorbed and excreted by humans, and may be a promising candidate for therapeutic intervention in AD [Yan et al., 2001]. Although ferulic acid has been demonstrated to be effective in vitro, the low lipophilicity impairs its in vivo efficiency, bioavailability, and stability. 

Because of their high acidity, phenols are often called carbolic acids. The phenol molecule is highly acidic because it has a partial positive charge on the oxygen atom due to resonance, and the anion that is formed by loss of a hydrogen ion is also resonance stabilized. 

Friedel-Crafts type reactions of strongly deactivated arenes have been the subject of several recent studies indicating involvement of superelectrophilic intermediates. Numerous electrophilic aromatic substitution reactions only work with activated or electron-rich arenes, such as phenols, alkylated arenes, or aryl ethers.5 Since these reactions involve weak electrophiles, aromatic compounds such as benzene, chlorobenzene, or nitrobenzene, either do not react, or give only low yields of products. For example, electrophilic alkylthioalkylation generally works well only with phenolic substrates.6 This can be understood by considering the resonance stabilization of the involved thioalkylcarbenium ion and the delocalization of the electrophilic center (eq 4). With the use of excess Fewis acid, however, the electrophilic reactivity of the alkylthiocarbenium ion can be... 

The enol form of phenol benefits from the resonance stabilization and aromaticity of the benzene ring, but the keto form, with the single sp3 center in the ring, cannot. 

Free Radical Scavengers. Hindered phenolics are the most important free radical scavengers used in polypropylene. These compounds function by hydrogen transfer reactions involving labile hydroxy-hydro-gen atoms in the molecule. The results of hydrogen transfer reactions are elimination of active radicals, and formation of resonance-stabilized phenoxy radicals which do not react with weak bonds in the polymer chain. 

The phenolic initially gives up its labile hydrogen, which in turn reacts with the various radicals produced in chain reactions then the phenoxy radical becomes stabilized owing to its ability to form resonance structures. The resonance-stabilized forms of the phenoxy radical will not attack tertiary carbon—hydrogen bonds in the polypropylene chain but will react with other radicals such as a peroxide, resulting in the elimination of a second free radical. 

Heat Stability. Polymeric phenolic phosphites seem to be efficient hydrogen transfer agents which provide excellent heat stability when used in conjunction with thiodipropionates. Presumably, they scavenge free radicals and form resonance-stabilized phenoxy radicals in much the manner as simple hindered phenolics (see reaction scheme on p. 214). 

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