Functional groups quinoidal

Formation and Elimination of Multiple Bond Functionalities. Reactions that involve the formation and elimination of multiple bond functional groups may significantly effect the color of residual lignin in bleached and unbleached pulps. The ethylenic and carbonyl groups conjugated with phenoHc or quinoid stmctures are possible components of chromophore or leucochromophore systems that contribute to the color of lignin. 

The compatibility with different functional groups, the remarkable regio-and stereoselectivity, and the development of asymmetric procedures have made benzannulation an attractive methodology for the synthesis of natural products with densely functionalized quinoid or fused phenolic substructures [13-20], Some pertinent examples are the syntheses of vitamins K and E [17], and the production of anthracyclinones or naphtoquinone antibiotics [13, 14a, 15, 21]. 

This initial attack of the ozone molecule leads first to the formation of ortho- and para-hydroxylated by-products. These hydroxylated compounds are highly susceptible to further ozonation. The compounds lead to the formation of quinoid and, due to the opening of the aromatic cycle, to the formation of aliphatic products with carbonyl and carboxyl functions. The nucleophilic reaction is found locally on molecular sites showing an electronic deficit and, more frequently, on carbons carrying electron acceptor groups. In summary, the molecular ozone reactions are extremely selective and limited to unsaturated aromatic and aliphatic compounds as well as to specific functional groups. 

For strong Pt fixation on CNM surface it is necessary to introduce functional groups, preferable from ones are -COOH, -OH and quinoid. The diversity of carbon allotropic forms and types of bonds between carbon atoms causes the different approaches to inculcation of a functional groups on CNM surface. 

Comparison of the surface density (pmol/m ) of the functional groups on active carbons oxidized by nitric acid in relation to carbon oxidized in air (see Table 4). shows a significantly higher (about 10 times) concentration of quinoid (carbonyl) structures. The gentler Nernst slope for CWN2—Ox is indicative of reduction with partial ionization of the hydroxyl group, according to the reaction... 

The results presented in Figure 1, Curve C, reveal that Illinois No. 6 coal undergoes an initial, very rapid reduction reaction. There is no initial, rapid reaction when the insoluble alkylation products are treated with potassium and naphthalene in tetrahydrofuran in a second alkylation reaction. The observations suggest that accessible acidic hydrogen atoms and other very readily reduced functional groups, e.g., quinoid... 

Practically the totality of the electron transfer reaction on the carbon surfaces are attributed to quinoid complexes only [40], Nevertheless, other functional groups with heteroatoms bearing an unshared pair of electrons (i.e. oxygen, nitrogen, sulfur, halogens) may also be involved in redox reactions [40, 178]. For instance, carbonyl and phenol groups are also present in lactone-like functionalities. In the case of lactones, they can easily undergo one-electron transfer process [40]. 

Organic compounds which are reduced at the mercury dropping electrode must contain a highly polar or unsaturated bond. Examples of the most common irreversibly reducible bonds and functional groupings are given in Table 2. Reversible electrode processes have been claimed for quinoid compounds (derivatives of 0- and p-benzoquinones, naphthoquinones, anthraquinones, -aminophenols, different types of dyestuffs like indophenols, thiazines, phenazines, indigosulphonates, and riboflavin etc.), for systems nitrosobenzene-N-phenylhydroxylamine, azobenzene-hydrazobenzene, alloxan-dialuric acid etc. 

Comparison of the TR spectra for HPDP in MeCN solvent to results from density functional theory calcnlations for the triplet state of HPDP indicates the triplet state has a qninoidal strnctnre with the carbonyl group about 16° out of the quinoidal plane and a delocalized nn character. Figure 3.29 compares ps-TR spectra of... 

The ct-amino group of the substrate SAM replaces that of active site lysine as the Schiff base partner of the cofactor (external aldimine. Scheme 2(b)) and the C-a proton of SAM is next abstracted by the e-NH2 function of active site lysine to form a quinoid intermediate (Scheme 2(c)). 

A reasonable model has been proposed to accommodate these results (2/y 23). The presence of quinoid functions in lignin would give rise to electron donor-acceptor complexes with existing phenolic groups. These complexes, like quinhydrone, would form stable radical anions (semiquinone anions) on basification, according to the scheme shown below. Both biological and chemical oxidation would create more quinone moieties, which in turn would increase the contribution of Reactions 1 and 2. Alternately, enzymatic (< ) and/or alkaline demethylation 16) would produce... 

Carbon black can increase the thermal stability of many polymers because of its properties. Phenoxyl and quinoid groups on the surface of carbon black function as antioxidants. These groups also participate in the catalytic decomposition of peroxides which contributes to a reduction in degradation rate. Quinone, polynuclear structures, polyconjugated double bonds, and carbonyl groups all scavenge radicals. Many polymers and rubbers benefit from these properties of carbon black. 

As to the underlying reaction mechanism, it appears that vacuum thermolysis of (545), (554) and (561) generates in each case, initially, a transient o-quinodimethane which cyclizes in the highly dilute gas phase. o-Quinoid vinylketene intermediate (562) could be regenerated in solution from the isolated benzocyclobutenone (563) on heating or irradiation and trapped by electron-poor dienophiles, e.g. (632) (Scheme 141). = Alternatively, benzocyclobutenones can be easily functionalized at the carbonyl group or at Ca, affording further o-quinodimethane precursors. 

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