Meta- and para-hydroxyls

Fig. 1. Chemical structure of (-)-epinephrine. Individual moieties, including the meta- and para-hydroxyls, catechol ring, protonated amine, alcoholic chiral p-carbon hydroxyl, and A-methyl group are indicated.

Fenoll LG, Rodriguez-Lopez JN, Varon R, Garcia-Ruiz PA, Garcia-Canovas F, Tudela J (2000) Action mechanism of tyrosinase on meta- and para-hydroxylated monophenols. Biol Chem 381 313-320... 

Some ortho substituted phenols such as o mtrophenol have significantly lower boiling points than those of the meta and para isomers This is because the intramolec ular hydrogen bond that forms between the hydroxyl group and the substituent partially compensates for the energy required to go from the liquid state to the vapor... 

Simple phenolics are substituted phenols. The ortho, meta and para nomenclature refers to a 1,2-, 1,3- and 1,4-substitution pattern of the benzene ring, respectively, where in this case one of the functional groups is the hydroxyl group. With three functional groups, the substitution pattern can be 1,3,5, which, when all three substituents are identical, is designated as a mt /fl-tri-substitution pattern, whereas the 1,2,6, substitution pattern is indicated by the prefix v/c (Figure 1-1). 

Although significant improvements have been made in the synthesis of phenol from benzene, the practical utility of direct radical hydroxylation of substituted arenes remains very low. A mixture of ortho-, meta- and para-substituted phenols is typically formed. Alkyl substituents are subject to radical H-atom abstraction, giving benzyl alcohol, benzaldehyde, and benzoic acid in addition to the mixture of cresols. Hydroxylation of phenylacetic acid leads to decarboxylation and gives benzyl alcohol along with phenolic products [2], A mixture of naphthols is produced in radical oxidations of naphthalene, in addition to diols and hydroxyketones [19]. 

The reactivities differ for ortho, meta, and para positions of toluene. In anthranilic acid (10), positions 3 and 5 in the molecule possess 49.8 and 27.4% of the total activity thus, tritium incorporation tends to be highest at positions of highest electron density.35 In isopropanol,35 the hydroxyl group incorporates approximately three times as much isotope as any other hydrogen position, while tritiation of the secondary C-H is slightly more efficient than the primary C-H. In the aliphatic esters of aromatic carboxylic acids, the aryl hydrogens tritiate ten times faster than alkyl hydrogens.10,36... 

Mono- and Poly-phenols.—As with the other ring substitution products of benzene and its homologues we have not only mono- but also di- tri- and other poly-substitution products, so also we have polyphenols in which more than one hydrogen atom of the ring is substituted by the hydroxyl group. In these poly phenols the conditions of position isomerism are present so that ortho, meta and para vicinal, symmetrical and unsymmetrical isomers are known. 

Eiigenole and Safrole.—The chief constituent of a more common essential oil is eugenole which occurs in oil of cloves. It is the monomethyl ether of di-hydroxy 7-phenyl propene in which the methoxy group is meta and the hydroxyl group is para to the propene chain. It is thus a propene derivative of the mono-methyl ether of pyro-catechinol. The alpha isomer is known as iso-eugenole. 

Hydroxylation of the aromatic ring of phenylalanine has been found to occur when aqueous solutions are subject to direct irradiation with ultra-violet light.Similar quantities of the ortho, meta and para isomers were formed the meta and para isomers were further hydroxylated to dopa. The reaction proceeds in the absence of oxygen and is inhibited by iodide, thiocyanate and thiourea, leading the authors to suggest that hydroxyl radical is involved. 

An example of structural discrimination was found in the mass spectra of isomeric N-nitrophenyl benzenesulfonamides 11781. Common features of the mass spectra of ortho, meta and para isomers were the presence of [M — H]- ions, more intense than the molecular anion, and the presence of fragments at m/z 137 and m/z 141, originating from N—S bond rupture. However, the ratio of the intensities of the latter ions was markedly higher for the ortho and para compounds, which reflected the stabilization of the m/z 137 anion by a nitro group in a conjugatively effective relationship. The ortho isomer was further differentiated by the elimination of hydroxyl radical, to form [M — OH] - ions. 

X-ray Induced Hydroxylation of Benzoic Acid. It is well known that irradiation of benzoic acid in dilute solutions gives rise to decarboxylation as well as to hydroxylation in the ortho- meta- and para-positions (10, 27). From these investigations it follows that radiation induced formation of hydroxybenzoic acids in neutral aqueous solutions accounts for about 50% of the total degradation. In order to shed more light on the ability of N20 to convert hydrated electrons into oxidizing radicals, it was decided to compare the effect of nitrous oxide with that of hydrogen peroxide under anoxic conditions. If Reaction 1 and 2 occur rapidly, the effect of nitrous oxide would be expected to be similar to that of hydrogen peroxide, except that the latter compound may also convert H atoms into OH radicals. 

---