Phenols isomeric

Carvacrol, Cj Hj O, is a phenol isomeric with thymol, with which it is frequently found associated, especially in certain types of thyme and origanum oils. 

Phenols in tobacco smoke (hydroxyquinone, resorcinol, catechol, phenol, isomeric cresols)... 

Chavicol and Estragole.—The para phenol isomeric with anol is known as chavicol and is found in hetel leaf oil. Its methyl ether is estragole which is a constituent of estragon oil. 

Thymol (2-isopropyl-5-methylphenol) is the main monoterpene phenol, isomeric with carvacrol, found in thyme essential oil. This compound has revealed several biological properties, such as antibacterial, antifungal, anti-inflammatory, antioxidant, and immunomodulatory."... 

Note 1. A slight excess of phenol was used. With a stoichiometric amount the solution may be too strongly basic, so that the further isomerization will become too fast. 

This reaction sequence is much less prone to difficulties with isomerizations since the pyridine-like carbons of dipyrromethenes do not add protons. Yields are often low, however, since the intermediates do not survive the high temperatures. The more reactive, faster but less reliable system is certainly provided by the dipyrromethanes, in which the reactivity of the pyrrole units is comparable to activated benzene derivatives such as phenol or aniline. The situation is comparable with that found in peptide synthesis where the slow azide method gives cleaner products than the fast DCC-promoted condensations (see p. 234). 

The preference for O acylation of phenols arises because these reactions are kmetically controlled O acylation is faster than C acylation The C acyl isomers are more stable how ever and it is known that aluminum chloride is a very effective catalyst for the conversion of aryl esters to aryl ketones This isomerization is called the Fries rearrangement... 

Bromination can be conveniently effected by transfer of bromine from one nucleus to another. As the Friedel-Crafts isomerization of bromoaromatic compounds generally takes place through an intermolecular mechanism, the migrating bromine atom serves as a source of positive bromine, thus effecting ring brominations (161,162). 2,4,6-Tribromophenol, for example, has been prepared by bromination of phenol with dibromobenzene. 

This process has been widely studied and led to the constmction of new and original industrial units. Interest in the reaction stems from the simplicity of the process as well as the absence of undesirable by-products. However, in order to be economically rehable, such a process has to give high yield of dihydroxybenzenes (based on hydrogen peroxide as well as phenol) and a great flexibiUty for the isomeric ratio of hydroquinone to catechol. This last point generated more research and led to original and commercial processes. 

Other Methods. A variety of other methods have been studied, including phenol hydroxylation by N2O with HZSM-5 as catalyst (69), selective access to resorcinol from 5-methyloxohexanoate in the presence of Pd/C (70), cyclotrimerization of carbon monoxide and ethylene to form hydroquinone in the presence of rhodium catalysts (71), the electrochemical oxidation of benzene to hydroquinone and -benzoquinone (72), the air oxidation of phenol to catechol in the presence of a stoichiometric CuCl and Cu(0) catalyst (73), and the isomerization of dihydroxybenzenes on HZSM-5 catalysts (74). 

Polyhydric phenols with more than two hydroxy groups (ie, the three positional isomers of benzenetriol, the three isomeric benzenetetrols, benzenepentol [4270-96-6] and benzenehexol [608-80-0]) are discussed in this article. The benzenediols are catechol, resorcinol, and hydroquinone (see Hydroquinone, RESORCINOL, AND CATECHOL). 

Pyrazoles, isoxazoles and isothiazoles with a hydroxyl group in the 3-position (491 Z = NR, O, S) could isomerize to 3-azolinones (492). However, these compounds behave as true hydroxy derivatives and show phenolic properties. They give an intense violet color with iron(III) chloride and form a salt (493) with sodium hydroxide which can be O-alkylated by alkyl halides (to give 494 R = alkyl) and acylated by acid chlorides (to give 494 R = acyl). 

The impurities present in aromatic nitro compounds depend on the aromatic portion of the molecule. Thus, benzene, phenols or anilines are probable impurities in nitrobenzene, nitrophenols and nitroanilines, respectively. Purification should be carried out accordingly. Isomeric compounds are likely to remain as impurities after the preliminary purifications to remove basic and acidic contaminants. For example, o-nitrophenol may be found in samples ofp-nitrophenol. Usually, the ri-nitro compounds are more steam volatile than the p-nitro isomers, and can be separated in this way. Polynitro impurities in mononitro compounds can be readily removed because of their relatively lower solubilities in solvents. With acidic or basic nitro compounds which cannot be separated in the above manner, advantage may be taken of their differences in pK values (see Chapter 1). The compounds can thus be purified by preliminary extractions with several sets of aqueous buffers... 

Aromatic compounds such as toluene, xylene, and phenol can photosensitize cis-trans interconversion of simple alkenes. This is a case in which the sensitization process must be somewhat endothermic because of the energy relationships between the excited states of the alkene and the sensitizers. The photostationary state obtained under these conditions favors the less strained of the alkene isomers. The explanation for this effect can be summarized with reference to Fig. 13.12. Isomerization takes place through a twisted triplet state. This state is achieved by a combination of energy transfer Irom the sensitizer and thermal activation. Because the Z isomer is somewhat higher in energy, its requirement for activation to the excited state is somewhat less than for the E isomer. If it is also assumed that the excited state forms the Z- and -isomers with equal ease, the rate of... 

Dihydrothebainone-A-5 6-methyl enolate, CjaHjjOjN, m.p. 164-165-5°, [a] ° — 115-7° (EtOH). Cold N/HCl converts it into dihydrothebainone hydrochloride. The isomeric dihydrothebainone-J-6 7-methyl enolate is formed on catalytic hydrogenation of phenolic dihydrothebaine. It has m.p. 127-8°, [a] ° — 8° (EtOH) and yields dihydrothebainone on acid hydrolysis. ... 

Eximidine, Ci,Hj40N(0Me)3. (Item 36 list, p. 172.) A phenolic base, m.p. 133°, yielding a methiodide, m.p. 218° (dec.). It is isomeric with corydine and possibly belongs to the aporphine group (Manske). ... 

---