Ethers, identification aromatic

Purely aromatic ethers e.g., diphenyl ether), which are commonly encountered, are very hmited in number. Most of the aromatic ethers are of the mixed aliphatic - aromatic type. They are not attacked by sodium nor by dilute acids or alkahs. When hquid, the physical proper-ties (b.p., d . and ) are useful constants to assist in their identification. Three important procedures are available for the characterisation of aromatic ethers. 

In 1979, it was stated that poiybrominated aromatic ethers have received little attention (ref. 1). That statement is still applicable. Analyses to characterize this class of commercial flame retardants have been performed using UV (refs. 1-2), GC (refs. 1-6), and GC-MS (refs. 1-4). The bromine content of observed peaks was measured by GC-MS, but no identification could be made. The composition of poiybrominated (PB) diphenyl ether (DPE) was predicted from the expected relationship with polyhalogenated biphenyl, a class which has received extensive attention. NMR (refs. 3-6) was successfully used to identify relatively pure material which had six, or fewer, bromine atoms per molecule. A high performance liquid chromatography (HPLC) method described (ref. 1) was not as successful as GC. A reversed phase (RP) HPLC method was mentioned, but no further work was published. 

Ethers are unaffected by sodium and by acetyl (or benzoyl) chloride. Both the purely aliphatic ethers e.g., di-n-butyl ether (C4H, )30 and the mixed aliphatic - aromatic ethers (e.g., anisole C3HSOCH3) are encountered in Solubility Group V the purely aromatic ethers e.g., diphenyl ether (C,Hj)20 are generally insoluble in concentrated sulphuric acid and are found in Solubility Group VI. The purely aliphatic ethers are very inert and their final identification may, of necessity, depend upon their physical properties (b.p., density and/or refractive index). Ethers do, however, suffer fission when heated with excess of 67 per cent, hydriodic acid, but the reaction is generally only of value for the characterisation of symmetrical ethers (R = R ) ... 

Other preliminary experiments on alkali lignin included oxidations by barium peroxide and alkali (5, 6), alkali fusion, and alkali fusions in the presence of calcium peroxide, sodium borate perhydrate, and monopersulfate compound. Ether extractives and water extractives were examined, but in all cases too many of the oxidation products obtained were new and unidentifiable, and it was impossible to evaluate the experiments adequately with the available techniques. Vanillic acid appeared to be the chief oxidation product under conditions which did not demethylate further or destroy the aromatic nature of the oxidation products. Some oxidation conditions yielded p-hydroxybenzyl moieties as products, and some gave no trace of these products whatever. More detailed studies of the ether-insoluble, water-soluble components of the several oxidation mixtures were postponed until adequate procedures were developed for analytical isolation and identification. 

The conditions of GLC for the methyl ether and methyl ester fractions have been selected to allow analysis of a wide range of molecular weights with a resultant loss of complete resolution of closely related compounds. In addition, identification of individual compounds in the various fractions has been directed exclusively to aromatic components. Results pertaining to methyl ethers are discussed first, followed by those for methyl esters. 

Identification as a previously reported ether is accomplished through the usual comparison of physical properties. This can be confirmed by cleavage with hot concentrated hydriodic acid (Sec. 17.7) and identification of one or both products. Aromatic ethers can be converted into solid bromination or nitration products whose melting points can then be compared with those of previously reported derivatives. 

It would appear that most of the benzene ring structures in humic substances have two or more substituents, such as carbonyl, carboxyl, hydrocarbon, hydroxyl, and methoxyl and other ether functional groups. The evidence from identification of the products of degradation reactions, and more recently from NMR data, suggests that aliphatic hydrocarbons and aliphatic and aromatic ether groups link the core components in the macromolecules, and that carbonyl, carboxyl, and hydroxyl substituents are likely to be attached to some of the aliphatic hydrocarbons (Hayes and Swift, 1978). 

The various classes of volatile organic compounds have different distribution coefficients, which aid in their separation and identification. Two equilibrations transfer all alkanes and cycloalkanes into the gas phase, leaving aromatics in the water. Alcohols, acids, aldehydes, and ethers partition little to the gas phase and generally do not interfere in hydrocarbon analyses. If present in amounts that interfere, they can be identified as nonhydrocarbons by their distribution coefficients. They can be analyzed if desired by greatly increasing the gas-to-water ratio. 

Ethers can be prepared under mild conditions from aromatic halogen compounds that contain ortho- or para-nitro groups.772"775 Alkyl 2,4-di-nitrophenyl ethers are obtained from 1 -fluoro-2,4-dinitrobenzene and alcohols in the presence of triethylamine,776 a reaction that can be used for identification of alcohols. 

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