Saturated/unsaturated compounds separation

The principal direct appHcation of furfural is as a selective solvent. It is used for separating saturated from unsaturated compounds in petroleum refining, for the extractive distillation of butadiene and other hydrocarbons in the manufacture of synthetic mbber and for the production of... 

Reagents which selectively retard certain chemical species can be incorporated into a thin-layer plate. Thus, silver nitrate, which forms weak 7t-complexes with unsaturated compounds, aids their separation from saturated compounds. 

Entries 25-31 show the results for the separation of saturated and unsaturated compounds. Although these compounds have similar molecular structures, the unsaturated compounds, except for the case of entry 30, were found to... 

James and Martin [14] were the first to use GC for checking the separation of methyl ethers of saturated fatty acids from those of the unsaturated acids in a separation method involving the use of lead salts. As a result of the use of the bromination reaction the molecular weight of the products formed from unsaturated compounds increases abruptly, and the products do not emerge from the column and do not appear on the chromatogram. Bromination was also used for isolating unsaturated compounds and to improve their separation from saturated compounds by GC [15]. 

Isolation of the hydrocarbons from other lipids The total lipid extract may be subjected to removal of elemental sulphur by passage through an activated copper column (Blumer, 1957) and then to chromatographic separation on adsorbent columns or thin layer plates. For column chromatography, silic el is used with a short alumina bed on the top of the silic el. Both adsorbents should be partially deactivated by the addition of water (2—5%) to prevent the formation of artifacts (Blumer, 1970). Elution with a non-polar solvent such as hexane or pentane and subsequently with mixtures of non-polar and polar solvents, e.g. benzene and methanol, permits the isolation of several fractions containing saturated, unsaturated, aromatic hydrocarbons and more polar compounds (methyl esters, alcohols, acids, phenols and heterocyclic compounds). The interference from esters encountered in the isolation of aromatic hydrocarbons can be avoided prior to separation by saponification of the esters of fatty acids, which are easily removed. 

The organization is fairly classical, with some exceptions. After an introductory chapter on bonding, isomerism, and an overview of the subject (Chapter 1), the next three chapters treat saturated, unsaturated, and aromatic hydrocarbons in sequence. The concept of reaction mechanism is presented early, and examples are included in virtually all subsequent chapters. Stereoisomerism is also introduced early, briefly in Chapters 2 and 3, and then given separate attention in a fuU chapter (Chapter 5). Halogenated compounds are used in Chapter 6 as a vehicle for introducing aliphatic substitution and elimination mechanisms and dynamic stereochemistry. 

The nature of the carrier gas strongly affects the separation of model mixtures. A six-component mixture of hydrocarbon gases can be separated completely on sodium zeolite at a column temperature of 20 C with carbon dioxide, whereas the conventional carrier gases such as hydrocarbon and helium are not able to separate this mixture (see Fig. 5-28). It is interesting to note that, in contrast to carbon dioxide, use of helium or nitrogen as carrier gas with this form of zeolite does not result in a temperature inversion of the elution order propane-ethylene and butane-propylene, i.e., an unsaturated compound is eluted after the corresponding saturated compound at any column temperature. 

The different absolute retention times will be actual reaction times of a possible hydrogenation reaction in the column. The peak area of propyne was normalized to a saturated hydrocarbon, propane, and listed in Table 7-1. From the results it is easy to see that for unsaturated compounds no hydrogenation occurs at temperatures of 200 °C. On using hydrogen as the carrier gas hydrocarbons can be separated up to n-decane. 

Mixtures of saturated and unsaturated members of a lipid class are fractionated on silver nitrate-impregnated layers according to number, configuration and position of the double bonds. Homologous saturated or unsaturated compounds cannot be separated from one another. 

Saturated and unsaturated compounds are not separated on boric acid-containing adsorbents. From Fig. 142d it is seen that saturated and unsaturated dihydroxy-esters are fractionated according to both the number of double bonds and the configuration of the hydroxyl groups, when chromatographed on a silica gel G layer, impregnated with both silver nitrate and boric acid. 

Solvents. The chromatograms are developed stepwise as a rule Petroleum ether (60—70° C)-diethyl ether (80 + 20) is suited to separation of saturated neutral lipids from the mercuric acetate adducts of the corresponding unsaturated compounds [119]. This solvent migrates 15 to 18 cm in 1.5—2 h on silica gel G layers. A second solvent, n-pro-panol-acetic acid (100+ 1), serves for fractionating the adducts of Upids with one, two, three or more double bonds. It migrates 12—14 cm in 3—4 h. The saturated compounds are found between the two solvent fronts (see Fig. 144). Hexane-dioxan (60 + 40) is better for separating adducts of neutral lipids which contain three, four and more double bonds, on silica gel G it migrates about 15 cm in an hour [219]. 

Thin-layer chromatographic separation of saturated lecithins from the adducts of the corresponding unsaturated compounds has facilitated enzymatic structure analysis of egg-, soya- and milk lecithins and also the gas chromatographic determination of their fatty acid composition [14]. 

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