Separating Mixtures of Organic Compounds

The common basis of the procedures most often used to separate mixtures of organic compounds is the difference in polarity that exists or may be induced in the components of the mixture. This difference in polarity is exploited in nearly all the separation techniques, including distillation, recrystallization, extraction, and chromatography. The greatest differences in polarity, which make for the simplest separations, are those that exist between salts and nonpolar organic compounds. Whenever one or more of the components of a mixture is convertible to a salt, it can be separated easily and efficiently from the nonpolar components by distillation (Chap. 4) or extraction (Chap. 5). 

Assuming that a mixture contains a carboxylic acid, a phenol, an amine, and a neutral compound, each of which is water-insoluble, the separation is initiated by dissolving the mixture in a suitable organic solvent such as diethyl ether. The ethereal solution of the mixture is first extracted with sodium bicarbonate solution, which removes the carboxylic acid by converting it to its water-soluble sodium salt. This extraction is followed by one with sodium hydroxide solution, which removes the water-soluble sodium salt of the phenol. Finally, the ethereal solution of the mixture is treated with hydrochloric acid, which reacts with the amine, converting it into a water-soluble ammonium salt. The ethereal solution that remains after removal of the aqueous solution contains the neutral compound. 

As you can see, each extraction is performed on the same ethereal solution that originally contained all the components of the mixture, and the sequence of extraction, namely, NaHC03, then NaOH, and finally HCl, is extremely important. The bases and acid each remove one type of organic compound from the mixture and leave the neutral compound in the other layer when the extractions have been completed. Each of the basic and acidic extracts is subsequently treated with acid or base to liberate the carboxylic acid, phenol, and amine from its salt each of these compounds is then removed from the respective aqueous solutions by extraction with ether or by collecting the solid by vacuum filtration. 

Many different layers and solutions are obtained in the experimental procedure that follows. This can lead to much confusion unless the flasks containing 

General scheme for separating a simple mixture of water-insoluble compounds. 

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If organic compounds occur in mixtures, separation of the mixture often must be done before the individual components can be identified. Techniques such as gas chromatography, liquid chromatography, and capillary electrophoresis are often used to separate mixtures of organic compounds prior to identification of the components. These methods are discussed in Chapters 11-13. 

We must now move on to consider how we can separate the various components contained in one layer after solvent separation and washing. There are several possible techniques that we could use to separate mixtures of organic compounds, and all have the added advantage that they also purify the compound at the same time. 

SCHEME FOR THE SEPARATION OF SIMPLE BINARY MIXTURES OF ORGANIC COMPOUNDS... 

The general method to be adopted for the analysis of mixtures of organic compounds is to separate them into their components and to... 

Separations based upon differences in the physical properties of the components. When procedures (1) or (2) are unsatisfactory for the separation of a mixture of organic compounds, purely physical methods may be employed. Thus a mixture of volatile liquids may be fractionally distilled (compare Sections 11,15 and 11,17) the degree of separation may be determined by the range of boiling points and/or the refractive indices and densities of the different fractions that are collected. A mixture of non-volatile sohds may frequently be separated by making use of the differences in solubilities in inert solvents the separation is usually controlled by m.p. determinations. Sometimes one of the components of the mixture is volatile and can be separated by sublimation (see Section 11,45). 

It is now common to couple an instrument for separating a mixture of organic compounds e.g. using gas chromatography (GC) or high performance liquid chromatography (HPLC), directly to the input of a mass spectrometer. In this way, as each individual compound is separated from the mixture, its mass spectrum can be recorded and compared automatically with the library of known compounds and identified immediately if it is a known compound. 

Chromatographic Analysis.—Chromatographic adsorption-analysis, the most delicate method of separation of closely related compounds, depends on the simultaneous adsorption and separation of mixtures of organic compounds, such as natural dyes, biochemical products, isomerides, hydrocarbons, etc., in suitable solvents such as petroleum, ether, chloroform, carbon disulphide and water. 

Nanotubes can also be filled with a variety of molecules to create nanometre-scale filters. These filters can be used to remove bacteria and viruses from water including the poliovirus, which is less than 30 nm in diameter [966,967]. Nanotube filters made from carbon have the disadvantage of being more brittle, but have the advantage of being much more heat-resistant (they can be autoclaved) when compared with conventional polymer or cellulose filters [966]. In addition to water purification, nanotube filters can be used to separate components from mixtures of organic compounds [966]. 

Example Ethanol is separated from a mixture of organic compounds by gas chromatography. The concentration of each component is proportional to its peak area. However, the chromatograph detector has a variable sensitivity from one run to the next. Is an internal standard required to determine the concentration of ethanol ... 

Z. Luo, Q. Zhang, Y. Oderaotoshi, and D. P. Curran, Fluorous mixture synthesis A fluorous-tagging strategy for the synthesis and separation of mixtures of organic compounds, Science, 291 (2001), 1766-1769. 

Since these bisulfite addition compounds are ionic water-soluble compounds and can be formed in up to 90% yield, they serve as a useful means of separating aldehydes and methyl ketones from mixtures of organic compounds. At high sodium bisulfite concentrations these adducts crystallize and can be isolated by filtration. The aldehyde or ketone can be regenerated by adding either a strong acid or base ... 

Before the complex mixtures of organic compounds in sediment can be analyzed, they must first be extracted and separated into fractions with similar chemical features. Organic solvents which do not alter chemical structures (such as hydrocarbons or chloroform) can extract only a small to moderate amount of the organic matter from sediments. If the initial organic matter has numerous hydrophilic functional groups, as is the case in recent sediments, it is more soluble in aqueous solutions, particularly when the pH and ionic strength are modified by addition of alkaline or acidic substances. 

Continuous separation of different isomeric mixtures of organic compounds has been studied by means of a HFCLMP. Detailed description of HFCLMP system and application data are presented in Chapter 5 and... 

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