Introduction to Alkanes and Cycloalkanes

Petroleum is a finite resource whose origin is under debate. At the La Brea Tar Pits in Los Angeles, many prehistoric animals perished in a natural vat containing hydrocarbons. 

We noted earlier that the family of organic compounds called hydrocarbons can be divided into several gronps on the basis of the type of bond that exists between the individual carbon atoms. Those hydrocarbons in which all of the carbon-carbon bonds are single bonds are called alkanes, those hydrocarbons that contain a carbon-carbon double bond are called alkenes, and those with a carbon-carbon triple bond are called alkynes. 

Alkanes and cycloalkanes are so similar that many of their properties can be considered side by side. Some differences remain, however, and certain structural features arise from the rings of cycloalkanes that are more conveniently studied separately. We shall point out the chemical and physical similarities of alkanes and cycloalkanes as we go along. 

The primary source of alkanes is petroleum. Petroleum is a complex mixture of organic compounds, most of which are alkanes and aromatic hydrocarbons (cf. Chapter 14). It also contains small amounts of oxygen-, nitrogen-, and sulfur-containing compounds. 

The first step in refining petroleum is distillation the object here is to separate the petroleum into fractions based on the volatility of its components. Complete separation into fractions containing individual compounds is economically impractical and virtually impossible technically. More than 500 different compounds are contained in the petroleum distillates boiling below 200°C, and many have almost the same boiling points. Thus the fractions taken contain mixtures of alkanes of similar boiling points (see the table below). Mixtures of alkanes, fortunately, are perfectly suitable for uses as fuels, solvents, and lubricants, the primary uses of petroleum. 

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Introduction to Alkanes and Cycloalkanes 143 THE CHEMISTRY OF... Petroleum Refining 143... 

One method (EPA 8020) that is suitable for volatile aromatic compounds is often referred to as benzene-toluene-ethylbenzene-xylene analysis, although the method includes other volatile aromatics. The method is similar to most volatile organic gas chromatographic methods. Sample preparation and introduction is typically by purge-and-trap analysis (EPA 5030). Some oxygenates, such as methyl-f-butyl ether (MTBE), are also detected by a photoionization detector, as well as olefins, branched alkanes, and cycloalkanes. 

The analysis of n-alkanes is important in the oil industry, hence the linearity of retention on various columns was studied extensively. The range of Cj to C7 n-alkanes was examined for linearity of the retention time vs carbon number . However, the resolution power (effective plates) of packed columns is not sufficient if all isomer mixtures of the C2-C9 alkanes and cycloalkanes are to be studied. The introduction of the capillary GC column SCOT (Surface Coated Open Tubular), as early as 1960, was followed immediately by changes in analytical methods The SCOT columns were made from either stainless steel or glass, presenting the analytical chemist with ca 150,000 effective... 

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