The Structures and Classes of Hydrocarbons

In this section, we dissect one group of compounds down to their skeletons and see how to name and draw them. Hydrocarbons, the simplest type of organic compound, are a large group of substances containing only H and C atoms. Some common fuels, such as natural gas and gasoline, are hydrocarbon mixtures. Hydrocarbons are also important feedstocks, precursor reactants used to make other compounds. Ethylene, acetylene, and benzene, for example, are feedstocks for hundreds of other substances. 

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Structural Complexity of Organic Molecules Chemical Diversity of Organic Molecules The Structures and Classes of Hydrocarbons Carbon Skeletons and Hydrogen Skins Alkanes... 

One large and structurally simple class of hydrocarbons includes those substances in which all the carbon-carbon bonds are single bonds. These are called saturated hydrocarbons, or alkanes. In the alkanes the carbon atoms are bonded to each other in chains, which may be long or short, straight or branched. 

Pitch is a "pseudo solution" of a wide variety of different generic classes of hydrocarbons ranging from paraffins at one extreme to very highly aromatic species at the other. By using the Theory of Solubility for Non-Electrolytes, specific fractions can be isolated from a pitch by properly selecting a solvent system and extraction conditions. "Tailored" precursors for carbon fiber and other carbon products, such as carbon/carbon matrices and bulk graphites, can thus be obtained. The technique of extraction, the characteristics of different precursors, and the structure and properties of carbon fiber and composites made from solvent extracted precursors will be discussed. 

We have already met a number of the techniques that have been used to study the structure and reactivity of chemisorbed hydrocarbons. Table 4.1 lists most of the important ones together with a few references that will help the reader to access the wider literature, and to draw attention to the names of the scientists who have made major contributions to their use. It will be appreciated that many of them are only applicable to one class of material, i.e. either to single crystals or to finely divided metal particles techniques that are equally applicable to both classes (e.g. vibrational spectroscopy) are particularly valuable in enabling comparisons between the two classes to be drawn. 

There is already a very considerable literature on the application of theoretical procedures to our understanding of the chemisorbed states of hydrocarbons, and no doubt its growth is set to continue. At the outset we need to ask ourselves what expectations we have of such work, and what questions and problems we would like to see addressed. There would seem to be two different classes of question demanding attention. The first is a detailed and profound insight into the structure and bonding of individual molecules and their fragments on specific crystal planes or adsorption sites. The second is a more qualitative and broad-brush look over the whole held, to discern what factors may explain satisfactorily the trends we have observed, with particular reference to apparently anomalous behaviour. It may well be that in the course of time the questions of the second type will be answered more precisely by the quantitative methods now being applied to questions of the first type, and indeed one can see already a start in this direction. 

Compoimds that contain only carbon and hydrogen are called hydrocarbons. In the most basic class of hydrocarbons, each carbon atom is bonded to foiu other atoms. These compoimds are called alkanes. The three simplest alkanes, which contain one, two, and three carbon atoms, respectively, are methane (CH4), ethane (C2Hg), and propane (C3H8). The structural formulas of these three alkanes are as follows ... 

The projects discussed in Section 20.2 have been centred around the spectroscopic probing of the structure and dynamics of surfactant systems and lyotropic mesophases, in particular hydrocarbon gels. Three related investigations not yet discussed in detail due to limitations in space shall be briefly mentioned. These are the study of the conformation of surfactant molecules by surface enhanced Raman spectroscopy, the development of a lipophilic dye probe for surfactant systems, and an outlook onto a different class of reaction gels created by hydrolysis of metal alkoxide precursors. 

The compounds that are identifiable in the sea represent a vast array of biochemicals attributable to the life and death of marine plants and animals. They are generally grouped into six classes based on structural similarities hydrocarbons, carbohydrates, lipids, fatty acids, amino acids, and nucleic acids. Because they represent compounds that can be quantified and understood for their chemical properties and known role in biological systems, a great deal of information has been accumulated over the years about these groups and the specific compounds found within them.7... 

Example The El mass spectmm of -decane is typical for this class of hydrocarbons (Fig. 6.18a). Branching of the aliphatic chain supports cleavage of the bonds adjacent to the branching point, because then secondary or tertiary carbenium ions and/or alkyl radicals are obtained (Fig. 6.18b,c). This allows for the identification of isomers to a certain degree. Unfortunately, hydrocarbon molecular ions may undergo skeletal rearrangements prior to dissociations, thereby obscuring structural information. 

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