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Introduction and General Aspects

Hydrocarbon chemistry is essentially abiological organic chemistry although methane and fossil fuels and derivatives have biological origin.

1.1 Hydrocarbons and Their Classes

Hydrocarbons, as their name indicates, are molecular compounds of carbon and hydrogen. As such, they represent one of the most significant classes of organic compounds (i.e., of carbon compounds).1 In methane (CH4), the simplest saturated alkane, a single carbon atom is bonded to four hydrogen atoms. In the higher homologs of methane (of the general formula CnH2n+2), all atoms are bound to each other by single [(sigma (s), two-electron, two-center] bonds with carbon displaying its tendency to form C-C bonds. Whereas in CH4 the H/C ratio is 4, in C2H6 (ethane) it is decreased to 3, in C3H8 (propane) to 2.67, and so on. Alkanes can be straight chain (each carbon attached to not more than two other carbon atoms) or branched (in which at least one of the carbon is attached to either three or four other carbon atoms). Carbon atoms can be aligned in open chains (acyclic hydrocarbons) or can form rings (cyclic hydrocarbons).

Cycloalkanes are cyclic saturated hydrocarbons containing a single ring. Bridged cycloalkanes contain one (or more) pair(s) of carbon atoms common to two (or more) rings. In bicycloalkanes, there are two carbon atoms common to both rings. In tricycloalkanes, there are four carbon atoms common to three rings such as in adamantane (tricyclo[3.3.1.13,7]decane) giving a caged hydrocarbon structure.

Carbon can also form multiple bonds with other carbon atoms. This results in unsaturated hydrocarbons such as olefins (alkenes, CnH2n), specifically, hydrocarbons containing a carbon-carbon double bond or acetylenes (alkynes, CnHn-2) containing a carbon-carbon triple bond. Dienes and polyenes contain two or more unsaturated bonds.

Aromatic hydrocarbons (arenes), a class of hydrocarbons of which benzene is parent, consist of cyclic arrangement of formally unsaturated carbons, which, however, give a stabilized (in contrast to their hypothetical cyclopolyenes) delocalized p system.

The H/C ratio in hydrocarbons is indicative of the hydrogen deficiency of the system. As mentioned, the highest theoretical H/C ratio possible for hydrocarbons is 4 (in CH4), although in carbocationic compounds (the positive ions of carbon compound) such as CH5+ and even CH62+ the ratio is further increased (to 5 and 6, respectively). On the other end of the scale, in extreme cases, such as the dihydro or methylene derivatives of C60 and C70 fullerenes discovered in the 1980s, the H/C ratio can be as low as ~0.03!

An index of unsaturation (hydrogen deficiency) i can be used in hydrocarbons, whose value indicates the number of ring and/or double bonds (a triple bond is counted as two double bonds) present (C and H = the number of carbon and hydrogen atoms); i = 0 for methane, for ethene i = 1 (one double bond), for acetylene (ethyne) i = 2, etc.

The International Union of Pure and Applied Chemistry (IUPAC) has established rules to name hydrocarbons. Frequently, however, trivial names are also used and will continue to be used. It is considered not very important to elaborate on the question of nomenclature. Systematic naming is mostly followed. Trivial (common) namings are, however, also well extended. Olefins or aromatics clearly are very much part of our everyday usage, although their IUPAC names are alkenes and arenes, respectively. Straight-chain saturated hydrocarbons are frequently referred to as n-alkanes (normal) in contrast to their branched analogs (isoalkanes). Similarly, straight-chain alkenes are frequently called n-alkenes as contrasted with branched isoalkenes (or olefins). What needs to be pointed out, however, is that one should not mix the systematic IUPAC and the still prevalent trivial (or common) namings. For example, (CH3)2C=CH2 can be called isobutylene or 2-methylpropene but should not be called isobutene as only the common name butylene should be affixed by iso. On the other hand, isobutane is the proper common name for 2-methylpropane [(CH3)3CH]. We discuss, for example, the isobutane-isobutylene alkylation for production of isooctane (a major component of high-octane gasoline) but it should not be called isobutane-isobutene alkylation.

1.2 Energy-Hydrocarbon Relationships

Every facet of human life is affected by the need for energy. The sun is the central energy source of our solar system. The difficulty lies in converting solar energy into other energy sources and also to store them for future use. Photovoltaic devices and other means to utilize solar energy are intensively studied and developed but at the enormous level of our energy demands, Earth-based major installations using the present day technology are inadequate. The size of collecting devices would necessitate to utilize large areas of the earth. Atmospheric conditions in most of the industrialized world are unsuitable to provide constant solar energy supply. Perhaps a space-based collecting system beaming energy back to Earth can be established at some time in the future, but except small-to-medium-scale installations, solar energy is of limited significance for the foreseeable future. Other unconventional energy sources, such as wind, ocean wave, tides of the seas, and geothermal energy as well as energy from the combustion of biomass represent a rapidly increasing yet still small fraction of our energy production. Nevertheless, search for alternate renewable energy sources to produce clean, safe, and sustainable energy is vital for the future sustenance of mankind.

Our major energy sources are fossil fuels (i.e., oil, gas, and coal) as well as atomic energy. Fossil energy sources are, however, nonrenewable (at least on our timescale) and their burning causes serious environmental problems. Increased carbon dioxide levels are considered to contribute to the "greenhouse" effect. The major limitation, however, is the limited nature of our fossil fuel resources. The world total proven coal reserves at the end of 2015 were estimated to be 892,000 M/t lasting about 114 years at the current rates of consumption.2 (The timeframe for the United States with the largest coal reserve of 237,000 M/t is 292 years.) The corresponding data for total petroleum oil and natural gas are 1,697,600 million barrels (50.4 years) and 186.9 trillion cubic meters (52.8 years). In human history, these are short periods and we will need to find new solutions.

The United States still relies overwhelmingly on fossil energy sources, with only 8.3% coming from atomic energy and 9.6% from renewable sources (Table 1.1). Other industrialized countries utilize to a much higher degree of nuclear and hydroenergy2 (Table 1.2). Since the 1980s, concerns about safety and difficulties in disposing fission by-products dramatically limited the growth of the otherwise clean atomic energy industry.

Table 1.1 U.S. Energy Consumption by Sources (%)3

Table 1.2 Power Generated in Industrial Countries by Nonfossil Fuels (2010)

A way to extend the lifetime of our fossil fuel energy reserves is to raise the efficiency of thermal power generation. Progress has been made in this regard, but the heat efficiency even in the most modern power plants is limited. Heat efficiency increased substantially from 19% in 1951 to 38% in 1970, but for many years since then 39% appeared to be the limit. Combined-cycle thermal power generation-a combination of gas turbines and steam...