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Hydrocarbon Chemistry

John Wiley & Sons

Chapter One

1.1. HYDROCARBONS AND THEIR CLASSES

Hydrocarbons, as their name indicates, are compounds of carbon and hydrogen. As such, they represent one of the most significant classes of organic compounds (i.e., of carbon compounds). In methane (C[H.sub.4]) the simplest saturated alkane, a single-carbon atom, is bonded to four hydrogen atoms. In the higher homologs of methane (of the general formula [C.sub.n][H.sub.2n+2]) all atoms are bound to each other by single [(sigma ([sigma]), two-electron two-center] bonds with carbon displaying its tendency to form C-C bonds. Whereas in C[H.sub.4] the H : C ratio is 4, in [C.sub.2][H.sub.6] (ethane) it is decreased to 3; in [C.sub.3][H.sub.8] (propane), to 2.67; and so on. Alkanes can be straight-chain (with each carbon attached to not more than two other carbon atoms) or branched (in which at least one of the carbons is attached to either three or four other carbon atoms). Carbon atoms can be aligned in open chains (acyclic hydro-carbons) 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 suchas in adamantane (tricyclo[3.3.1] 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, [C.sub.n][H.sub.2n]), specifically, hydrocarbons containing a carbon-carbon double bond or acetylenes (alkynes, [C.sub.n][H.sub.n-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 [pi] 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 C[H.sub.4]), although in carbocationic compounds (the positive ions of carbon compound) such as C[H.sup.+.sub.5] and even C[H.sup.2+.sub.6] 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 recently (at the time of writing) discovered [C.sub.60] and [C.sub.70] fullerenes, 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, and so on:

i = (2C + 2) - H / 2

The International Union of Pure and Applied Chemistry (IUPAC) established rules to name hydrocarbons. Frequently, however, trivial names are also used and will continue to be used. It is not considered necessary to elaborate here 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, i-alkanes). Similarly straight-chain alkenes are frequently called n-alkenes as contrasted with branch 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, ([C[H.sub.3]).sub.2]C=C[H.sub.2] can be called isobutylene or 2-methyl-propene. It, however, 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 [[(C[H.sub.3]).sub.3]CH]. Consequently we discuss isobutane-isobutylene alkylation for production of isooctane: high-octane gasoline (but it should not be called isobutane-isobutene alkylation).

1.2. ENERGY-HYDROCARBON RELATIONSHIP

Every facet of human life is affected by our 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 level of our energy demands, Earth-based major installations by present-day technology are not feasible. The size of collecting devices would necessitate utilization of large areas of the Earth. Atmospheric conditions in most of the industrialized world are unsuitable to provide a 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 for small-scale installation, solar energy is of limited significance for the foreseeable future. Unfortunately, the same must be said about wind, ocean waves, and other unconventional energy sources.

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 (see Section 1.5). The most realistic estimates put our overall worldwide fossil resources as lasting for not more than 200 or 300 years, of which oil and gas would last less than a century. In human history this is a short period, and we will need to find new solutions. The United States relies overwhelmingly on fossil energy sources, with only 8% coming from atomic energy and 4% from hydro energy (Table 1.1).

Other industrialized countries utilize to a much higher degree of nuclear and hydroenergy (Table 1.2). Since 1980, concerns about safety and fission byproduct disposal difficulties, however, dramatically limited the growth of the otherwise clean atomic energy industry.

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 respect, 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-was allowed in Japan to further increase heat efficiency from 35-39% to as high as 43%. Conservation efforts can also greatly contribute to moderate worldwide growth of energy consumption, but the rapidly growing population of our planet (5.4 billion today, but should reach 7-8 billion by 2010) will put enormous pressure on our future needs.

Estimates of the world energy consumption until 2020 are shown graphically in Figure 1.1 in relationship to data dating back to 1960. A rise in global energy consumption of 50-75% for the year 2020 is expected compared with that for 1988. Even in a very limited growth economic scenario the global energy demand is estimated to reach 12 billion tons of oil equivalent (t/oe) by the year 2020.

Our long-range energy future clearly must be safe nuclear energy, which should increasingly free still remaining fossil fuels as sources for convenient transportation fuels and as raw materials for synthesis of plastics, chemicals, and other substances. Eventually, however, in the not too distant future we will need to make synthetic hydrocarbons on a large scale.

1.3. HYDROCARBON SOURCES AND SEPARATION

All fossil fuels (coal, oil, gas) are basically hydrocarbons, deviating, however, significantly in their H : C ratio (Table 1.3).

1.3.1. Natural Gas

Natural gas, depending on its source, contains-besides methane as the main hydrocarbon compound (present usually in concentrations >80-90%)-some of the higher homologous alkanes (ethane, propane, butane). In "wet" gases the amount of [C.sub.2]-[C.sub.6] alkanes is more significant (gas liquids). Typical composition of natural gas of various origin is shown in Table 1.4.

Natural-gas liquids are generally of thermal value only but can be used for dehydrogenation to alkenes. Their direct upgrading to gasoline-range hydrocarbons is also pursued. Natural gas as we know it is of biological origin (not unlike petroleum oil). Large gas reservoirs were discovered and utilized in the twentieth century. Increasingly deeper wells are drilled and deposits under the seas are explored and tapped. An interesting but as yet unproved theory by Gold holds that hydrocarbons may also be formed by slow outgassing of methane from vast deep deposits dating back to the origin of our planet. Besides biologically derived oil and gas, "deep" carbon compounds trapped in the Earth's crust are subject to intense heat, causing them to release hydrocarbons that migrate toward the Earth's surface, where they are trapped in different stratas. Seepage observed at the bottom of the oceans and finds of oil during drilling into formations (such as granite) where no "biogenic" oil was expected are cited as proof for "abiogenic" hydrocarbons. If abiogenic methane and other hydrocarbons exist (although most geologists presently disagree), vast new reserves would become available when improved drilling technology is developed to reach deeper into the Earth's crust.

Other vast yet untapped reserves of natural gas (methane) are locked up as hydrates under the permafrost in Siberia. Methane gas hydrates are inclusion compounds of C[H.sub.4]*n]H.sub.2]O composition. Their amount is estimated to equal or exceed known conventional natural-gas reserves. Their economical utilization, however, remains a challenge. Significant amounts [[less than or equal to] 500 million tons per year (Mt/y)] of natural methane is also released into the atmosphere from varied sources ranging from marsh lands to landfills, to farm animals. Methane in the atmosphere represents only a small component, although its increase can cause a significant greenhouse effect.

1.3.2. Petroleum or Crude Oil

Petroleum or crude oil is a complex mixture of many hydrocarbons. It is characterized by the virtual absence of unsaturated hydrocarbons consisting mainly of saturated, predominantly straight-chain alkanes, small amounts of slightly branched alkanes, cycloalkanes, and aromatics. Petroleum is generally believed to be derived from organic matter deposited in the sediments and sedimentary rocks on the floor of marine basins. The identification of biological markers such as petroporphyrins provides convincing evidence for the biological origin of oil (see, however, the abovementioned possibility for abiogenic "deep" hydrocarbons). The effect of time, temperature, and pressure in the geological transformation of the organics to petroleum is not yet clear. However, considering the low level of oxidized hydrocarbons and the presence of porphyrins, it can be surmised that the organics were acted on by anaerobic microorganisms and that temperatures were moderate, <200ºC. By comparing the elemental composition of typical crude oils with typical bituminous coals, it is clear why crude oil is a much more suitable fuel source in terms of its higher H : C atomic ratio, generally lower sulfur and nitrogen contents, very low ash content, (probably mostly attributable to some suspended mineral matter and vanadium and nickel associated with porphyrins), and essentially no water content.

Finally, it is interesting to mention that the most recent evidence shows that even extraterrestrially formed hydrocarbons can reach the Earth. The Earth continues to receive some 40,000 tons of interplanetary dust every year. Mass-spectrometric analysis revealed the presence of hydrocarbons attached to these dust particles, including polycyclic aromatics such as phenanthrene, chrysene, pyrene, benzopyrene, and pentacene of extraterrestrial origin (indicated by anomalous isotopic ratios).

Petroleum-a natural mineral oil-was referred to as early as in the Old Testament. The word petroleum means "rock oil" [from the Greek petros (rock) and elaion (oil)]. It had been found over the centuries seeping out of the ground, for example, in the Los Angeles basin (practically next door to where this review is written) and what are now the La Brea Tar Pits. Vast deposits were found in varied places ranging from Europe, to Asia, to the Americas, and to Africa. In the United States the first commercial petroleum deposit was discovered in 1859 near Titus-ville in western Pennsylvania when Edwin Drake and Billy Smith struck oil in their first shallow (~20-m-deep) well. The well yielded 400 gallons (gal) of oil a day (about 10 barrels). The area was known before to contain petroleum that residents skimmed from a local creek's surface, which was thus called "oil creek." The oil-producing first well opened up a whole new industry. The discovery was not unexpected, but provided evidence for oil deposits in the ground that could be reached by drilling into them. Oil was used for many purposes, such as in lamp illumination and even for medical remedies. The newly discovered Pennsylvania petroleum was soon also marketed to degrease wool, prepare paints, fuel steam engines, and power light railroad cars and for many other uses. It was recognized that the well oil was highly impure and had to be refined to separate different fractions for varied uses (see Section 1.4). The first petroleum refinery, a small stilling operation, was established in Titusville in 1860. Petroleum refining was much cheaper than producing coal oil (kerosene), and soon petroleum became the predominant source for kerosene as an illuminant. In the 1910s the popularity of automobiles spurred the production of gasoline as the major petroleum product. California, Texas, Oklahoma, and more recently Alaska provided large petroleum deposits in the United States, whereas areas of the Middle East, Asia, Russia, Africa, South America, and more recently of the North Sea became major world oil production centers.

The daily consumption of crude oil in the United States is about 16-17 million barrels.

Continues...

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Excerpted from Hydrocarbon Chemistry by George A. Olah Árpad Molnar Excerpted by permission.
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