Chemical Reactions of Alkanes. Combustion of Alkanes

As a class, alkanes generally are unreactive. The names saturated hydrocarbon, or "paraffin," which literally means "not enough affinity" [L. par(um), not enough, + affins, affinity], arise because their chemical "affinity" for most common reagents may be regarded as "saturated" or satisfied. Thus none of the C−H or C−C bonds in a typical saturated hydrocarbon, for example ethane, are attacked at ordinary temperatures by a strong acid, such as sulfuric acid (H2SO4), or by an oxidizing agent, such as bromine (in the dark), oxygen, or potassium permanganate (KMnO4). Under ordinary conditions, ethane is similarly stable to reducing agents such as hydrogen, even in the presence of catalysts such as platinum, palladium, or nickel.

However, all saturated hydrocarbons are attacked by oxygen at elevated temperatures and, if oxygen is in excess, complete combustion to carbon dioxide and water occurs. Vast quantities of hydrocarbons from petroleum are utilized as fuels for the production of heat and power by combustion, although it is becoming quite clear that few of the nations of the world are going to continue to satisfy their needs (or desires) for energy through the use of petroleum the way it has been possible in the past.

Petroleums differ considerably in composition depending on their source. However, a representative petroleum1 on distillation yields the following fractions:

1. Gas fraction, boiling up to 40^o, contains normal and branched alkanes from C1 to C5. Natural gas is mainly methane and ethane. "Bottled" gas (liquefied petroleum gas) is mainly propane and butane.
2. Gasoline, boiling point from 40^o to 180^o, contains mostly hydrocarbons from C6 to C10. Over 100 compounds have been identified in gasoline, and these include continuous-chain and branched alkanes, cycloalkanes, and alkylbenzenes (arenes). The branched alkanes make better gasoline than their continuous-chain isomers because they give less "knock" in high-compression gasoline engines.
3. Kerosine, boiling point 180^o to 230^o, contains hydrocarbons from C11 to C12. Much of this fraction is utilized as jet engine fuels or is "cracked" to simpler alkanes (and alkenes).
4. Light gas oil, boiling point 230^o to 305^o, C13 to C17, is utilized as diesel and furnace fuels.
5. Heavy gas oil and light lubricating distillate, boiling point 305^o to 405^o, C18 to C25.
6. Lubricants, boiling point 405^o to 515^o, C26 to C388, familiarly encountered as paraffin was and petroleum jelly (Vaseline).
7. The distillation residues known as asphalts.

The way in which petroleum is refined and the uses for it depend very much on supply and demand, which always are changing. However, the situation for the United States in 1974 is summarized in Figure 4-3, which shows roughly how much of one barrel of oil (160 liters) is used for specific purposes.

Figure 4-3: Sources and uses of petroleum in the United States in 2011.

In the past three decades, petroleum technology has outpaced coal technology, and we now are reliant on petroleum as the major source of fuels and chemicals. Faced with dwindling oil reserves, however, it is inevitable that coal again will become a major source of raw materials. When coal is heated at high temperatures in the absence of air, it carbonizes to coke and gives off a gaseous mixture of compounds. Some of these gases condense to a black viscous oil (coal tar), others produce an aqueous condensate called ammoniacal liquors, and some remain gaseous (coal gas). The residue is coke, which is used both as a fuel and as a source of carbon for the production of steel. The major component in coal gas is methane. Coal tar is an incredible mixture of compounds, mostly hydrocarbons, a substantial number of which are arenes. Coal and coal tar can be utilized to produce alkanes, but the technology involved is more complex and costly than petroleum refining. It seems inevitable that the cost of hydrocarbon fuel will continue to rise as supply problems become more difficult. And there is yet no answer to what will happen when the world's limited quantities of petroleum and coal are exhausted.