Hydrocarbon

A hydrocarbon is any chemical compound whose molecules are made up of nothing but carbon and hydrogen atoms.

Carbon atoms have the unique ability to form strong bonds to each other, atom after atom. Every hydrocarbon molecule is built upon a skeleton of carbon atoms bonded to each other either in the form of closed rings or in a continuous row like links in a chain. A chain of carbon atoms may be either straight or branched. In every case, whether ring or chain, straight or branched, all the carbon bonds that have not been used in tying carbon atoms together are taken up by hydrogen atoms attached to the carbon skeleton. Because there is no apparent limit to the size and complexity of the carbon skeletons, there is in principle no limit to the number of different hydrocarbons that can exist.

Hydrocarbons are the underlying structures of all organic compounds. All organic molecules can be thought of as being derived from hydrocarbons by substituting other atoms or groups of atoms for some of the hydrogen atoms and occasionally for some of the carbon atoms in the skeleton.

Gasoline

Probably the most important product of the fractional distillation of petroleum is gasoline, a mixture of alkanes containing six to ten carbon atoms in their molecules: hexane (C₆H₁₄), heptane (C₇H₁₆), octane (C₈H₁₈), nonane (C₉H₂₀), and decane (C₁₀H₂₂), plus small amounts of higher-molecular weight alkanes. More than six trillion gallons of gasoline are burned each year in the United States.

Gasoline must have certain properties in order to work well in automobile engines. If the gasoline-air mixture does not explode smoothly when ignited by the spark in the cylinder, that is, if it makes a fast, irregular explosion instead of a fast but gentle burn, then the explosive force will hit the piston too soon, while it is still trying to move down into the cylinder. This clash of illtimed forces jars the engine, producing a metallic clanking noise called a knock, which is especially audible when the engine is laboring to climb a hill. Extensive knocking can lead to serious engine damage, so gasolines are formulated to minimize this effect.

Of all the hydrocarbons that can be in gasoline, normal (straight-chain) heptane, C₇H₁₆, has been found to make auto engines knock worst. It has been assigned a value of zero on a scale of gasoline desirability. The hydrocarbon that knocks least is a branched-chain form of octane, C₈H₁₈, called iso-octane. It has been rated 100. Every gasoline blend is assigned an octane rating between zero and 100, according to how much knocking it produces under standard test conditions. Most automobile fuels sold have octane ratings above 85. High-octane gasolines that are even better than iso-octane because of anti-knock additives can have ratings above 100.

The C₆ to C₁₀ hydrocarbons make up only about 20-30% of crude oil, which is far from enough to supply the world's appetite for gasoline. But even if there were enough of it, the natural mixture has an octane rating of only about 40 to 60—not good enough for modern engines. Refineries therefore modify the natural mixture of molecules by breaking down big molecules into smaller ones (cracking) and by reshaping some of the smaller molecules into forms that knock less (reforming).

By the time gasolines get to the pump, they are no longer pure hydrocarbon mixtures; they have been blended with additives. Lead-containing antiknock compounds such as tetraethyl lead, Pb(C₂H₅)₄, are no longer used because lead is a toxic air pollutant; methyl-tert-butyl ether (MBTE) is used instead. Other additives remove harmful engine deposits, prevent gum formation, inhibit rusting, prevent icing, clean the carburetor, lubricate the cylinders, and dye the gasoline distinctive colors for identification purposes.