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Hydrocarbons Crude oil is a liquid that comes from reservoirs below the earth's surface. It is called crude oil because it must be processed or "refined" into useable products like gasoline. Natural gas is a gas or vapor that is also stored in reservoirs below the ground. Crude oil is often called petroleum. Crude oil and natural gas are composed of molecules containing carbon and hydrogen atoms. Hydrogen is most commonly found in water which is composed of two hydrogen atoms and one oxygen atom. The chemical formula for water is written as H2O. The carbon atom, represented by the letter C, is found in the mineral coal or in the carbon dioxide which we exhale when we breathe. Carbon dioxide is composed of two atoms of oxygen and one atom of carbon. The chemical formula for carbon dioxide is written as CO2. Molecules that are formed by the union of carbon and hydrogen atoms are called hydrocarbons ("hydrogen" and "carbons"). Large hydrocarbon molecules, such as those found in petroleum, are formed by joining or bonding of many hydrogen and carbon atoms. The properties of these hydrocarbons depend upon the number of and arrangement of the carbon atoms in their molecules. Crude oil is composed of not just one molecule but a mixture of many molecules composed of different numbers of carbon and hydrogen atoms.
Ethylene is composed of two carbon atoms but only four hydrogen atoms. The difference between these two molecules is that in the case of ethane the carbon atoms are tied together with a single atomic bond while in the case of ethylene the carbon atoms are joined by a double bond. Compounds consisting solely of carbon and hydrogen atoms joined together by single bonds are called saturated hydrocarbons. Those that are tied together by double bonds are called unsaturated. Ethylene is an important chemical building block used to make plastics such as polyvinyl chloride for PVC piping, ethylene glycol for automotive antifreeze, and acetic acid for medicine. As more carbon atoms are added, the molecule's weight and properties change. A hydrocarbon with three carbon atoms is called propane, C3H8, which is used for heating, cooking and manufacturing chemicals. Many of us have had hamburgers and hot dogs cooked on propane grills. Four carbon atom chains form butane, C4H10, molecules. Butane is used to make gasoline, prepare chemicals and provide energy for heating and cooking. As we add more carbons the hydrocarbon becomes heavier and heavier and its properties change. We move from methane which is a gas to hexane which is a liquid. The components of crude oil include heavier hydrocarbons. We separate crude oil into fractions by distillation. As the hydrocarbons become heavier their boiling point increases and they produce more heat when they are burned. In distillation, fractions boiling at different temperatures are separated. Ethane boils at lower temperature than hexane. The distillation process allows use to refine crude oil into its different fractions to produce products like gasoline, jet fuel, diesel oil, home heating oil, residual fuel oil, lubricating oils, and asphalt.
Hydrocarbon Classifications Carbon atoms linked together in chains are called paraffins. Paraffin chains can have from one to one hundred carbon atoms. Paraffins having four or more carbon atoms can have different structures. The carbon atoms can be in a straight chain or form branches off of the main chain. The more carbon atoms that a paraffin contains the more form different structures it can contain. These different structures of the same compound are called isomers. Isomers have different properties. The butane molecule can be straight chain (normal or n-butane) and branched chain (iso or i-butane). Paraffins are also known as saturated hydrocarbons because they have the maximum number of hydrogen atoms. Olefins are chains of carbon atoms which include two carbon atoms attached with a double bond. These compounds are called unsaturated because they do not have the maximum number of hydrogen atoms. Olefins are not present in crude oil or natural gas but are formed when crude oil is processed in a refinery. When the carbon chains contain five or more carbon atoms, the carbons can form "rings" or "circles." These cyclic compounds are called cycloparaffins or naphthenes. The most common cycloparaffins is cyclohexane, C6H12, which is composed of six carbons joined together in the shape of a hexagon. These cyclical compounds can be modified even further by removing some hydrogen atoms and joining some of the carbon atoms with double bonds to form aromatics. Benzene, C6H6, is formed by removing six hydrogen atoms from cyclohexane. The most important aromatics are benzene, toluene, and xylene. Petroleum can also contain molecules containing oxygen, nitrogen, sulfur, and metals. Crude oil can contain up to 2% oxygen and 1% nitrogen. Petroleum also contains sulfur compounds that must be removed. The sulfur level can be as high as 5%. Sulfur compounds in crude oil give the oil a distinctive sour odor. Therefore if a crude contains high levels of sulfur, it is called a sour crude and if it has little or no sulfur it is called a sweet crude. Crude oil also contains metals such as nickel and vanadium. Natural gas contains sulfur, oxygen, nitrogen and carbon dioxide that are removed before the gas is sold.
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