D8.6 Petroleum Chemistry

Petroleum (from Latin, petra: “rock”, oleum: “oil”) consists primarily of naturally occurring hydrocarbons, predominantly alkanes and cycloalkanes. The alkane chains can be quite long, and properties such as melting point and boiling point usually vary smoothly and predictably as a function of the number electrons in various linear alkane molecules.

Alkane Molecular Formula Number of Electrons Melting Point (°C) Boiling Point (°C) Phase at Room Temperature
methane CH4 10 –182.5 –161.5 gas
ethane C2H6 18 –183.3 –88.6 gas
propane C3H8 26 –187.7 –42.1 gas
butane C4H10 34 –138.3 –0.5 gas
pentane C5H12 42 –129.7 36.1 liquid
hexane C6H14 50 –95.3 68.7 liquid
heptane C7H16 58 –90.6 98.4 liquid
octane C8H18 66 –56.8 125.7 liquid
nonane C9H20 74 –53.6 150.8 liquid
decane C10H22 82 –29.7 174.0 liquid
tetradecane C14H30 114 5.9 253.5 solid
octadecane C18H38 146 28.2 316.1 solid
Table: Melting and boiling points of alkanes.

Petroleum is the main source of hydrocarbon fuels, such as LP gas, gasoline, and fuel oil. These are separated by fractional distillation, a process in which petroleum is boiled and the different hydrocarbons condense to liquids at different temperatures (Figure below). Fractional distillation takes advantage of the differences in boiling-point of the various component substances. The different boiling points arise from the differences in the London dispersion forces between molecules.

This figure contains a photo of a refinery, showing large columnar structures. A diagram of a fractional distillation column is also shown. Near the bottom of the column, an arrow pointing into the column from the left shows a point of entry for heated crude oil. The column contains several layers at which different components are removed. At the very bottom, residue materials are removed through a pipe as indicated by an arrow out of the column. At each successive level, different materials are removed through pipes proceeding from the bottom to the top of the column. In order from bottom to top, these materials are fuel oil, followed by diesel oil, kerosene, naptha, gasoline, and refinery gas at the very top. To the right of the column diagram, a double sided arrow is shown that is blue at the top and gradually changes color to red moving downward. The blue top of the arrow is labeled, “Small molecules: low boiling point, very volatile, flows easily, ignites easily.” The red bottom of the arrow is labeled, “Large molecules: high boiling point, not very volatile, does not flow easily, does not ignite easily.”
Figure: Fractional Distillation. In a column for the fractional distillation of crude oil, oil heated to about 425 °C in a furnace vaporizes when it enters the base of the tower. The vapors rise up the tower, cooling at each successive level. At first, mixtures of compounds with higher boiling points condense and are drawn off. As the vapors gradually cool, mixtures with lower boiling points condense to liquids. All compounds in a mixture with about the same boiling point are called a petroleum fraction. (credit left: modification of work by Luigi Chiesa)

Gasoline is a liquid mixture of linear and branched alkanes, each containing five to twelve carbon atoms. Gasoline often contain various additives to improve its performance as a fuel. Kerosene, diesel fuel, motor oil, and fuel oil are primarily mixtures of alkanes made of larger molecules with more electrons than gasoline molecules.

Provided there is plenty of oxygen available, combustion converts almost all the carbon in the alkane fuel to carbon dioxide and water. Thus combustion of alkanes invariably adds water vapor and CO2 to the atmosphere—a human contribution to global warming.

Because there is greater demand for gasoline than for other components of petroleum, catalytic cracking is used in petroleum refining to break up larger molecules into smaller molecules, some of which are within the gasoline range of 5–12 carbon atoms. Catalytic cracking involves temperatures of 480–550 °C and a catalyst—conditions that can break (crack) carbon-carbon bonds and rearrange molecular structures. The hydrocarbon molecules are broken up in a fairly random way to produce mixtures of smaller molecules, some of which have carbon-carbon double bonds. One possible reaction involving C15H32 might be:

C15H32 ⟶ 2 C2H4 (ethene) + C3H6 (propene) + C8H18 (octane)

The alkene products, ethene and propene, are important for producing other organic chemicals. The octane product is a component of gasoline. Note that catalytic cracking involves temperatures higher than fractional distillation as well as a catalyst in order to break carbon-carbon bonds (as opposed to overcoming LDFs between hydrocarbon molecules).

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Chemistry 109 Fall 2021 Copyright © by John Moore; Jia Zhou; and Etienne Garand is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.