The alkanes are also called the saturated hydrocarbons, because all the bonds that are not used to make the skeleton itself are filled to their capacity—saturated—with hydrogen atoms. They are also known as the paraffin hydrocarbons, from the Latin parum affinis, meaning "little affinity," because these compounds are not very chemically reactive.
The three smallest alkane molecules, containing one, two, and three carbon atoms, are shown in three ways.
The structural formulas are one way in which simple organic molecules can be depicted in two dimensions on paper; each line indicates a single covalent bond-a shared pair of electrons. The three-dimensional ball-and-stick models and space-filling models, in which the balls represent the carbon and hydrogen atoms (roughly to scale) and the sticks represent the bonds, are used by chemists to study the shapes of molecules.
The names and formulas of the first eight normal (not branched) alkanes are: Methane (CH4); Ethane (C2H6); Propane (C3H8); Butane (C4H10); Pentane (C5H12); Hexane (C6H14); Heptane (C7H16); Octane (C8H18).
While the first four alkanes were named before their structures were known, the rest have been named with Greek roots that tell how many carbon atoms there are in the chain: pent = five, hex = six, and so on, all ending in the "family name," -ane. The chemical formula of an alkane hydrocarbon can be obtained quickly from the number of carbon atoms, n, in its skeleton: the formula is CnH2n+2 . This method works because every carbon atom has two hydrogen atoms attached except for the two end carbon atoms which have two extra ones. As an example, the formula for pentane is C5H12.
The branched alkanes are named by telling what kinds of branches—methyl, ethyl or propyl groups, etc.—are attached to the main chain and where. For example,
is named 2-methyl pentane; the 2 indicates that the methyl group (-CH3) branches off the second carbon atom from the nearest end of the pentane chain.
The four lightest normal alkanes, having the smallest (lowest molecular weight) molecules, are gases at room temperature and pressure, while the heavier ones are oily liquids, and still heavier ones are waxy solids. Alkanes, which are the major constituents of crude oil, do not mix with water and float on its surface. The wax that we call paraffin and make candles from is a mixture of alkanes containing between 22 and 27 carbon atoms per molecule.
All hydrocarbons burn in air to form carbon dioxide and water. Methane, CH4, as the major constituent of natural gas, is widely used as a heating fuel. Also known as marsh gas, methane occurs naturally in marshes and swamps, being produced by bacteria during the decomposition of plant and animal matter. It can form explosive mixtures with air, however, and is therefore a hazard when present in coal mines. On the positive side, bacteria-produced methane has prospects for being developed as a commercial source of fuel.
Propane, C3H8, and butane, C4H10, are compressed into tanks, where they liquefy and can be used as portable fuels for such applications as barbecue grills, mobile-home cooking, and disposable cigarette lighters. Because these compounds are pure and burn cleanly, they are being explored as fuels for non-polluting automobile engines. They are often referred to as LPG-liquefied petroleum gas.
Cycloalkanes are alkanes whose carbon atoms are joined in a closed loop to form a ring-shaped molecule. Cyclopropane which contains three carbon atoms per molecule has molecules that are in the shape of a three-membered ring, or triangle. Cyclohexane, with six carbon atoms, has hexagonal molecules; it is used as a good solvent for many organic compounds.