Methane and Its Three-Dimensional Structure

Methane, the simplest hydrocarbon, is a gas of molecular formula CH₄, which occurs naturally in underground pockets in the petroleum-producing areas of the world. Colorless and odorless (the odor associated with natural gas in our homes is caused by the deliberate addition of traces of organic sulfur compounds), methane is an ultimate product of the decay of organic matter in the absence of air. It rises in bubbles from beneath the surface of swamps and marshes, hence its name "marsh gas." Since it burns cleanly and with the release of a great deal of heat (Fig. 1-1), it is one of the best and least polluting of all our common sources of energy.

Figure 1-1. The combustion of methane is one of the principal sources of energy in modern civilization. The combustion of 16 g of methane (1 mole) releases 213,000 cal of heat, enough to raise the temperature of 2.13 liters of water from 0 to 100°C.

Despite the ease with which it burns, methane is, by the standards of organic chemistry, unreactive. By this we mean that at ordinary temperatures methane combines only slowly or not at all with most chemical reagents. For example, it can be bubbled unchanged through concentrated acid or base and heated without effect with most oxidizing and reducing agents. Because the carbon in methane is combined with the maximum number of hydrogens possible, four, we say that it is a saturated hydrocarbon.

Under sufficiently vigorous conditions, for instance, in the heat of a match, saturated hydrocarbons will react. One of their most useful reactions is with the halogens, especially chlorine or bromine. If a sample of methane is mixed with chlorine gas, no reaction occurs, but in the presence of sunlight a reaction does take place and a mixture of organic compounds results (Fig. 1-2). Gaseous hydrogen chloride is also formed.

Figure 1-2. Methane reacts with chlorine in sunlight to form a mixture of chloro-, dichloro-, trichloro-, and tetrachloromethanes. Only one compound of each formula is known.

The various products of the reaction all have different boiling points (b.p.) and may be readily separated from one another and identified. As their molecular formulas indicate, the products obviously correspond to compounds in which successively one, two, three, and four hydrogen atoms of methane are replaced by chlorine atoms. These compounds may be named chloromethane (or, alternatively, methyl chloride), dichloromethane (or methylene chloride), trichloromethane (chloroform), and tetrachloromethane (or carbon tetrachloride). It is important to note that all of these products, as well as methane itself, have four bonds to the carbon atom. This tetravalence of carbon is characteristic of nearly all of its compounds. We shall see in the next section how the valences of the smaller atoms, including carbon, can easily be predicted from the electronic theory of bonding.

Next, consider the implications for the structure of methane that only one compound of each structure exists, that is, one methyl chloride, one methylene chloride, and so on. For example, the simplest explanation for the fact that only one molecule CH₃Cl exists is that hydrogens in CH₃, are equivalent, so that no matter which is replaced by chlorine, the same molecule results. Such would be the case, for instance, if methane were square (Fig. 1-3); but other possibilities exist, such as the tetrahedral model also shown in Fig. 1-3.

Figure 1-3. Two possible structures for methane and chloromethane which correctly predict that only one compound of the formula CH₃Cl should exist. In the square model all four bonds from carbon are in the same plane (that of the page). In the tetrahedral model, the solid lines indicate bonds in the plane of the page, the dotted line, behind the page, and the heavy line, in front.

Although both square and tetrahedral spatial formulas predict only one chloromethane, the square arrangement predicts that two dichloromethanes, CH₂Cl₂, should exist (Fig. 1-4). In one of these molecules the two chlorine atoms would be adjacent to each other, and in the other they would be on opposite sides of the carbon atom. Note that the distance between the two chlorine atoms would differ for these two compounds. The tetrahedral model, on the other hand, correctly predicts the existence of only a single dichloromethane. Some practice may be required until this can be clearly recognized. All aspects of the sterochemistry (isomerism due to spatial arrangement of atoms) of organic molecules may be explained only if it is assumed that whenever a carbon atom is surrounded by four atoms, these atoms are tetrahedrally arranged. This conclusion, first reached on strictly organic chemical grounds in 1874, has been fully confirmed by more recent physical methods.

Figure 1-4. (a.) If methane were square planar, two dichloromethanes, should exist.

(b.)The tetrahedral structure correctly predicts the existence of only one molecule with the formula CH₂Cl₂.

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