The C1, and C2, hydrocarbons
Hydrocarbons are compounds that contain only carbon and hydrogen. We shall consider in this chapter the four simplest known hydrocarbonsthose with the lowest molecular weights-and we shall see that they represent three classes of compounds: the alkanes, in which each carbon atom has four single bonds; the alkenes, in which two carbon atoms are joined by a double bond (two electron pairs); and the alkynes, in which two carbon atoms are joined by a triple bond (three electron pairs). We shall also see that these classes of compounds are physically similar but chemically rather different.
There are four stable hydrocarbons of molecular weight 30 or less. They are all gases at room temperature, and analyses for carbon and hydrogen content coupled with determinations of their molecular weights show them to have the formulas CH4, , C2H6, C2H2,, and C2H2 . The first of these is methane, CH4,, whose physical properties and molecular shape were discussed in Chapter 1. The other three are all C2, compounds and are called, respectively, ethane, ethene, and ethyne (ethyne rhymes with brine). These are the systematic names approved by the International Union of Pure and Applied Chemistry, IUPAC1.' However, ethene is often called ethylene and ethyne called acetylene. It is to be hoped that both of these older names will pass out of use in time.
If the carbon atoms in each of the three C, compounds are tetravalent, then there is only one possible way to bond the atoms together in each case:
The tendency of carbon to form bonds at the tetrahedral angle results in compounds such as methane and ethane being nonplanar. The two-dimensional representation of ethane, above, is thus misleading and it is just as informative (and quicker) to write the formula as CH3-CH3, . (Or, indeed, as C2H6, since there is only one stable compound known with this formula. We shall see that with some C3 hydrocarbons and with all hydrocarbons having four or more carbons, some indication of structure is necessary because a designation such as C4,H8, is ambiguous, there being five known compounds with this formula.)
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Because of the importance of molecular structure in organic chemistry, we shall consider the three-dimensional shapes of these compounds in the next section.
molecular shape of CH4,, C2H6, C2H4, and C2H2
You can illustrate the shape of a tetrahedral molecule such as methane with ball-and-stick models (Figure 2.1)
With ethene and ethyne, the model's carbon-to-carbon bonds are constructed from stiff metal springs or flexible or curved plastic connectors because more than one bond exists between the carbon atoms (Figure 2-2).
These simple mechanical models are surprisingly good for predicting the shapes of molecules and, indeed, their reactivity. Ethene is known from spectroscopic measurements to be planar, and this is the shape the model naturally takes. The electronic analogy here is that the orbitals for each pair of electrons extend as far away from one another as possible. Ethyne, likewise, is known to be linear. The strain involved in making "bent bonds " for these models is reflected in a higher degree of chemical reactivity for these compounds than for ethane.
The arrangement of the linkages in the ethene model suggests that dne CH2, group cannot twist with respect to the other CH2, group without gross distortion from the favored geometry. We shall see that this conclusion, too, is borne out by chemical evidence (Section 2.6B). By contrast, the model of the saturated compound, ethane, suggests that free rotation should be possible about the single bond joining the two carbon atoms if the sticks representing the bonds are allowed to rotate in the holes of the balls representing the atoms. Such rotation is considered in more detail in the next section.