Chemistry: Structural Formulas

The sixth element in the periodic table, carbon, has the electron configuration 1 s² 2 s² 2 p² and, thus, has 4 valence electrons in the unfilled orbitals of its second electron shell. To fill these orbitals to a stable set of 8 valence electrons, a single carbon atom may share electrons with 2, 3, or even 4 other atoms. No other element forms such strong bonds to as many other atoms as carbon does. Moreover, multiple carbon atoms readily link together with single, double, or triple bonds. These factors make element number 6 unique in the entire periodic table. The number of carbon-based compounds is many times greater than the total of all compounds lacking carbon.

All types of life are based on carbon compounds, so the study of the chemistry of carbon is called organic chemistry. You should realize, however, that organic compounds are not necessarily derived from plants and animals. Hundreds of thousands of them have been synthesized (built) in the laboratory from simpler substances.

Figure 1 is an illustration of propane, one of the simplest organic compounds:

Figure 1

The structural formula of propane.

This representation is called a structural formula, in which lines depict two electron bonds between atoms. Look at the propane structure and observe that the 4 bonds to each carbon complete its valence orbitals with 8 electrons.

In the diagram of propane, the most important feature is the chain of 3 carbons. Such carbon-carbon bonding is what generates the incredible variety of organic compounds. This linkage of carbon atoms can continue without limit. Just as propane has 3 bonded carbons, you can imagine organic compounds with 4 or 5 or 500 carbons in an extensive chain or network.

The structural formula for propane shows 3 axial carbon atoms and 8 peripheral hydrogen atoms. The composition of propane can be more compactly expressed as C₃ H₈. This representation is a molecular formula. Such a formula does not directly tell how the various atoms are interbonded.

Compare two different compounds that have 4 linked carbon atoms. Refer to Figure 2 .

Figure 2

Isomers of C₄H₁₀.

Although these two compounds have the same molecular formula (and, therefore, have identical chemical compositions), their structural formulas reveal a difference in the way that the 4 carbons are assembled. Structure is just as essential as composition in organic chemistry.

The two varieties of C₄H₁₀ are called isomers, meaning that they have the same composition but differing structures. Structure affects both the physical properties and chemical reactivity of isomers. In the example of C₄H₁₀ isomers, both exist as gases at room temperature, but they can easily be condensed to liquids by cooling or compression. The two liquids have different temperatures at which they boil. (See Table 1 .)

TABLE 1

Structure and Boiling Point

*Isomer*	*Boiling Point*
Butane	−1° C
Isobutane	−12° C

The boiling behavior is consistent with their structures. The longest carbon chain in butane is 4 atoms, whereas the longest such chain in isobutane is only 3 atoms. The more compact molecules of isobutane escape from the liquid more readily, so the more volatile isobutane has a lower boiling point.

Chemists frequently write condensed structural formulas that omit the carbon-hydrogen bonds, as shown in Figure 3 .