Structural Formulas

The term Formula in chemistry can have a variety of meanings. The empirical formula of a compound is that which is obtained through laboratory research. By analyzing the chemical composition of a compound in the laboratory and knowing the atomic weights of the elements in the compound, one can calculate the simplest, or empirical, formula for that compound. For example, the analysis of ethane shows that the ratio of carbon to hydrogen atoms in the molecule is 1:3. Therefore, the empirical formula for ethane, is CH3.

If one also knows the molecular weight of a compound, the molecular formula for that compound can be determined. Knowing the molecular weight of ethane to be 30, for example, allows one to calculate the molecular formula for the compound to be C2H6.

Finally, the structural formula of a compound shows not only the kind and actual number of atoms, but also their arrangement in space. The structural formula for ethane, for example, is H H H - C - C - H H H.

Structural formulas are exceedingly important to chemists, especially organic chemists, since they make it possible to design schemes of synthesis and to predict the properties of compounds. The writing of structural formulas was not possible until at least some chemists were convinced that chemical formulas were more than a convenience in representing compounds, and that they could actually show the structure of atoms in a molecule. That situation developed in the late 1850s and early 1860s. The term chemical structure was first coined in 1861 by the Russian chemist Alexander Butlerov (1828-1892). The first true structural formulas were written by Friedrich Kekulé and Archibald Couper.

In 1857, Kekulé suggested that every carbon atom was capable of combining with ("bonding" to) four other atoms. The number four was specific and definite. A carbon atom never joined with one, two, three, five, or some other number of atoms. Further, Kekulé said, in many instances, carbon atoms joined with each other in long chains.

Kekulé represented the molecules formed in this way with small circles and ellipses that some colleagues jokingly referred to as "Kekulé's sausages." Couper's suggestion of using dotted lines and short dashed lines for the bonds between atoms was a much better system and was soon widely adopted. Other chemists struggled to find ways of representing the structure of molecules in three dimensions. Kekulé and Couper had shown how to depict the arrangement of atoms in a molecule in two dimensions, on a piece of paper. But it was obvious to others that molecules really exist in three dimensional space and had to be represented as such.

Among the first efforts to move in this direction was that of August Wilhelm von Hofmann (1818-1892). In 1865, Hofmann made models of molecules using sticks and croquet balls. However, those models showed only planar arrangements of atoms. The first real breakthrough in this field came with the work of Joseph Le Bel (1847-1930) and Jacobus van't Hoff.

Working independently, the two chemists developed the concept of a tetrahedral carbon atom. They pointed out that the four bonds in a carbon atom were directed in three dimensions, toward the four corners of a tetrahedron. A major success of this model was its ability to explain an unknown phenomenon of optical isomers that had been known since the time of Louis Pasteur. With the work of Le Bel and van't Hoff, the theory of structural formulas essentially reached the form in which it is used today.