It is necessary to draw structural formulas for
organic compounds because in most cases a molecular formula does not uniquely
represent a single compound. Different compounds having the same molecular
formula are called isomers, and the prevalence of organic isomers
reflects the extraordinary versatility of carbon in forming strong bonds to
itself and to other elements.
The procedure for
drawing structural formulas rests on simple covalent bond theory (sharing of
electron pairs) and the octet rule. A full discussion of these important topics
can be found in most introductory chemistry texts. It is recommended that a
novice first draw Lewis structures, showing the shared and unshared valence
shell electrons in a clear and unambiguous fashion. Next, each shared electron
pair (covalent bond) can be replaced by a line joining the bonded atoms to give
a Kekulé formula. A single line represents a s-covalent
bond. Two lines (a double bond) represent one s-bond
and one p-bond, and three lines (a triple bond)
indicate one s-bond and two p-bonds. Some simple examples of such an approach are shown
in the following table.
| Common Name | Molecular Formula | Lewis Formula | Kekulé Formula |
|---|
| Methane | CH4 |  |
| Ammonia | NH3 | |
| Ethane | C2H6 | |
| Methyl Alcohol | CH4O | |
| Ethylene | C2H4 | |
| Formaldehyde | CH2O | |
| Acetylene | C2H2 | |
| Hydrogen Cyanide | CHN |
Further simplification of structural formulas may be achieved without any loss of the information they convey. In condensed structural formulas the bonds to each carbon are omitted, but each distinct structural unit (group) is written with subscript numbers designating multiple substituents, including the hydrogens. Shorthand (line) formulas omit the symbols for carbon and hydrogen entirely. Each straight line segment represents a bond, the ends and intersections of the lines are carbon atoms, and the correct number of hydrogens is calculated from the tetravalency of carbon. These notations are illustrated for the seven C4H10O isomers listed in the following table. Non-bonding valence shell electron pairs are omitted in these formulas.
| Kekulé Formula | Condensed Formula | Shorthand Formula |
|---|---|---|
 | ||
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