Drawing Organic Structures

Molecules are actual, three-dimensional entities. Their structure is a major factor that determines their physical properties and the way one molecule interacts with another molecule. Because molecules are normally too small to see, chemists have devised ways to visually represent molecules. One way is by using a two-dimensional structural formula like that of the hydrocarbon heptane.

A two-dimensional structural formula of a hydrocarbon shows all of the atoms with all of their bonds in the plane of the page.

Hydrocarbons provide the backbone of all organic compounds. Each carbon atom in a hydrocarbon forms a total of four bonds. These bonds are combinations of single bonds with hydrogen atoms and single or multiple bonds with other carbon atoms.

For molecules that contain a large number of atoms or complex structures, drawing every bond and every atom is time and space consuming. A common notation developed to abbreviate the drawing without sacrificing the clarity of the structure is the condensed structural formula shown below for heptane:

CH3-CH2-CH2-CH2-CH2-CH2-CH3  (Heptane)

Hydrocarbons are compounds composed only of carbon and hydrogen atoms.

A condensed structural formula includes all of the atoms but uses line bonds to emphasize the main structural characteristics of the molecule.

Taking out the lines representing the carbon—carbon bonds condenses this formula still more:

CH3CH2CH2CH2CH2CH2CH3  (Heptane)

Heptane has five repeating —CH2— groups, called methylene groups. Because many organic molecules have such repetitive groups, an even more condensed notation shows these repeating units. Using this notation, the formula for heptane is as follows:
CH3(CH2)5CH3  Heptane

The bond-line structural formula is the notation that most organic chemists prefer to use. Bond-line formulas are easy to draw and quickly convey the essential structure of a molecule. Both the ends and the angles of the structure represent the carbon atoms. C—H bonds are not shown, but you should assume that the appropriate number of hydrogen atoms is present to complete the four bonds required by carbon to have its octet of electrons. The bond-line formula for heptane looks like this:

Bond-line formulas represent the carbon atoms as the intersection of lines and as line ends. You assume all the hydrogens needed to complete carbon’s valences.

Not all hydrocarbons are straight chains; many are rings. Chemists use the same structural formulas for them. Because the illustration of the two-dimensional structural formula of methylcyclopentane is so cluttered, it does not clearly show the ring.

Methylcyclopentane

The condensed structural formula is clearer.

Methylcyclopentane

The bond-line structural formula is even clearer. Thus, chemists use it most frequently.

Often, chemists combine the bond-line and condensed notations to clarify a structure or emphasize specific features. This formula also represents methylcyclopentane.