8.1 Structural Formula

A structural formula shows how different atoms are joined up or arranged in a molecule or compound. You can represent structural formulas in two ways. You can draw it as a displayed structural formula or as a condensed structural formula.

A displayed formula is a full structural formula that shows all the bonds between the atoms in a molecule as individual lines. The bonds are represented by lines. The strength of the bond is indicated by the number of lines. Single bonds are represented by one line. Double bonds are represented by two lines. In some molecules, three lines are possible. These formulas are very useful because they show as much information as possible about the molecule.

Methane is the simplest alkane. Other examples of alkanes are ethane, propane, butane etc. The displayed formulas of methane and ethane:

Methane

Ethane

Note that displayed formulas are always drawn with the molecule straightened out and flattened. They, however, show exactly how all the atoms are joined together in a condensed structural formula.

However, they are very large and time- consuming to draw. Condensed structural formulas try to solve this problem by omitting some of the bonds and grouping some of the atoms together.

8.1.1 The normal way to draw a structural formula

Except for the smallest molecules, drawing a fully displayed formula is very time- consuming. You can simplify the formula by listing the main chain carbon atoms and the hydrogen atoms attached to them in the sequence they appear, for example, ${\mathrm{C}\mathrm{H}}_3$$\mathrm{CH}_{3}$ or ${\mathrm{C}\mathrm{H}}_2$$\mathrm{CH}_{2}$ instead of showing all the carbon- hydrogen bonds. So, the structural formula for methane could be written as ${\mathrm{C}\mathrm{H}}_4$$\mathrm{CH}_{4}$ and that of ethane as ${\mathrm{C}\mathrm{H}}_3-{\mathrm{C}\mathrm{H}}_3$$\mathrm{CH}_{3} - \mathrm{CH}_{3}$ or as ${\mathrm{C}\mathrm{H}}_3{\mathrm{C}\mathrm{H}}_3$$\mathrm{CH}_{3}\mathrm{CH}_{3}$

The condensed structural formulas of these alkanes are

A condensed or normal structural formula uses established abbreviations for various groups of chains. In the displayed formula, we list the main chain carbon atoms and the hydrogen atoms attached to them in the sequence in which they appear in the naming system. It means that a condensed or normal formula is a simplified version that omits some or all of the bonds. It involves grouping atoms that are connected to the same carbon atom.

For instance,

Table 8.1: Condensed or normal structural formulas of some alkanes

Example 8.1

Give the displaced and normal structural formula for pentane.

Problem Solving Strategy

i) The stem pent - means five carbon atoms.

ii) The ending - ane indicates an alkane.

iii) Write a string or chain of five carbon atoms.

iv) Attach hydrogen atoms to the carbons to give each carbon atom four bonds. This requires three hydrogen atoms on each end of carbon and two each on others.

v) For the structural formula, write each carbon atom's set of hydrogen atoms next to the carbon.

Solution