If you are drawing methane, ethane, or propane, you probably noticed that the carbon and hydrogen atoms can be arranged in only one way.

What about drawing diagrams for larger organic molecules? For example, consider the molecular formula C₄H₁₀. What would your diagram look like?

Perhaps you would draw this:

Does the following structural diagram also represent C₄H₁₀? Count the number of carbon atoms and hydrogen atoms.

The compounds shown above have quite different structures although they have the same molecular formula. Each structure is named differently and is considered a different entity. Chemists refer to these compounds as structural isomers.

Because structural isomers have differing structures, they have different physical and chemical properties. For example, a branched alkane has a lower boiling point than the corresponding straight chain alkane does. Why does branching decrease boiling point?

Branching decreases boiling point because branched chains are shorter and fatter than straight chains. The "bulky" branched molecules cannot lie as close together as can long, thin straight chain alkanes. Therefore, the intermolecular forces between the branched isomers are not as strong. Recall from Chemistry 20 that London dispersion forces are formed as a result of the attraction between the positively charged atomic nuclei and negatively charged electrons of a neighbouring molecule. The further away the neighbouring molecule is, the weaker the London dispersion forces.

Check Your Understanding

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