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THINK ABOUT IT

The structural formula is a valuable source of information about a molecule. You can use the structural formula to identify functional groups in a molecule. Knowing the functional group also helps you to predict molecular properties, including smell. However, sometimes only knowing the functional group is not enough to predict the smell of a compound.

What three-dimensional features of a molecule are important in predicting smell?

To answer this question, you will explore

EXPLORING THE TOPIC

New Smell Molecules

Below are three compounds, each with a distinctive smell. In spite of their wide range of smells, each of the three compounds contains the same functional group.

Take a moment to locate their functional groups. Each of these molecules has a hydroxyl functional group, and each name ends in “-ol.” All three molecules are alcohols. So, why are the three smells so different? Something besides just functional group affects the smell of a compound. The answer lies in the three-dimensional shape of the molecules of each compound.

Three-Dimensional Models

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It is difficult to show a three-dimensional drawing of a molecule on a flat piece of paper. With a molecular model set, you can build a three-dimensional structure.

Notice that the bonds in a model kit look like little sticks. And the atoms are small spheres. These models are called ball-and-stick models.

A picture of a ball-and-stick model for ethyl acetate, C₄H₈O₂, is shown here. The carbon atoms are shown in black, the hydrogen atoms are white, and the oxygen atoms are red.

Illustrations of ball-and-stick models have some drawbacks, in that some atoms may be partially visible or entirely hidden. However, the illustrations convey more information about molecular shape than structural formulas do. Using a real model is the best way to examine the three-dimensional shape of a molecule. The two representations for a molecule of citral, C₁₀H₁₆O, are shown for comparison. Look for similarities and differences between them.

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Notice that the structural formula shows the citral molecule as flat with all of the carbon atoms in a line. The ball-and-stick representation, on the other hand, shows that the carbon atoms are not arranged in a line, but are connected in a zigzag fashion. Both representations contain much of the same information, but the ball-and-stick model adds information about the way the atoms are arranged in space.

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Ball-and-stick models for the three compounds introduced at the beginning of this lesson are below.

The overall shapes of the molecules differ greatly. The carbon atoms in geraniol are connected in a long chain, while the menthol and fenchol molecules both contain ring structures. Perhaps these overall shapes are related to the different smells of these three alcohols.

LESSON SUMMARY

What three-dimensional features of a molecule are important in predicting smell?

Molecules are three-dimensional. A ball-and-stick model kit is a tool that you can use to construct models of molecules. Ball-and-stick models allow you to see how the various atoms in molecules are arranged in space, as well as the overall shape of each molecule. Molecular shape may have something to do with smell.