Intermolecular Attractions and Solubility: A Classroom Demonstration For A Difficult Topic

An understanding of solubility, melting points, and boiling points based on intermolecular attractions, or intermolecular forces (IMF), is critical in academic research as well as industrial work. For example, I know that the following facts:

  • Methylene chloride is a better solvent for the extraction of the products of LAH reductions than diethyl ether.
  • Stearic acid is a solid at room temperature while isostearic acid is a liquid.
  • 1-butanol is basically immiscible in water.

Knowing these facts has been critical at particular points in my career. (If I had only learned the last one sooner, I could have prevented the only uncontrolled explosion I have had in a chemistry lab.) Consequently, I consider IMF an important topic.

In the current draft of his textbook, Joel spends over 20 pages in the second chapter on the topic of IMF. This depth of treatment is an exceptional aspect of the text, and one I would like to see in all organic textbooks.

Why this suggestion? Because IMF is a difficult topic for instructors to teach and for students to understand. There are many reasons for this, but I will mention two basic problems. First, while practicing chemists acquire information about IMF, solubility, melting/boiling points, etc., through laboratory experience and the evaluation of specific situations (such as those listed in the first paragraph), we instructors attempt to summarize this information in algorithms for students to learn and apply. Second, IMF requires students to visualize what is happening at a molecular level, and this presents a challenge.

In an attempt to address both of these difficulties, I incorporated a lecture demonstration this fall that previously was part of a laboratory experiment devoted to intermolecular forces. The demonstration has three parts:

1. Combine approximately 10 mL of hexane and 10 mL of methanol in a large (22 mm x 200 mm) test tube. At this point, the students will see two layers, although not of equal volume.

2. Add approximately 10 mL of 1-butanol and mix. The students will see that the contents of the tube are homogeneous.

3. Add approximately 10 mL of water and mix. A layer, approximately one-fourth of the total volume, will form on the top of the tube.

After each step, you can ask students to explain what they observe using IMF. The results in step two allow you to discuss the different roles of the hydroxyl group and the alkyl chain in 1-butanol. My students this year correctly identified the shallow, top layer that formed in step three as hexane. One group asked for a continuation of the experiment, adding more 1-butanol and then more methanol.

In a follow-up activity, I plan to have students provide drawings of the molecular interactions at each step in order for them to explain what they observed. This, combined with the demonstration, allows for a discussion of IMF based on experimental observation and also asks students to depict what occurs at the molecular level.

— Steve Pruett

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