Liberation in the Laboratory

One of the unexpected benefits of Joel’s organization was the discovery that students, once they learn the elementary steps common in organic reactions, can perform a limitless—at least in theory—range of experiments in the lab. (In the upcoming textbook, elementary steps are discussed in Chapter 7, midway through the first semester.) Since students have examined elementary steps in class, learned to identify the individual steps in multistep mechanisms, and predicted the products of elementary steps, there really isn’t a reaction I can’t use in lab.

For example, in the second half of this upcoming semester, my students will perform a lab that compares and contrasts natural and synthetic dyes. Students will extract carminic acid from cochineal beetles for the natural dye and will synthesize indigo from 2-nitrobenzaldehyde, acetone, and sodium hydroxide. They will subsequently dye strips of multifiber with both dyes and then compare the dyes’ performance.

Obtaining the carminic acid is, granted, a simple extraction. But rather than simply providing step-by-step instructions for students to follow, I have them create and detail their own procedure that is based on an informal description of the process given to me by a colleague. This “bug juice” experiment is a colorful way to introduce natural products and the history of carminic acid and its use as a colorant in food. It also makes for interesting out-of-class reading.

The Baeyer-Drewson synthesis of indigo, as seen in the figure below, is a potential goldmine as an experiment.

First, it allows you to introduce synthesis early in the course by using a reaction that is virtually foolproof and creates a product with which students come in contact daily. Second, even an abbreviated explanation of the reaction steps, such as in this procedure, allows you to ask students to visualize and draw the mechanism, as in this post-lab question:

The first equation in the handout, which shows 2-nitrobenzaldehye, acetone, and sodium hydroxide as the reactants, is actually three steps: 

            i. A proton-transfer step with sodium hydroxide and acetone as reactants
            ii. Addition of a nucleophile (from step i) to a polar pi bond
            iii. A proton transfer from water to the product of step ii.

 Show the reactants, products, and the movement of electrons for each of the above steps.

A more detailed mechanism, while daunting, includes other steps you can ask students to identify. Third, the dyeing process involves redox chemistry, another topic for post-lab questions:

Assign oxidation numbers to the carbons in indigo and leucoindigo and determine which carbons are reduced when indigo reacts with dithionite anion to become leucoindigo.

Finally, the reaction allows you to introduce enolate anions and aldol-type reactions early in the course.

In short, students have the opportunity to synthesize a familiar natural product, describe their observations, and go further to consider the question, “How did this happen?”

— Steve Pruett

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