Lets’ Get Real: The Law of Definite Proportions

Michael Jansen | Wed, 10/04/2023 - 14:50

I’m not a fan of asking students to memorize rules or laws for the sake of memorizing rules or laws. There has to be a point—a reason, or at least a decent "this hits home” example of the rule or law.

Stay with me . . .

The Law of Definite Proportions

When I was a kid, I remember hearing that water was discovered on Mars.¹ Wow, I thought—is that special Martian water? Is it green?

Nope.

It turns out that water is always 89% oxygen and 11% hydrogen by mass, and has the formula H₂O. This is true regardless of the source of the water: whether it’s produced by the combustion of a hydrocarbon, or as the result of a biochemical process, or is synthesized in a Martian’s lab—it doesn’t matter.²

This is the Law of Definite Proportions, LDP. Also known as Proust's law, or the Law of Constant Composition, it states that a given chemical compound always contains its component elements in fixed mass ratio and does not depend on its source and method of preparation.³

And so “Water is Water is Water” is how I pitched the LDP to my grade 11 Chemistry students.

This is a good story, but it lacks observable evidence. And it is observable evidence that students need.

Some History: Friedrich Wöhler’s Synthesis of Urea

In an excellent series of videos on Organic Chemistry produced by TV Ontario in the 1980s, one episode featured the LDP^4,5.

It explains how the German Chemist Friedrich Wöhler, working in the 1820s, accidentally prepared urea, H₂NCONH₂, while attempting to produce ammonium isocyanate, NH₄OCN, using the seemingly straightforward double displacement reaction:

AgOCN(aq) + NH₄Cl(aq) → AgCl(s) + NH₄OCN(aq) Equation 1

After filtering off the silver chloride precipitate, he gently heated the ammonium cyanate solution to dry it. Unbeknownst to Wohler—at least initially—the solid product obtained was not ammonium cyanate, but urea, an isomer.

NH₄OCN(aq) + heat (60°C) → H₂NCONH₂(s) Equation 2

urea_3d_ball.png

Figure 1. The structure of urea⁶

To make a long story short, Wöhler, initially trained as a physician, likely had experience with urea, H₂NCONH₂, a metabolite contained in mammalian urine. When Wöhler examined the properties of what he thought would be ammonium cyanate, he noticed that they were identical to those of urea. I’m pretty sure that he also noticed that ammonium cyanate and urea have the same chemical formula, too.

This was such an important discovery that Germany printed a postage stamp to commemorate this event.⁷

Here’s the bottom line: It doesn’t matter how urea is prepared—in a horse or a goat or in the lady next door—OR IN A CHEMISTRY LAB—it is the same—H₂NCONH₂. We see that it is possible to prepare a so-called “organic” compound in a laboratory. Whether Wöhler singlehandedly dispelled the theory of Vitalism is immaterial: his synthesis of urea is the LDP writ large.⁸

How to Demonstrate the LDP in Your Classroom

Fine, you say—all good. But how can I make this come alive in my classroom, given that I don’t have access to (expensive and toxic) silver cyanate to replicate Wöhler’s urea synthesis?

We can demonstrate the LDP quickly and easily, using either of two easily prepared esters:

A. Methyl Salicylate (aka oil of wintergreen)

methyl_salicylate.png

Figure 2. Methyl salicylate, the methyl ester of salicylic acid¹⁰

B. Pentyl Acetate (aka pentyl ethanoate or amyl acetate)

pentyl_acetate_.png

stick structure of Pentyl Acetate molecule

Figure 3. Pentyl acetate (aka pentyl ethanoate or amyl acetate)¹¹

Each of the following activities allows students to employ “nasal spectroscopy”¹², which is a fancy way of “smelling something”.

Depending on the depth of discussion, this demonstration/lesson is suitable for 11 or 12/AP Chemistry, and should take 30 – 45 minutes.

A. Methyl Salicylate

Methyl salicylate, or oil of wintergreen, has a distinctive “minty” smell. It is the active ingredient in Wint-o-green Lifesavers and in muscle-ache ointments such as Ben-Gay¹³.

i. You may choose to begin by giving students the opportunity to smell a sample of Ben Gay Arthritis Extra Strength. This contains their highest concentration of methyl salicylate—30%, and 8% menthol, presumably by mass.¹⁴ This product smells strongly of methyl salicylate.¹⁵Ask students the source of the methyl salicylate in the ointment. That is, is it taken from a plant or from an animal, or was it synthesized in a lab? (Answer: We don’t know. But for sure, it’s whichever is the most cost-effective.)

ii. Methyl salicylate can be prepared from the reaction of salicylic acid and methanol, in the presence of a sulfuric acid catalyst, as shown below¹⁶.

preparation_of_methyl_salicylate_.png

Equation 3

This synthesis can be done in either a teacher-demonstration or in a mini-lab¹⁷:

Rather than heating reactant mixture in a water bath—as implied above, which can take some time—the synthesis can be carried out using a microwave oven¹⁸:

I suggest that you prepare several samples of the following at once, to facilitate passing them around the room. You may also choose to have several pre-made samples available, similar to what we see on cooking shows.

To a test tube, add ca 1g of salicylic acid. (Pass the sample around the room to allow students to notice that this solid acid is odorless.) Add ca 1 mL of methanol, followed by 3 drops of concentrated sulfuric acid, H₂SO₄. Place the test tube(s) in an empty 250 mL or 400 mL beaker to keep it upright. “Zap” in a microwave oven for 10-15 seconds.

After “zapping”, add several mL of ice-cold water to the test tube(s); you will see solid methyl salicylate float; note its distinctive odor. This shows students the laboratory synthesis of methyl salicylate, and the qualitative identification of methyl salicylate by its odor.

If you prepare several of these samples simultaneously, it will facilitate passing them around for students to smell.

iii. To show students naturally produced methyl salicylate, you’ll need a wintergreen plant (see figure 4).

wintergreen_plant.png

Figure 4. A wintergreen plant. The red berries may not always be present.¹⁹

These small plants have green oval leaves, and can have red berries depending on the time of year. Wintergreen is native to certain regions of North America²⁰.

(I bought one at a garden center last year. After planting, it survived the winter in my Toronto-backyard.)

Remove a leaf; tear it in half; invite students to smell it. While the odor is not overwhelming, it is more-than-detectable. To quote my son Karl, who at the age of four, on sniffing the torn-open leaf, declared it “minty”. This shows, at least to a first approximation, that methyl salicylate—the same as what was just prepared in the lab—can also be made by a plant.

And so the LDP is illustrated, albeit in a less-than-rigorous manner.

Since I retired, I no longer have access to a chemistry lab. It was my intention, before abdicating my teaching position, to attempt an extraction of methyl salicylate from wintergreen leaves (or berries) using an organic solvent. If you happen to try this—and succeed—I’d be obliged if you’d let me know what you did.

B. Pentyl Acetate

This ester is contained in bananas and contributes to its characteristic scent. According to the ingredient list of a banana²¹, pentyl acetate is present in bananas in a small amount.

Pentyl acetate is synthesized from pentanol acid and acetic acid—in a condensation reaction similar to the preparation of methyl salicylate, as shown below.

preparation_of_pentyl_acetate.png

Equation 4

To synthesize pentyl acetate, follow the procedure for the preparation of methyl salicylate, substituting ca 1 mL each of pentanol and glacial acetic acid.

Similarly to methyl salicylate, you may ask your students to compare the scent of the ester to that of a honest-to-goodness, good ‘ol banana, to arrive at a qualitative verification of the LDP.

SAFETY NOTES:

a) Whichever ester you choose, try the synthesis yourself first, with no students around.

b) Wear eye protection and latex/nitrile gloves, especially when handling concentrated sulfuric acid and glacial acetic acid.

c) Use a microwave oven dedicated to chemistry experiments, not the one you use to re-heat lunch.

d) Students are not to handle concentrated sulfuric acid; it is corrosive and nasty. Concentrated H₂SO₄ is to be added BY THE TEACHER only, who is wearing gloves. The acid must be added to the reactant mixture that is already in the test tube.²² I advise that students wear latex/nitrile gloves when dispensing glacial acetic acid.

e) Place the microwave oven inside a fume hood; lower the safety shield of the ‘hood during “zapping”.

f) Put a beaker of water (ca 250 mL) inside the microwave oven to act as a “sink” for unabsorbed microwaves.

Discussion Points

This activity raises important questions. And it is questions like these that encourage students to think for themselves. I suggest that you begin by asking students to poke holes in this demonstration/lesson; they could pretend to be lawyers and “cross-examine” you. If that doesn’t work, ask the questions yourself.

Regarding methyl salicylate:

-Does the odor of menthol—or any other ingredient—present in Ben Gay mask the smell of methyl salicylate? (It’s possible; how much we don’t know off-hand.)

- Are there other compounds that smell like methyl salicylate? Could such a compound, or compounds, be present in a wintergreen leaf? (It’s possible)

-Does salicylic acid have an odor? (No, students had a chance to smell it prior to the synthesis.)

-How could the minty smelling compound—methyl salicylate—in the wintergreen leaf be isolated and characterized? (Perhaps by extraction using an organic solvent. It would then need to be purified before being analyzed according to, say, its melting point/boiling point, or by spectroscopic means. One could also do a side-by-side comparison with a known sample of methyl salicylate)

-In the microwave-oven synthesis of methyl salicylate, could other compounds be produced in the reaction? (At this point in a student’s career, let’s say it’s possible)

If so, could these compounds smell like methyl salicylate? (It’s possible)

-How could the lab-preparation of methyl salicylate be improved? (We could isolate pure methyl salicylate and characterize it using means other than smell, such as mp/bp)

Regarding pentyl acetate:

-Modify the above questions to refer to naturally produced and synthetically produced pentyl acetate

Aside from discussing the LDP, the important-ness of the post-activity discussion lies in encouraging students to think for themselves—to ask questions. Simply because the teacher demonstrates something, or that a given point is in a textbook, in an internet post, in a newspaper/magazine/journal article, or on television, doesn’t mean that it’s (100 %) true²³.

Now, more than ever, critical, empirically based thinking is essential.

Finally, if you prepare either of these esters for 11 Chemistry students—or if they do it themselves—you may want to mention that you will re-visit this type of synthesis, called a condensation reaction, in 12/AP Chemistry, in light of Organic Chemistry and Chemical Equilibrium.

You may be interested in ready my previous article published in Chem13News: Chance Favours the Prepared Mind (accessed 8/7/2023).

RESOURCES

A student-ready handout (11 or 12/AP) prepared by Michael Jansen: “The Birth of Synthetic Organic Chemistry”
A student-ready handout (AP Chemistry) prepared by Michael Jansen, regarding the Urea molecule, incorporating: melting point, molecular geometry, formal charge, resonance
An Organic Chemistry Laboratory Activity (12/AP) on the qualitative, microwave oven synthesis of three esters, prepared by Michael Jansen