What's in the Flask?

[kevin_revell]

One of the challenges of teaching organic is getting students to analyze chemical reactions based on the entire chemical system.  Let me give three examples, and share a technique that I’ve found helpful in helping students think through these systems.

1.  Free Radical Halogenation

Think about the classic mechanism for this reaction:

Students struggle especially with the second step of the propagation.  If Cl₂ has already been used in the initiation step, how can it be present in the second step of the propagation?  Why not just write

CH₃ + Cl• à CH₃Cl?

To answer this question, students need to understand the chemical environment in which the reaction is taking place. When we introduce this in class, I ask students “what’s in the flask?”  I often draw a large flask on the whiteboard, and then ask students to name the different species that are present.  What results is a picture something like this:

This image helps students recognize that the reaction vessel is filled with CH₄ and Cl₂ – but only a small amount of radical is present.  This leads to the important follow-up:  What will the CH₃ radical more likely to encounter – a Cl radical or molecular Cl₂?  The drawing seems to help students understand the context of the reaction beyond simply the balanced equation.  It also helps drive home the idea that  the propagation sequence occurs until all of the starting materials are consumed.

2.  Mechanisms

As a second example, consider the acid-catalyzed dehydration of water.   As we talk through mechanisms, I sometimes gives students a sketch like this:

 

I usually get plenty of questions from my organic students, like "Why isn’t “H₃O⁺” written in the products?" or "Where does the H₂SO₄ go?"   And then there’s the test.  Every year, at least one student writes hydroxide as the base that abstracts the H⁺.  Not in a sulfuric acid solution!

The “what’s in the flask” question is a nice way to deal with these questions and misperceptions.  I often lead with an acid-base equilibrium: 

Does this equilibrium lie to the right or to the left?  Based on this, what’s in our flask?

This leads to some great questions:  Where does the “H⁺” written in the first step really come from?  What base is most likely to remove the H⁺ in the last step?  Why is it wrong to show a hydroxide ion pulling off the proton?

3.  Write the reagent

Students are sometimes overwhelmed by the myriad of ways that a single reagent can be written.  For example, we want an alkoxide base for an elimination – do we write this as NaOCH₃, or just CH₃O^-? Or how about NaOMe/MeOH, or KOMe/MeOH?  If we use ethoxide instead of methoxide, we double the possibilities.  For many of us who teach organic, the casual use of chemical synonyms can be problematic. 

In this situation, I again find the “what’s in the flask” technique to be useful.  I like to talk with my students about how these reagents are made, then sketch the components of the flask (i.e., methanol, sodium ion, methoxide ion).  Then we talk about all the different ways this might be written (on my test, on the ACS final, on the MCAT or DAT, etc.), and how to focus on the key features of the reaction.

Outlook

I was pleased when a student recently asked me to draw a flask on the board and show what was in a solution.  It was an encouraging sign.  While hard to quantify, I think this little technique is positively impacting the way my students think about chemical reactions.