While teaching chapters 17-18, I have shown students the versatility of carbonyls and enolate chemistry. The discussion in Karty’s book is arranged well and does a nice job of spotlighting the chemistry unique to carbonyls, especially as it ranges from selective addition (direct or conjugate) to the use of enolates for alkylation and halogenation.
In the second semester, my audience is filled with a portion of previous students that I had in organic chemistry 1, as well as those from my colleague who simultaneously taught the other section. Those students who had me previously were exposed to the concepts and importance of pKa values. My colleague did not discuss this topic in such detail. It is not uncommon that items are stressed more or less, as some professors tend to have a stronger affinity for one topic or another.
However, I have found the students asking why the reagents move in the way that they do in the various mechanisms (ie. why is a proton removed rather than the carbonyl being attacked.) This brought to my attention the question of “Are pKa’s necessary to succeed in the classroom?”
I remind students that it does not matter about the name of a reaction or topic, rather it matters how the electrons move. This is why I wonder about the necessity of pKa values and the appropriate knowledge base of students. I remember in my undergraduate career, I never used pKa values (let alone memorized them) but I cannot understand how I succeeded without them.
The concept of pKa allows the chemist to determine what proton to remove. For example, in Section 18.9, we were discussing aldol reactions with nitriles and nitroalkanes, and the students tried to justify the electron movement. The book does a phenomenal job of detailing the pKa values, yet it is the instructor who compliments the learning process by guiding the students, like a tour guide on a trip, to comprehend the “why” of each electronic step. Let’s use problem 18.36 as an example for this topic. In the problem, there is nitrile, an aldehyde and sodium hydroxide (NaOH) in ethanol (EtOH).
I try to ask questions that trigger active participation…Where does the NaOH attack? How is the NaOH used? Oftentimes, I get replies of …it’s a base…it will take a hydrogen. But the question is really rooted in the why! Why does the NaOH take hydrogen?
My questions follow with …Why does the NaOH attack the alpha hydrogen to the nitrile? Why does it not attack the carbonyl’s hydrogen atoms? Does it attack the EtOH?
Each of these three components…the nitrile, the formaldehyde and EtOH…all vary by their pKa and the freely ionize-able hydrogen. The alpha carbon to the nitrile has a pKa that is least acidic (~10) due to the resonance stabilization of the nitrile and aromatic ring. The hydrogens on the formaldehyde will be very tough to remove (pKa: ~25-30). Not to mention that the removal of a hydrogen atom from the formaldehyde would destabilize the molecule, because there would be a negative charge on a sp2 hybridized atom. Collectively, the formaldehyde is the electrophile in this reaction. EtOH has a pka of 16, but it is in excess and used to help stabilize the charges in this reaction.
This entire thought process is essential to know how to mediate through the mechanism and identify the major organic product. In retrospect, I think half the battle of organic chemistry is knowing how to use the reagents and when to move the species. I impress upon students the need to work problems and build this electronic perspective. I equate this process to the idea of sports… a football team practices constantly and runs routine drills in preparation for game day. In like manner, I ask the students to practice constantly and run through routine organic problems, so that test day is a complete success!
-Kerri Taylor, Columbus State University
Feel free to share your thoughts in the “Comments” section below!