pKa Values: A Chemist’s Best Friend to Predicting a Reaction

Proton transfer reactions are described in Chapter 6 of Karty and are the students’ first experience with a general reaction. When introducing this material to my students last week, they were a little nervous when I said the word “reactions.” I told them that my job, in guiding them along their organic chemistry journey, was to help build their expertise in determining the direction of an overall reaction. I told them that their text presents this material in a simple step-wise mechanism, and that they would be “getting their feet wet in the shallow end of a swimming pool.”

My goal is to equip my students with necessary skills as they progress through the lecture course. As a new instructor, I see why my previous mentors and professors emphasized understanding pKa values. I have grown to appreciate and utilize the acid dissociation constant correspondent to different functional group on organic compounds. I now emphasize pKa values to my students: Acidity of an organic compound describes how well an acid can give up a proton, particularly in the context of an acid-base reaction.

Karty does a thorough job of presenting proton transfers through the use of pKa values and the electron transfer from nucleophile to electrophile. At the beginning of the lecture, I present the pKa values, based on Table 6-1, to the class and explain the trends of the various functional groups and ionizable protons. I remind the students that the reaction will favor the side with the weakest acid (highest pKa value) and ask them to think about how to show the electron movement to support that direction of each chemical reaction.

Table 6-1

I typically begin lectures by presenting information, followed by an application of that topic. The purpose is to theoretically move the students from memorization to comprehension. I encourage students to utilize critical thinking and to think about the content in the mechanisms. In class, following the presentation of the pKa values, I explain how to apply the chemical trends and integrate pKa values to demonstrate how students can determine if a reaction favors the products or reactants. Focusing on mechanisms, Karty empowers students by training them to push electrons in a reaction. A mechanistically organized course teaches students how to recognize if organic species are electron rich or electron poor, and showing the application at each step of the reaction provides reinforcement. Mechanisms aid students in developing the skillset of distinguishing the nucleophile from the electrophile.

I observed that the students quickly understood the concepts of electron pushing and drawing the arrow from “negative to positive.” Many students commented that their successful comprehension of the proton transfer mechanism was due to its similarity to moving pi electrons involved in resonance. In general, the students found the pKa values helpful, allowing them to confirm the direction of each reaction as they learned how to push electrons and show the arrows’ movement from negative to positive. As an instructor, I appreciate teaching organic chemistry in a mechanistically organized course and try to get students to look at a reaction as a movement of electrons, rather than having them memorize types of products.

-Kerri Taylor, Columbus State University
Click here to learn more about Dr. Taylor

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