1. Examples of Student Responses
Olivia M. Crandell, Melanie M. Cooper, Justin H. Carmel, and Ryan L. Stowe
Department of Chemistry, Michigan State University, East Lansing, Michigan
A Preliminary Investigation into Students’ Use of Mechanistic Reasoning in Undergraduate Organic Chemistry Courses
Background
Coding Scheme
Examples of Student Responses
Organic chemistry is a complex discipline that works to understand chemical reactivity at the molecular level. Chemists use their knowledge of chemical principles to understand observed chemical reactivity and make predictions for novel reactions.1 To predict chemical reactivity, chemists must consider the many causal elements that give rise to a chemical reaction as well as the particulate level processes that bring about a chemical transformation (called a chemical mechanism). Chemical reactions occur because of electrostatic interactions between chemical species (a causal element). Chemical bonds break and form as a result of electron movement (the chemical mechanism). Combining causal and mechanistic elements together to explain an observed phenomenon is called mechanistic reasoning.2 Students in undergraduate organic chemistry courses often struggle to incorporate both causal and mechanistic components to engage in mechanistic reasoning when predicting and explaining chemical reactivity.3,4
Mechanistic
“The electrons on the oxygen attack the carbon, and then the bond between the carbon and the bromine is broken so the electrons from the bond move to the bromine.” – Traditional student mid-test response
Causal
“…the Br is electronegative which polarizes the bond between it and the carbon, causing the carbon to be partially positive… “ – OCLUE student mid-test response
Non-normative (NN)
Descriptive
(D)
Causal
(C)
Mechanistic
(M)
Causal Mechanistic (CM)
Response does not address the reaction given in the prompt.
Response gives evidence that the student recognizes a bond is breaking or a bond is forming.
Student correctly discusses an electrostatic attraction between the –OH group and the carbon. They may identify the carbon as partial positive because of the electronegativity of the neighboring bromine atom.
Methods
Causal Mechanistic
“Bromine pulls electron density towards itself from the carbon it is attached to. The hydroxide molecule attacks the electron deficient carbon and the bromine, which is electron rich and more stable than OH- leaves…. Electrons on oxygen attack electron deficient carbon and [the] bromine atom takes electrons from the C-Br bond and leaves with a negative charge.” – Traditional student mid-test response
Student correctly discusses electron movement processes. They discuss the electrons from the –OH group moving to the carbon atom and/or discuss the electrons in the C-Br bond moving onto the bromine.
Helping students develop mechanistic reasoning is a primary goal of a two-semester transformed undergraduate organic chemistry course that is currently being piloted at Michigan State University called Organic Chemistry, Life, the Universe, and Everything (OCLUE). As part of our previous work4, we developed an activity to elicit student understanding of the causal and mechanistic components of a chemical reaction. We have administered a variation of this activity to students enrolled a traditional organic chemistry course as well as to students enrolled in OCLUE. The activity was administered at the start of 1st semester in Fall ‘16 (pre-test) and again at the start of the 2nd semester in Spring ‘17 (mid-test). A post-test will be administered at the conclusion of the 2nd semester in Spring ‘17. Pre- and mid- responses from random cohorts of 100 OCLUE students and 100 traditional students have been analyzed for their use of mechanistic reasoning.
Student includes correct mechanistic and causal elements in their explanation of the chemical reaction.
First Semester Changes
Reaction Details
Changes in Student Explanations
Polarized bond
Electronegative atom
Partial positive charge on carbon
Nucleophile
Key Components of OCLUE
Pre-test
Mid-test
Coulombic Interactions
Intramolecular
Interactions
Intermolecular
41 students stayed CM 16 16 4 9 11 10
Preliminary Results 2
A preliminary comparison of these two cohorts shows that the number of students advancing to a causal mechanistic response from a less sophisticated response is not that different for these two cohorts. As such, the cohorts were combined into a single representation (Figure 1) to show the overall change between the coding bins from the pre-test to the mid-test. Overall, we still see that the change from a less sophisticated response to a more sophisticated response is still not that different for these students. Some students improve in this first semester (33) but nearly the same number of students gave less sophisticated responses after the first semester (35).
Description of the Activity
Ongoing Work
In this preliminary investigation, we saw that there are many different ways that students discuss the causal and mechanistic components of the reaction. Students often give partial mechanistic responses or only mention some of the causal elements highlighted in the Reaction Details above. We intend to investigate the consistency of students’ explanation of the mechanism (the multi-step, particulate-level process) within their given response as well as looking at the consistency of their written responses with their drawn mechanistic arrows. We also intend to gather more data in a post-test to see how student responses change over the 2nd semester of organic chemistry.
Figure 1. A Sankey diagram showing the changes between the coding bins from the pre-test to the mid-test. Both cohorts are combined in this representation (N=200).
References
Acknowledgements
Bhattacharyya, G. From Source to Sink: Mechanistic Reasoning Using the Electron-Pushing Formalism. J. Chem. Educ. 2013, 90(10),
Kris, C.; Schwarz, C.; Reiser, B.J. What Kind of Answer Are We Trying to Provide? Identifying and Supporting Mechanistic Reasoning in Science Learning. Manuscript in preparation.
Anderson, T.L.; Bodner, G.M. What can we do about ‘Parker’? A case study of a good student who didn’t ‘get’ organic chemistry. Chem. Educ. Res. Pract. 2008, 9(2),
Cooper, M.M.; Kouyoumdjian, H.; Underwood, S.M. Investigating Students’ Reasoning about Acid-Base Reactions. J. Chem. Educ. 2016, 93,
We would like to thank the professors CEM 251 and 252 for allowing us access to their students and also thank the students for participating in this research. Furthermore, we would like to thank the National Science Foundation (# , PI: M. Cooper) for funding Organic Chemistry, Life, the Universe, and Everything (OCLUE) and this research pertaining to mechanistic reasoning.