Learning to Think in Mechanistic Alternatives: S_N1 vs. E1 and the Gibbs-Helmholtz Equation

Catharina Schmitt¹, Lukas Kaiser¹ and Michael Schween^1,

¹Faculty of Chemistry, Philipps-Universität Marburg, Marburg, 35032, Germany

Cite this paper:
Catharina Schmitt, Lukas Kaiser and Michael Schween. Learning to Think in Mechanistic Alternatives: S_N1 vs. E1 and the Gibbs-Helmholtz Equation. World Journal of Chemical Education. 2019; 7(2):102-108. doi: 10.12691/wjce-7-2-10

Abstract

One of the biggest challenges for learners of organic chemistry is learning to think in competing mechanistic alternatives and using cross-linked chemical knowledge. An outstanding subject for this is the competition between S_N1 and E1 reactions. In this case, it is special that the competing reactions have an identical first step and separate into different paths only from the intermediate, the second step, of the reaction. Learners who are familiar with the S_N1 mechanism have the opportunity to work out the fact that in addition to the S_N1 reaction, the E1 reaction is also proceeding, which more or less dominates, depending on the substrate, nucleophile or solvent and the temperature. This differentiation is undertaken with a set of experimental learning opportunities we have developed using simple qualitative analytics. Our learning opportunities are designed as contrasting cases, one of them with a variation of temperature in one setup. These allow the question why a chemical reaction can occur even though it is enthalpically unfavorable, i.e. has a positive reaction enthalpy (ΔH>0), to be answered. The latter helps the learners to realize that in addition to the enthalpy of reaction, there must be another energetic quantity that determines the thermodynamics of chemical reactions: Entropy. In the end, this leads to the discussion of the Gibbs-Helmholtz equation and, thus, to basic insights into chemical thermodynamics.

Keywords:
high school first-year undergraduate laboratory instruction mechanistic reasoning in organic chemistry substitution vs. elimination thinking in mechanistic alternatives Gibbs-Helmholtz equation

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References:

[1]	Graulich, N. and Schween, M. “Concept-oriented task design: making purposeful case comparisons in organic chemistry”. Journal of Chemical Education, 95, 376-383. 2018.
[2]	Cooper, K.A., Hughes, E.D., Ingold, C.K. and MacNulty, B.J., “Mechanism of elimination reactions. Part VII […]”. Journal of the Chemical Society, 2043-2049. 1948.
[3]	Grove, N.P. and Bretz, S.L., “Perry’s scheme of intellectual and epistemological development as a framework for describing student difficulties in learning organic chemistry”. Chemistry Education Research and Practice, 11, 207-211. 2010.
[4]	Popova, M. and Bretz, S.L., “‘It’s only the major product that we care about in organic chemistry’: an analysis of students’ annotations of reaction coordinate diagrams”. Journal of Chemical Education, 95 (7), 1086-1093. 2018.
[5]	Schwartz, D.L., Chase, C.C., Oppezzo, M.A. and Chin, D.B. “Practicing versus inventing with contrasting cases: The effects of telling first on learning and transfer”. Journal of Educational Psychology, 103 (4), 759-775. 2011.
[6]	Schmitt, C., Wißner, O. and Schween, M. “Carbenium-Ionen als reaktive Zwischenstufen. (Experimenteller) Zugang zu einem tiefergehenden Verständnis organischer Reaktionen”. CHEMKON, 20, 59-65. 2013.
[7]	Schmitt, C., Seel, L. and Schween, M. “SN1-Reaktionen anhand von Konkurrenzreaktionen verstehen. Experimentelles Design zum Einfluss von Substratstruktur und Abgangsgruppenqualität”. NiU-Chemie, 160, 32-38. 2017.
[8]	Schmitt, C. and Schween, M. “Using trityl carbocations to introduce mechanistic thinking to German high school students”. World Journal of Chemical Education, 6, 18-23. 2018.
[9]	Newton, T.A., Hill, B.A. and Olson, J. “Using conductivity devices in nonaqueous solutions I: demonstrating the SN1 mechanism”. Journal of Chemical Education, 81 (1), 58-60. 2004.
[10]	Hedinger. Chemophon – Akustischer Leitfähigkeitsprüfer. Available: https://www.der-hedinger.de/produkte/ph-und-leitfaehigkeitsmessung/leitfaehigkeitsmessung/artikel/65361.html. [Accessed January 22, 2018].
[11]	Trabert, A., Eckert, F. and Schween, M. Kohlenstoff-Kohlenstoff-Bindungen knüpfen lernen – ein Modellexperiment zum konzeptbasierten Problemlösen, CHEMKON 23 (4), 163-169. 2016.