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

Copyright

This work is licensed under a Creative Commons Attribution 4.0 International License. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/

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