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 SN1 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 SN1 mechanism have the opportunity to work out the fact that in addition to the SN1 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 (H>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.
Highlights
All chemical reactions basically always raise questions of selectivity
In order to enable this in the entanglement of theory and experimental practice, we present a newly developed learning opportunity based on the concept of Inventing-with-Contrasting-Cases. [5]
The experiments focusing on the variation of solvent and the variation of leaving group showed that the use of isopropanol as a solvent and tert-butyl bromide provided the best results for investigating the competing SN1 and E1 reaction
Summary
All chemical reactions basically always raise questions of selectivity. Two or more alternative products will result from the same reactants. Answering the question regarding which portions of these products are formed is only possible by a detailed energetic consideration of the reaction paths (Figure 1). [1] Chemoselectivity is the most important of the various selectivities. This means that reactants may form a specific product through a particular mechanism and, through another mechanism, an alternative, typically non-constitutive isomeric product. As with almost no other example, students are challenged to reflect on the diverse structural and energetic implications of chemical reactions using organic chemistry concepts, or, more generally speaking, to think in mechanistic alternatives
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