Stereochemistry of E2 Elimination Reactions
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This Demonstration considers the different conformations of a substituted cyclohexane, trans-1-chloro-2-isopropyl-cyclohexane, in the course of ring flipping and elimination reactions. Two different chair conformations are stable, the more stable being the one with the lower steric bulk. Select "ring flipping" and the "reaction progress" slider to observe the transition between the two chair conformations . In this transition the equatorial and axial ligands interchange. Select "chair seat" to see the common seat in the two chair conformations.[more]
Select "reaction coordinates" to show the potential energy curve between the substituted and (less stable) unsubstituted cyclohexanes .
The curve of the substituted cyclohexane is nearly symmetrical. There is a small maximum associated with the boat conformation and two small minima associated with the so-called twist-boat conformation.
Statistical thermodynamics determines the population ratios of the different conformations in the course of chemical reactions, as molecules change into the more stable conformations. The elimination reaction is not reversible because the cyclohexene (isopropylcyclohexene), highlighted in red, is the only chemical compound remaining after the other products vaporize.
By selecting "elimination reaction" you can observe reaction kinetics characteristic of E2 elimination reactions of 6-carbon cyclic compounds. The elimination reaction in this case proceeds exclusively in anti mode and therefore a hydrogen is detached from the molecule according to Hofmann's rule , even if the most stable cyclohexene (by Zaitsev's rule ) does not form.
As the reaction takes place, the hydrogen (gray sphere) and chlorine (green sphere) are detached and dispersed in the reaction vessel. They go off screen as we are not interested in them anymore; while chlorine remains in the solution, the hydrogen tends to form a new bond with a strong base (bond in blue color).
Use the "reaction progress" slider to observe how the C-Cl and C-H bonds break simultaneously to form two new bonds: the double bond and the H-B bond between the hydrogen and the nucleophile that behaves as a strong base.[less]
Contributed by: D. Meliga, V. Giambrone, L. Lavagnino and S. Z. Lavagnino (January 2023)
Open content licensed under CC BY-NC-SA
Snapshot 1: substituted cyclohexane change from a more stable configuration to a less stable one as a consequence of steric effects
Snapshot 2: the leaving group () is in anti-periplanar position to the hydrogen and after elimination a double bond is formed
Snapshot 3: Elimination reaction further along
 J. Ashenhurst. "The Cyclohexane Chair Flip–Energy Diagram." (Nov 18, 2022) www.masterorganicchemistry.com/2014/06/06/the-cyclohexane-chair-flip-energy-diagram.
 X. Liu, "4.3 Conformation Analysis of Cyclohexane," Organic Chemistry I, (Nov 18, 2022) kpu.pressbooks.pub/organicchemistry/chapter/4-3-conformation-analysis-of-cyclohexane-and-substituted-cyclohexanes.
 S. V. Lavagnino. Eliminazione secondo Hofmann [Video]. (Nov 18, 2022) www.youtube.com/watch?v=nZDIQsNCpjM&list=PLswwssc6Q2yYoP_INHmbmouyxW8oP_Gib&index=46.
 J. Ashenhurst. "Antiperiplanar Relationships: The E2 Reaction and Cyclohexane Rings." (Nov 18, 2022) www.masterorganicchemistry.com/2012/10/18/the-e2-reaction-and-cyclohexane-rings.