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Transition States and Intermediates in E1-E1cB-E2 Elimination Reaction Sequence

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elimination reaction
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This Demonstration shows various types of elimination reactions for alkyl halides. Three cases can occur, depending on whether the hydrogen in the
β
-position detaches earlier (E1cB), simultaneously (E2) or later than the halogen of the molecule (E1). These three possibilities are influenced by the leaving group (halogen). E1cB and E1 reactions take place in two steps, and therefore exhibit two transition states (TS1 and TS2). The E2 reaction takes place in a single step, with a reaction intermediate (I)[1–3].
We studied a generic ethyl halide (
CH
3
CH
2
X
), in which the elimination mechanism changes from E1B to E1, passing through E2. It is possible to visualize how TS1, TS2 and I change: each elimination mechanism differs in the double bond because it is influenced by the different hybridization of the carbon (in two-step reactions, one carbon will be hybridized
2
sp
; in a one-step reaction, the detachment of halogen and hydrogen occurs synchronously), meaning that the double bond will be stronger or weaker, depending on the elimination mechanism.
The elimination reaction starts out first by the E1cB mechanism, then using the "progress" slider you can move to another mechanism. The two-dimensional diagram helps you understand which reaction and the relative strength of the double bond ("1/2=" means it is half-formed). The three-dimensional plot shows the potential energy, where at the top is an E1cB elimination, in the middle an E2 elimination, and at the bottom an E1 elimination. In between are mixed cases. Then three different paths can be selected, and it is possible to see changes from E1cb, then in E2 and finally in E1: the first one focuses on TS1, the second one focuses on I and the third one focuses on TS2.
The "focus" checkbox shows which bonds are involved and their geometrical changes due to the hybridization.

Details

Snapshot 1: First transition state (TS1) of the E1cB elimination; as the CH bond weakens, the double bond starts forming. The energy in the three-dimensional plot is a minimum at the intersection between the curves.
Snapshot 2: Intermediate state (I) of the E1cB elimination; as the CH bond weakens, the double bond begins to form. The energy is again a minimum at the intersection between the curves.
Snapshot 3: Second transition state (TS2) of the E1cB elimination; as the CH bond weakens, the double bond is almost formed. The energy in the three-dimensional plot is a maximum at the intersection.

References

[1] H. Hart, L. E. Craine and D. J. Hart, Organic Chemistry: A Short Course, 10th ed., Boston: Houghton Mifflin, Co., 1999.
[2] S. Z. Lavagnino, Eliminazione secondo Hofmann[Video]. (Jun 26, 2023) www.youtube.com/watch?v=nZDIQsNCpjM&list=PLswwssc6Q2yYoP_INHmbmouyxW8oP _Gib&index=47.
[3] J. Ashenhurst. "E1cB–Elimination (Unimolecular) Conjugate Base." Master Organic Chemistry. (Jun 26, 2023) www.masterorganicchemistry.com/2020/02/11/e1cb-elimination-unimolecular-conjugate-base.

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