An elimination will
Lead to formation of
(E2 results from build
Abstraction and leaving,
This post is from 8 April 2022 and marks the last of the mechanism-themed poems from NaPoWriMo2022. These verses were fun to write but, as with both the kinetics and enthalpy “series” in previous months, the resulting essays deal with themes that can seem remarkably abstract! Next week marks a return to some less involved topics, for the remainder of the semester.
This last poem addresses two new types of reaction mechanisms that often compete with the nucleophilic substitution reactions seen in previous posts (SN1 and SN2). These two new types of reaction pathways are called eliminations, wherein a base reacts with (often) an alkyl halide to “eliminate” a hydrogen atom and the leaving group, ultimately yielding the formation of an alkene, a compound with a double bond. As with the nucleophilic substitutions, eliminations (represented generally as E) can occur via a two-step process (E1, for unimolecular elimination) or a single-step, concerted process (E2, for bimolecular elimination). Both are shown in the diagram below, using conventions of electron-pushing mechanisms.
Via E1, the bond to the leaving group breaks first, yielding a carbocation as the bromide ion leaves. As shown here, a neutral base with a lone electron pair then abstracts (removes) a proton, so that the electrons originally in that C-H bond form a new pi bond between two carbon atoms. Besides the bromide ion, which departed in Step 1, the other side product is the now-protonated generic base, from Step 2.
Via E2, the negatively-charged bulky base doesn’t have enough room to attack the alkyl halide as a nucleophile. Instead, it abstracts a proton, and the subsequent formation of a pi bond then causes the departure of the bromide ion as the leaving group, all in one reaction step. The leftover side products here are the now neutral tert-butanol and the bromide ion, both from the single reaction step.
As with the other poems from this month, before launching into the essay, it’s worth acknowledging the motivation: the “why do we care about this in the first place?” aspect of such complicated topics. These four reaction patterns (SN1, SN2, E1, and E2) are evident in a tremendous number of settings. Chemistry students traditionally begin their extensive training in chemical and biochemical mechanisms by learning these four options and learning how to predict which of the four is likely to predominate given a set of reaction conditions. The reactions have massive implications for organic synthesis, biochemistry, and many other branches of chemistry. However, trying to learn them in the first place can be imposing. This poem takes several aspects of the elimination mechanisms and presents them in a rhymed format, which ideally might be memorable for students learning the material.
“Rivaling SN, /
An elimination will /
Lead to formation of /
Nucleophilic substitution reactions and elimination reactions “rival” one another: they involve comparable reactants that can accomplish multiple mechanistic steps, competing for the most likely pathway in a given situation. The reactant molecules in the reactions shown here could also theoretically undergo SN1 or SN2 reactions, respectively.
Why is this? Both bases and nucleophiles use electron pairs to achieve mechanistic ends: many molecules act as one or the other interchangeably. How a negatively charged species will act comes down to its own bulkiness and other reaction conditions. Does it have enough room to attack as a nucleophile, or is the organic molecule crowded (sterically hindered), so that abstraction of a proton is more feasible? Is it neutral or negatively charged? Many such questions help students make the “call” of whether SN2, SN1, E2, or E1 is occurring in a given scenario.
An elimination pathway yields a “newfound alkene”: a molecule containing a double bond.
“(E2 results from build /
Abstraction and leaving, /
Discerning between E1 and E2 mechanisms means considering characteristics of the reactant molecules, the base, the solvent, and other factors, in processes reminiscent of discerning between SN1 and SN2.
The new consideration for eliminations is that E2 has a geometric constraint (required 3-D arrangement) in the organic substrate. The proton that is abstracted from the alkyl halide and the leaving group must be “anticoplanar” to one another: in the same plane, on opposite sides of the molecule. “Abstraction and leaving [are] coincident”: these two steps happen in a concerted fashion, via E2.