E2 (Elimination, bimolecular) is a type of organic reaction mechanism where a base removes two atoms, typically a hydrogen and a leaving group, from adjacent carbon atoms in a single step, resulting in the formation of a new carbon-carbon double bond.
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The E2 mechanism involves the concerted removal of a hydrogen atom and a leaving group from adjacent carbon atoms, resulting in the formation of a new carbon-carbon double bond.
E2 reactions typically occur with strong, unhindered bases and substrates containing a good leaving group on a primary or secondary carbon.
The stereochemistry of the E2 reaction is anti-periplanar, meaning the hydrogen being removed and the leaving group must be on opposite sides of the molecule.
E2 reactions are often in competition with SN2 reactions, and the mechanism that predominates depends on factors such as the nature of the base, leaving group, and substrate.
In the context of biochemistry, the E2 mechanism is observed in the conversion of pyruvate to acetyl-CoA, a key step in the citric acid cycle.
Review Questions
Explain the key features of the E2 reaction mechanism and how it differs from other elimination reactions like E1.
The E2 mechanism is a bimolecular elimination reaction where a base removes a hydrogen atom and a leaving group from adjacent carbon atoms in a single, concerted step. This results in the formation of a new carbon-carbon double bond. The E2 mechanism is distinct from the E1 mechanism, which proceeds through a carbocation intermediate. The E2 reaction requires a strong, unhindered base and a substrate with a good leaving group, typically on a primary or secondary carbon. The stereochemistry of the E2 reaction is anti-periplanar, meaning the hydrogen being removed and the leaving group must be on opposite sides of the molecule.
Describe how the E2 mechanism is involved in the conversion of pyruvate to acetyl-CoA, and explain the significance of this reaction in the context of cellular metabolism.
The E2 mechanism is observed in the conversion of pyruvate to acetyl-CoA, a key step in the citric acid cycle. In this reaction, the enzyme pyruvate dehydrogenase acts as a base, removing a hydrogen atom and the carboxylate group (the leaving group) from pyruvate, resulting in the formation of acetyl-CoA. Acetyl-CoA is a central metabolite that can then enter the citric acid cycle, where it is further oxidized to generate ATP and reducing equivalents (NADH and FADH2) for the electron transport chain. This conversion of pyruvate to acetyl-CoA is a critical step in cellular respiration, as it allows the complete oxidation of carbohydrates to CO2 and H2O, providing the majority of the cell's energy in the form of ATP.
Analyze the factors that influence whether an organic reaction will proceed via an E2 or an SN2 mechanism, and explain the implications of each mechanism for the stereochemistry of the product.
The choice between an E2 or SN2 mechanism depends on a variety of factors, including the nature of the base, the leaving group, and the substrate. Generally, strong, unhindered bases favor the E2 mechanism, while weaker, more hindered bases tend to promote the SN2 pathway. Additionally, substrates with good leaving groups on primary or secondary carbons are more likely to undergo E2 elimination, while substrates with poor leaving groups or tertiary carbons are more susceptible to SN2 substitution. The stereochemical outcomes of these two mechanisms also differ. The E2 reaction proceeds with anti-periplanar stereochemistry, resulting in the formation of a new carbon-carbon double bond with trans geometry. In contrast, the SN2 mechanism proceeds with inversion of stereochemistry at the reaction center. Understanding the factors that influence the E2 versus SN2 pathways and their respective stereochemical outcomes is crucial for predicting and analyzing the products of organic reactions.
A type of organic reaction where two atoms or groups are removed from adjacent atoms, resulting in the formation of a new carbon-carbon double or triple bond.