Ch. 12 - Substitution and Elimination: Reactions of Haloalkanes

Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition

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Ch. 12 - Substitution and Elimination: Reactions of Haloalkanes

Problem 78

Chapter 11, Problem 78

Of the three possible elimination mechanisms (Figure 12.50), this chapter focused on two of them (E1 and E2). The third possibility occurs in situations like the one below. What makes this mechanism favored under these conditions?

Verified step by step guidance

Understand the three elimination mechanisms: E1, E2, and E1cB. E1 involves a two-step process with carbocation formation, E2 is a concerted one-step process, and E1cB involves a carbanion intermediate. The problem hints at a third mechanism, likely E1cB, so we will focus on its conditions.

Analyze the given conditions. E1cB is favored when the leaving group is poor, the β-hydrogen is acidic (due to electron-withdrawing groups nearby), and the intermediate carbanion is stabilized by resonance or inductive effects.

Examine the structure of the substrate. Look for electron-withdrawing groups (e.g., -NO₂, -CN, -COOH) near the β-carbon, which can stabilize the carbanion intermediate through resonance or inductive effects.

Consider the reaction conditions. E1cB is typically favored in basic conditions, where a strong base can deprotonate the β-hydrogen to form the carbanion intermediate.

Conclude why this mechanism is favored. The combination of a poor leaving group, a stabilized carbanion intermediate, and basic conditions makes E1cB the preferred elimination pathway under these circumstances.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Elimination Mechanisms

Elimination mechanisms in organic chemistry refer to the processes by which atoms or groups are removed from a molecule, resulting in the formation of a double bond. The two primary types discussed are E1 (unimolecular elimination) and E2 (bimolecular elimination), each with distinct mechanisms and conditions. Understanding these mechanisms is crucial for predicting the products of reactions and the conditions under which they occur.

E1 Mechanism

The E1 mechanism involves a two-step process where the leaving group departs first, forming a carbocation intermediate, followed by deprotonation to form a double bond. This mechanism is favored in polar protic solvents and with substrates that can stabilize the carbocation, such as tertiary alkyl halides. The rate of the reaction depends only on the concentration of the substrate, making it unimolecular.

E2 Mechanism

The E2 mechanism is a concerted process where the leaving group and a hydrogen atom are removed simultaneously, resulting in the formation of a double bond. This mechanism requires a strong base and is favored in polar aprotic solvents. The reaction rate depends on both the substrate and the base, making it bimolecular, and it typically occurs with primary or secondary substrates where steric hindrance is less of a concern.

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