Ch. 7 - Structure and Synthesis of Alkenes; Elimination

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch. 7 - Structure and Synthesis of Alkenes; Elimination

Problem 31b

Chapter 7, Problem 31b

Give the expected product(s) of E2 elimination for each reaction. (Hint: Use models!)
(b)

Verified step by step guidance

Step 1: Identify the substrate and the leaving group. The substrate is a cyclohexane derivative with a bromine atom (Br) as the leaving group. The reaction involves E2 elimination, which requires a strong base (NaOCH3) and an anti-periplanar geometry between the β-hydrogen and the leaving group.

Step 2: Determine the anti-periplanar β-hydrogens. In E2 elimination, the β-hydrogen must be anti-periplanar (opposite in the plane) to the leaving group. Analyze the structure to locate the β-hydrogens adjacent to the carbon bonded to Br. Note that one of the β-hydrogens is a deuterium (D).

Step 3: Predict the elimination products. The elimination will form double bonds between the α-carbon (the carbon bonded to Br) and the β-carbon (the carbon with the β-hydrogen). Since there are two β-hydrogens (one H and one D), two different alkenes can form: one with a hydrogen eliminated and one with a deuterium eliminated.

Step 4: Consider the stereochemistry of the products. The anti-periplanar geometry dictates the stereochemistry of the resulting double bonds. Draw the two possible alkenes, ensuring the correct placement of substituents and double bonds.

Step 5: Verify the regioselectivity and stability of the products. Compare the two alkenes formed in terms of their stability (e.g., substitution level of the double bond) and regioselectivity. Both products are expected due to the presence of two anti-periplanar β-hydrogens.

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Key Concepts

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

E2 Elimination Mechanism

The E2 elimination mechanism is a concerted reaction where a base removes a proton (H) from a β-carbon while a leaving group (like Br) departs from the α-carbon, resulting in the formation of a double bond. This mechanism requires the base to be strong and the substrate to be appropriately oriented, typically leading to the formation of alkenes.

Stereochemistry in E2 Reactions

Stereochemistry plays a crucial role in E2 reactions, as the elimination must occur in an anti-periplanar arrangement for optimal overlap of orbitals. This means that the hydrogen atom being removed and the leaving group must be positioned opposite each other, influencing the stereochemical outcome of the products formed.

Regioselectivity in Elimination Reactions

Regioselectivity refers to the preference of a chemical reaction to yield one structural isomer over others. In E2 eliminations, the formation of more stable alkenes (often the more substituted alkene) is favored, which can lead to the generation of multiple products depending on the substrate's structure and the base used.

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Related Practice

Textbook Question

Predict which of the following compounds will undergo elimination with KOH faster, and explain why. Predict the major product that will be formed.

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Textbook Question

Give the expected product(s) of E2 elimination for each reaction. (Hint: Use models!)

(a)

990

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Textbook Question

Predict the major and minor elimination products of the following proposed reactions (ignoring any possible substitutions for now). In each case, explain whether you expect the mechanism of the elimination to be E1 or E2.

(b)

923

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Textbook Question

Two stereoisomers of a bromodecalin are shown. Although the difference between these stereoisomers may seem trivial, one isomer undergoes elimination with KOH much faster than the other. Predict the products of these eliminations, and explain the large difference in the ease of elimination.

1334

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Textbook Question

Predict the products and mechanisms of the following reactions. When more than one product or mechanism is possible, explain which are most likely.

a. 1−bromohexane + sodium ethoxide in ethanol

(a)