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Ch. 7 - Structure and Synthesis of Alkenes; Elimination
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh. 7 - Structure and Synthesis of Alkenes; EliminationProblem 69b,c,d
Chapter 7, Problem 69b,c,d

When 2-bromo-3-phenylbutane is treated with sodium methoxide, two alkenes result (by E2 elimination). The Zaitsev product predominates.
b. When one pure stereoisomer of 2-bromo-3-phenylbutane reacts, one pure stereoisomer of the major product results. For example, when (2R,3R)-2-bromo-3-phenylbutane reacts, the product is the stereoisomer with the methyl groups cis. Use your models to draw a Newman projection of the transition state to show why this stereospecificity is observed.
c. Use a Newman projection of the transition state to predict the major product of elimination of (2S,3R)-2-bromo-3-phenylbutane.
d. Predict the major product from elimination of (2S,3S)-2-bromo-3-phenylbutane. This prediction can be made without drawing any structures, by considering the results in part (b).

Verified step by step guidance
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Step 1: Begin by understanding the mechanism of E2 elimination. E2 elimination is a bimolecular process where the base (sodium methoxide in this case) abstracts a proton from a β-carbon, and the leaving group (bromine) departs simultaneously, forming a double bond. The Zaitsev rule states that the more substituted alkene is the major product.
Step 2: For part (b), analyze the stereochemistry of (2R,3R)-2-bromo-3-phenylbutane. In E2 elimination, the anti-periplanar geometry is required for elimination to occur. Draw a Newman projection of the molecule along the C2-C3 bond, ensuring that the β-hydrogen and the bromine are anti-periplanar. This geometry leads to the formation of the Zaitsev product with the methyl groups cis.
Step 3: For part (c), consider the stereochemistry of (2S,3R)-2-bromo-3-phenylbutane. Again, use a Newman projection along the C2-C3 bond to identify the anti-periplanar β-hydrogen and bromine. Predict the major product based on the anti-periplanar elimination and the Zaitsev rule, ensuring the stereochemistry aligns with the transition state.
Step 4: For part (d), analyze the stereochemistry of (2S,3S)-2-bromo-3-phenylbutane. Based on the results from part (b), predict the major product without drawing structures. Consider the anti-periplanar requirement and the stereochemical relationship between the β-hydrogen and bromine.
Step 5: Summarize the stereochemical outcomes for each case, emphasizing the importance of anti-periplanar geometry in E2 elimination and how it dictates the stereospecificity and regioselectivity of the products. Highlight the Zaitsev rule and its role in determining the major product.

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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 from a β-carbon while a leaving group departs from the α-carbon, resulting in the formation of a double bond. This mechanism is characterized by its stereospecificity, meaning the spatial arrangement of substituents around the double bond is influenced by the orientation of the reactants. Understanding E2 is crucial for predicting the products of reactions involving alkyl halides.
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Zaitsev's Rule

Zaitsev's Rule states that in elimination reactions, the more substituted alkene (the Zaitsev product) is typically favored over the less substituted one. This preference arises because more substituted alkenes are generally more stable due to hyperconjugation and the inductive effect. Recognizing this rule helps in predicting the major product when multiple elimination pathways are possible.
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Newman Projections

Newman projections are a way to visualize the conformation of molecules by looking down the bond connecting two carbon atoms. This representation is particularly useful for analyzing steric interactions and torsional strain in conformers. In the context of elimination reactions, drawing Newman projections of the transition state can help illustrate the spatial arrangement of groups and predict the stereochemistry of the resulting alkenes.
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Related Practice
Textbook Question

When 2-bromo-3-phenylbutane is treated with sodium methoxide, two alkenes result (by E2 elimination). The Zaitsev product predominates.

a. Draw the reaction, showing the major and minor products.

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Textbook Question

Write a mechanism that explains the formation of the following product. In your mechanism, explain the cause of the rearrangement, and explain the failure to form the Zaitsev product.

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Textbook Question

Explain the dramatic difference in rotational energy barriers of the following three alkenes. (Hint: Consider what the transition states must look like.)

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Textbook Question

One of the following dichloronorbornanes undergoes elimination much faster than the other. Determine which one reacts faster, and explain the large difference in rates.

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Textbook Question

When (±)−2,3−dibromobutane reacts with potassium hydroxide, some of the products are (2S,3R)-3-bromobutan-2-ol and its enantiomer and trans-2-bromobut-2-ene. Why is no cis-2-bromobut-2-ene formed?

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Textbook Question

When (±)−2,3−dibromobutane reacts with potassium hydroxide, some of the products are (2S,3R)-3-bromobutan-2-ol and its enantiomer and trans-2-bromobut-2-ene. Give mechanisms to account for these products.

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