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 71

Chapter 7, Problem 71

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

Verified step by step guidance

Step 1: Analyze the structures of the cis- and trans-dichloronorbornanes. The cis isomer has both chlorine atoms on the same side of the bicyclic ring system, while the trans isomer has the chlorine atoms on opposite sides of the ring system.

Step 2: Consider the elimination reaction mechanism. The reaction involves the removal of a hydrogen atom and a chlorine atom to form a double bond. This is typically an E2 elimination mechanism, which requires the hydrogen and chlorine atoms to be anti-periplanar (i.e., opposite sides of the same plane).

Step 3: Evaluate the geometry of the cis isomer. In the cis isomer, the hydrogen atom and chlorine atom that are candidates for elimination are not anti-periplanar due to the rigid bicyclic structure. This makes elimination less favorable.

Step 4: Evaluate the geometry of the trans isomer. In the trans isomer, the hydrogen atom and chlorine atom are anti-periplanar, which aligns perfectly for the E2 elimination mechanism. This makes elimination much faster for the trans isomer.

Step 5: Conclude that the trans isomer undergoes elimination much faster than the cis isomer due to the anti-periplanar geometry required for the E2 mechanism, which is achievable in the trans isomer but not in the cis isomer.

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Key Concepts

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

Elimination Reactions

Elimination reactions involve the removal of a small molecule from a larger one, typically resulting in the formation of a double bond. In organic chemistry, these reactions can follow either an E1 or E2 mechanism, which differ in their steps and conditions. Understanding the mechanism is crucial for predicting the rate and outcome of the reaction.

Steric Hindrance

Steric hindrance refers to the repulsion between bulky groups in a molecule that can impede reactions. In the context of elimination reactions, steric hindrance can affect the accessibility of the leaving group and the formation of the double bond. The spatial arrangement of substituents, such as in cis and trans isomers, plays a significant role in determining the reaction rate.

Cis-Trans Isomerism

Cis-trans isomerism is a form of stereoisomerism where the relative positioning of substituents differs around a double bond or a ring structure. In the case of dichloronorbornanes, the cis isomer may allow for a more favorable transition state during elimination due to less steric hindrance compared to the trans isomer, leading to a faster reaction rate.

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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

The following reaction is called the pinacol rearrangement. The reaction begins with an acid-promoted ionization to give a carbocation. This carbocation undergoes a methyl shift to give a more stable, resonance-stabilized cation. Loss of a proton gives the observed product. Propose a mechanism for the pinacol rearrangement.

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Textbook Question

Deuterium (D) is the isotope of hydrogen of mass number 2, with a proton and a neutron in its nucleus. The chemistry of deuterium is nearly identical to the chemistry of hydrogen, except that the C―D bond is slightly (5.0 kJ/mol, or 1.2 kcal/mol) stronger than the C―H bond. Reaction rates tend to be slower if a C―D bond (as opposed to a C―H bond) is broken in a rate-limiting step. This effect on the rate is called a kinetic isotope effect. (Review PROBLEM 4-57)

a. Propose a mechanism to explain each product in the following reaction.

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Textbook Question

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).

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