Ch. 8 - Alkenes I: Properties and Electrophilic Additions

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

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Mullins 1st Edition

Ch. 8 - Alkenes I: Properties and Electrophilic Additions

Problem 55e

Chapter 7, Problem 55e

Predict the major product(s) of the following elimination reactions, paying close attention to the stereochemical outcome of the reactions.
(e)

Verified step by step guidance

Step 1: Identify the type of elimination reaction. The presence of a strong base, such as tert-butanol (t-BuOH), suggests that this reaction will likely proceed via an E2 mechanism. In an E2 reaction, the base abstracts a proton from a β-carbon, and the leaving group (Cl) departs simultaneously, forming a double bond.

Step 2: Analyze the stereochemistry of the substrate. The molecule contains a cyclohexane ring with a chlorine atom as the leaving group. For an E2 reaction, the β-hydrogen and the leaving group must be anti-periplanar (i.e., opposite sides of the plane). Check the stereochemistry of the β-hydrogens relative to the chlorine atom.

Step 3: Determine the β-hydrogens available for elimination. In this case, the β-hydrogens are located on the carbons adjacent to the carbon bearing the chlorine atom. Evaluate which β-hydrogen is anti-periplanar to the chlorine atom and can be abstracted by the base.

Step 4: Predict the major product based on Zaitsev's rule. Zaitsev's rule states that the most substituted alkene is generally the major product in elimination reactions. Consider the possible double bonds that can form and identify the most stable (most substituted) alkene.

Step 5: Consider stereochemical outcomes. Since the reaction proceeds via an E2 mechanism, the stereochemistry of the product will depend on the anti-periplanar geometry of the β-hydrogen and the leaving group. Draw the major product with the correct stereochemistry and confirm that it follows the rules of elimination.

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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 atoms or groups from a molecule, resulting in the formation of a double bond. Common types include E1 and E2 mechanisms, which differ in their reaction pathways and conditions. Understanding the mechanism is crucial for predicting the products, as it influences the stereochemistry and regioselectivity of the reaction.

Stereochemistry

Stereochemistry refers to the spatial arrangement of atoms in molecules and how this affects their chemical behavior. In elimination reactions, the stereochemical outcome can lead to different products, such as cis or trans isomers. Recognizing the stereochemical implications is essential for accurately predicting the major product of the reaction.

Regioselectivity

Regioselectivity is the preference of a chemical reaction to yield one structural isomer over others when multiple products are possible. In elimination reactions, this concept helps determine which alkene product is favored based on factors like stability and sterics. Understanding regioselectivity is key to predicting the major product in elimination reactions.

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Related Practice

Textbook Question

Unlike hydroboration–oxidation, the addition of H₂O catalyzed by H₃O⁺ is not stereospecific. Thinking carefully about the mechanism of the reaction, give two reasons why.

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Textbook Question

The first step of oxymercuration–reduction is stereospecific, and yet this fact wasn't emphasized in that discussion. Show the stereospecificity of the first step for the following alkene and then explain why the stereospecificity becomes unimportant after the second step.

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Textbook Question

Suggest an alkene to undergo hydroboration–oxidation (1. BH₃ 2. NaOH, H₂O₂) to give exclusively the alcohols shown. Pay close attention to the relative (but not absolute) stereochemical outcome.

(d)

(c)

Calculate the index of hydrogen deficiency for the following molecular formulas and structures.

(a)

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