Ch. 9 - Substitution and Elimination Reactions of Alkyl Halides

Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook

Bruice 8th Edition

Ch. 9 - Substitution and Elimination Reactions of Alkyl Halides

Problem 96c,d

Chapter 10, Problem 96c,d

For each of the following alkyl halides, indicate the stereoisomer that would be obtained in greatest yield in an E2 reaction.
c. 3-bromo-2,3-dimethylpentane
d. 3-bromo-3,4-dimethylhexane

Verified step by step guidance

Step 1: Recall the mechanism of an E2 reaction. E2 reactions are bimolecular eliminations where a base removes a β-hydrogen, and the leaving group departs simultaneously, forming a double bond. The reaction is stereospecific, favoring the anti-periplanar geometry of the β-hydrogen and the leaving group.

Step 2: Analyze the structure of 3-bromo-2,3-dimethylpentane. Identify the β-hydrogens available for elimination. Look for hydrogens on carbons adjacent to the carbon bonded to the bromine atom. Consider the anti-periplanar geometry to determine which β-hydrogen can be eliminated.

Step 3: Determine the stereoisomer formed in the elimination of 3-bromo-2,3-dimethylpentane. The anti-periplanar geometry dictates that the elimination will occur in a specific direction, forming the most stable alkene. Stability is influenced by factors such as substitution (more substituted alkenes are more stable) and steric hindrance.

Step 4: Analyze the structure of 3-bromo-3,4-dimethylhexane. Similar to the previous case, identify the β-hydrogens adjacent to the carbon bonded to the bromine atom. Again, focus on the anti-periplanar geometry to determine which β-hydrogen can be eliminated.

Step 5: Determine the stereoisomer formed in the elimination of 3-bromo-3,4-dimethylhexane. Consider the anti-periplanar geometry and the stability of the resulting alkene. The most substituted alkene will generally be favored, and steric hindrance should also be taken into account.

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Key Concepts

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

E2 Reaction Mechanism

The E2 (bimolecular elimination) reaction is a concerted process where a base abstracts a proton while a leaving group departs, resulting in the formation of a double bond. This mechanism typically requires a strong base and occurs in a single step, leading to the formation of alkenes. The stereochemistry of the substrate plays a crucial role, as the reaction favors anti-periplanar geometry for optimal overlap of orbitals.

Stereochemistry and Stereoisomers

Stereochemistry refers to the spatial arrangement of atoms in molecules and how this affects their chemical behavior. Stereoisomers are compounds with the same molecular formula and connectivity but different spatial orientations. In E2 reactions, the stereochemistry of the alkyl halide influences which stereoisomer is formed preferentially, often favoring the more stable trans or E isomer due to steric factors.

Zaitsev's Rule

Zaitsev's Rule states that in elimination reactions, the more substituted alkene (the one with more alkyl groups attached to the double bond) is typically the major product. This is because more substituted alkenes are generally more stable due to hyperconjugation and the inductive effect. Understanding this rule helps predict which stereoisomer will be formed in greater yield during E2 reactions involving alkyl halides.

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Defining Zaitsev’s Rule

Related Practice

Textbook Question

Which of following ethers cannot be made by a Williamson ether synthesis?

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Textbook Question

For each of the following alkyl halides, indicate the stereoisomer that would be obtained in greatest yield in an E2 reaction.

a. 3-bromo-2,2,3-trimethylpentane

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Textbook Question

When 2-bromo-2,3-dimethylbutane reacts with a strong base, two alkenes (2,3-dimethyl-1-butene and 2,3-dimethyl-2-butene) are formed.

a. Which of the bases (A, B, C, or D) would form the highest percentage of the 1-alkene?

b. Which would give the highest percentage of the 2-alkene?

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Textbook Question

For each of the following alkyl halides, indicate the stereoisomer that would be obtained in greatest yield in an E2 reaction.

b. 4-bromo-2,2,3,3-tetramethylpentane

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Textbook Question

Rank the following from most reactive to least reactive in an S_N1 reaction:

For each of the following alkyl halides, indicate the stereoisomer that would be obtained in greatest yield in an E2 reaction.

d. 3-bromo-3,4-dimethylhexane

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For each of the following alkyl halides, indicate the stereoisomer that would be obtained in greatest yield in an E2 reaction.c. 3-bromo-2,3-dimethylpentaned. 3-bromo-3,4-dimethylhexane

Verified video answer for a similar problem:

Key Concepts

E2 Reaction Mechanism

Stereochemistry and Stereoisomers

Zaitsev's Rule

For each of the following alkyl halides, indicate the stereoisomer that would be obtained in greatest yield in an E2 reaction.
c. 3-bromo-2,3-dimethylpentane
d. 3-bromo-3,4-dimethylhexane