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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 20a
Chapter 7, Problem 20a

Give the substitution and elimination products you would expect from the following reactions.
a. 3-bromo-3-ethylpentane heated in methanol

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Analyze the structure of the substrate, 3-bromo-3-ethylpentane. The carbon attached to the bromine is tertiary, making it prone to undergo both substitution (SN1) and elimination (E1) reactions in the presence of a polar protic solvent like methanol.
Understand the role of methanol (CH3OH). Methanol is a polar protic solvent, which stabilizes the carbocation intermediate formed during the reaction. This makes the SN1 and E1 mechanisms more favorable.
For the SN1 reaction: The bromine atom leaves, forming a tertiary carbocation. Methanol acts as a nucleophile and attacks the carbocation, leading to the formation of a substitution product where the bromine is replaced by a methoxy group (-OCH3).
For the E1 reaction: After the formation of the tertiary carbocation, a beta-hydrogen is removed by methanol acting as a base. This results in the formation of an alkene. The major product will follow Zaitsev's rule, where the more substituted alkene is favored.
Consider the reaction conditions (heat). Heating typically favors elimination (E1) over substitution (SN1). Therefore, the elimination product (alkene) is likely to be the major product, while the substitution product (ether) will be the minor product.

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Key Concepts

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

Nucleophilic Substitution Reactions (SN1 and SN2)

Nucleophilic substitution reactions involve the replacement of a leaving group (like Br) by a nucleophile (like methanol). SN1 reactions are unimolecular and proceed through a carbocation intermediate, favoring tertiary substrates, while SN2 reactions are bimolecular and involve a direct attack by the nucleophile, favoring primary substrates. The choice between SN1 and SN2 depends on the substrate structure and reaction conditions.
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Nucleophiles and Electrophiles can react in Substitution Reactions.

Elimination Reactions (E1 and E2)

Elimination reactions involve the removal of a leaving group and a hydrogen atom, resulting in the formation of a double bond. E1 reactions are unimolecular and also proceed through a carbocation intermediate, while E2 reactions are bimolecular and occur in a single concerted step. The type of elimination reaction favored depends on the substrate and the strength of the base used in the reaction.
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Understanding the properties of E1.

Reaction Conditions and Solvent Effects

The choice of solvent and reaction conditions significantly influences the outcome of substitution and elimination reactions. Polar protic solvents, like methanol, stabilize carbocations and favor SN1 and E1 mechanisms, while polar aprotic solvents favor SN2 and E2 mechanisms. Heating the reaction can also promote elimination pathways, leading to the formation of alkenes alongside substitution products.
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Related Practice
Textbook Question

The solvolysis of 2-bromo-3-methylbutane potentially can give several products, ­including both E1 and products from both the unrearranged carbocation and the rearranged ­carbocation. Mechanisms 6-6 and 7-2 show the products from the rearranged carbocation. Summarize all the possible products, showing which carbocation they come from and whether they are the products of E1 or reactions.

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Textbook Question

SN1 substitution and E1 elimination frequently compete in the same reaction.

b. Compare the function of the solvent (methanol) in the E1 and SN1 reactions.

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Textbook Question

Finish Solved Problem 7-3 by showing how the rearranged carbocations give the four products shown in the problem. Be careful when using curved arrows to show ­deprotonation and/or nucleophilic attack by the solvent. The curved arrows always show ­movement of electrons, not movement of protons or other species.

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Textbook Question

Give the substitution and elimination products you would expect from the following reactions.

c. 1-bromo-2-methylcyclohexane + silver nitrate in water (AgNO3 forces ionization)

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Textbook Question

Give the substitution and elimination products you would expect from the following reactions.

b. 1-iodo-1-phenylcyclopentane heated in ethanol

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Textbook Question

When (1-bromoethyl)cyclohexane is heated in methanol for an extended period of time, five products result: two ethers and three alkenes. Predict the products of this reaction, and propose mechanisms for their formation.

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