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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 24b
Chapter 7, Problem 24b

Which of these reactions are likely to produce both elimination and substitution products?
a. 2-bromopentane + NaOCH3
b. 3-bromo-3-methylpentane + NaOMe. (Me = methyl, CH3)
c. 2-bromo-3-ethylpentane + NaOH

Verified step by step guidance
1
Step 1: Understand the nature of the alkyl halide in each reaction. The structure of the alkyl halide determines whether the reaction will favor substitution (SN1 or SN2) or elimination (E1 or E2). For example, primary, secondary, and tertiary alkyl halides behave differently due to steric hindrance and carbocation stability.
Step 2: Analyze the nucleophile/base used in each reaction. Sodium methoxide (NaOCH3) and sodium hydroxide (NaOH) are strong bases and nucleophiles, which can promote both substitution and elimination reactions. Strong bases favor elimination (E2), while strong nucleophiles favor substitution (SN2).
Step 3: Consider the reaction conditions. For secondary alkyl halides, such as 2-bromopentane and 2-bromo-3-ethylpentane, both substitution and elimination are possible because the secondary carbon can undergo SN2 substitution or E2 elimination depending on the strength of the base/nucleophile and steric factors.
Step 4: Examine the structure of 3-bromo-3-methylpentane. This is a tertiary alkyl halide, which is highly sterically hindered. Tertiary alkyl halides typically undergo elimination (E2) rather than substitution (SN2) when reacting with strong bases like NaOMe. However, substitution (SN1) is possible under certain conditions due to carbocation stability.
Step 5: Summarize the likelihood of elimination and substitution for each reaction. For (a) 2-bromopentane + NaOCH3, both SN2 and E2 are likely. For (b) 3-bromo-3-methylpentane + NaOMe, elimination (E2) is more likely due to steric hindrance, but SN1 substitution could occur under specific conditions. For (c) 2-bromo-3-ethylpentane + NaOH, both SN2 and E2 are possible due to the secondary alkyl halide and strong base/nucleophile.

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Key Concepts

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

Nucleophilic Substitution Reactions

Nucleophilic substitution reactions involve the replacement of a leaving group in a molecule by a nucleophile. In organic chemistry, these reactions can occur via two main mechanisms: SN1 and SN2. The choice of mechanism depends on factors such as the structure of the substrate and the strength of the nucleophile. Understanding these mechanisms is crucial for predicting the products of reactions involving alkyl halides.
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Nucleophiles and Electrophiles can react in Substitution Reactions.

Elimination Reactions

Elimination reactions involve the removal of a small molecule, often HX or H2O, from a larger molecule, resulting in the formation of a double bond. The two primary types of elimination reactions are E1 and E2. E1 reactions are unimolecular and typically occur in two steps, while E2 reactions are bimolecular and occur in a single concerted step. Recognizing conditions that favor elimination over substitution is essential for predicting reaction outcomes.
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Regioselectivity and Stereoselectivity

Regioselectivity refers to the preference of a chemical reaction to yield one structural isomer over others, while stereoselectivity indicates the preference for one stereoisomer over another. In reactions that can produce both substitution and elimination products, understanding these concepts helps predict which products will be favored based on the substrate structure and reaction conditions. Factors such as sterics and electronics play a significant role in determining the regio- and stereochemical outcomes.
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Related Practice
Textbook Question

Show that the (S,S) enantiomer of this (R,R) diastereomer of 1-bromo-1,2-diphenylpropane also undergoes E2 elimination to give the cis diastereomer of the product. (We do not expect these achiral reagents to distinguish between enantiomers.)

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Textbook Question

For each reaction, decide whether substitution or elimination (or both) is possible, and predict the products you expect. Label the major products.

a. 1−bromo−1−methylcyclohexane + NaOH in acetone

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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 which of the three alkenes is the major elimination product.

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Textbook Question

Predict the elimination products of the following reactions. When two alkenes are possible, predict which one will be the major product. Explain your answers, showing the degree of substitution of each double bond in the products.

a. 2-bromopentane + NaOCH3

b. 3-bromo-3-methylpentane + NaOMe (Me = methyl,CH3)

c. 2-bromo-3-ethylpentane + NaOH

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Textbook Question

For each reaction, decide whether substitution or elimination (or both) is possible, and predict the products you expect. Label the major products.

c. chlorocyclohexane+NaOCH3 in CH3OH

d. chlorocyclohexane + NaOC(CH3)3 in (CH3)3COH

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Textbook Question

Under second-order conditions (strong base/nucleophile), SN2 and E2 reactions may occur simultaneously and compete with each other. Show what products might be expected from the reaction of 2-bromo-3-methylbutane (a moderately hindered 2° alkyl halide) with sodium ethoxide.

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