Ch.6 - Alkyl Halides; Nucleophilic Substitution

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch.6 - Alkyl Halides; Nucleophilic Substitution

Problem 9b

Chapter 6, Problem 9b

The light-initiated reaction of 2,3-dimethylbut-2-ene with N-bromosuccinimide (NBS) gives two products:

b. The bromination of cyclohexene using NBS gives only one major product, as shown on the previous page. Explain why there is no second product from an allylic shift.

Verified step by step guidance

Step 1: Understand the reaction mechanism. The reaction involves the allylic bromination of cyclohexene using N-bromosuccinimide (NBS) under light (hv). NBS is a source of bromine radicals, which selectively react at the allylic position of alkenes.

Step 2: Identify the allylic positions in cyclohexene. The allylic position is the carbon atom adjacent to the double bond. In cyclohexene, there are two equivalent allylic positions due to the symmetry of the molecule.

Step 3: Explain the formation of the major product. The bromine radical reacts with one of the allylic hydrogens, forming a resonance-stabilized allylic radical. This radical is symmetric, meaning the bromine can attach to either side of the allylic radical, but due to symmetry, only one product is formed.

Step 4: Address why no second product is formed. In cyclohexene, the allylic radical formed is symmetric, and there is no possibility for an allylic shift to create a different resonance structure that would lead to a second product. This symmetry ensures that only one major product is formed.

Step 5: Conclude the reasoning. The lack of a second product is due to the structural symmetry of cyclohexene, which prevents the formation of distinct allylic radicals or resonance structures that could lead to additional products.

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Key Concepts

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

Bromination Mechanism

Bromination of alkenes typically involves the addition of bromine across the double bond. In the presence of N-bromosuccinimide (NBS) and light, the reaction proceeds via a radical mechanism, where the bromine radical adds to the double bond, forming a brominated product. This mechanism is crucial for understanding how the reaction proceeds and why certain products are favored.

Allylic Rearrangement

Allylic rearrangement refers to the migration of a substituent from an allylic position (adjacent to a double bond) to a more stable position, often resulting in different product distributions. In the case of cyclohexene, the structure does not allow for a stable allylic radical to form after bromination, which limits the possibility of rearrangement and results in a single major product.

Stability of Radicals

The stability of radicals plays a significant role in determining the products of radical reactions. Tertiary radicals are more stable than secondary or primary radicals due to hyperconjugation and inductive effects. In the case of cyclohexene, the formation of a secondary radical during bromination does not lead to a more stable tertiary radical, thus preventing the formation of a second product from an allylic shift.

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Related Practice

Textbook Question

For each pair of compounds, predict which compound has the higher boiling point. Check [TABLE 6-2] to see if your prediction was right; then explain why that compound has the higher boiling point.

c. 1-bromobutane and 1-chlorobutane

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Textbook Question

Show how free-radical halogenation might be used to synthesize the following compounds. In each case, explain why we expect to get a single major product.

(d)

Textbook Question

Show how free-radical halogenation might be used to synthesize the following compounds. In each case, explain why we expect to get a single major product.

(c)

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Textbook Question

Show how free-radical halogenation might be used to synthesize the following compounds. In each case, explain why we expect to get a single major product.

(a) 1-chloro-2,2-dimethylpropane (neopentyl chloride)

(b) 2-bromo-2-methylbutane

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Textbook Question

Show how you might use S_N2 reactions to convert 1-chlorobutane into the following compounds.

b. 1-fluorobutane

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Textbook Question

The light-initiated reaction of 2,3-dimethylbut-2-ene with N-bromosuccinimide (NBS) gives two products:

a. Give a mechanism for this reaction, showing how the two products arise as a consequence of the resonance-stabilized intermediate.

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The light-initiated reaction of 2,3-dimethylbut-2-ene with N-bromosuccinimide (NBS) gives two products: b. The bromination of cyclohexene using NBS gives only one major product, as shown on the previous page. Explain why there is no second product from an allylic shift.

Verified video answer for a similar problem:

Key Concepts

Bromination Mechanism

Allylic Rearrangement

Stability of Radicals

The light-initiated reaction of 2,3-dimethylbut-2-ene with N-bromosuccinimide (NBS) gives two products:

b. The bromination of cyclohexene using NBS gives only one major product, as shown on the previous page. Explain why there is no second product from an allylic shift.