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 41a

Chapter 6, Problem 41a

Allylic halides have the structure

a. Show how the first-order ionization of an allylic halide leads to a resonance-stabilized cation.

Verified step by step guidance

Step 1: Recognize the structure of an allylic halide. The given structure contains a halogen atom (X) attached to a carbon atom that is adjacent to a double bond. This is characteristic of an allylic halide.

Step 2: Understand the first-order ionization process. In this process, the bond between the halogen (X) and the carbon atom breaks heterolytically, resulting in the formation of a carbocation and a halide ion (X⁻).

Step 3: Analyze the resulting carbocation. The carbon atom that loses the halogen becomes positively charged, forming an allylic carbocation. This carbocation is resonance-stabilized due to the delocalization of the positive charge over the π-electron system of the double bond.

Step 4: Draw the resonance structures. The positive charge can shift between the two carbons of the double bond, creating two resonance structures. Use curved arrows to show the movement of electrons from the π-bond to the positively charged carbon.

Step 5: Explain the stabilization. Resonance stabilization occurs because the positive charge is delocalized over multiple atoms, reducing the energy of the carbocation and making it more stable compared to a non-resonance-stabilized carbocation.

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Key Concepts

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

Allylic Halides

Allylic halides are organic compounds where a halogen atom is bonded to a carbon atom adjacent to a carbon-carbon double bond. This structure allows for unique reactivity patterns, particularly in nucleophilic substitution reactions, due to the presence of the double bond which can stabilize intermediates formed during reactions.

First-Order Ionization

First-order ionization refers to a reaction mechanism where a single molecule undergoes a transformation to form an ion, typically involving the departure of a leaving group. In the case of allylic halides, the halogen leaves, generating a carbocation that can be stabilized by resonance with the adjacent double bond.

Resonance Stabilization

Resonance stabilization occurs when a molecule can be represented by multiple valid Lewis structures, allowing for the delocalization of electrons. In allylic halides, the carbocation formed after ionization can be stabilized by resonance with the π electrons of the adjacent double bond, leading to a more stable cationic intermediate.

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Related Practice

Textbook Question

Allylic halides have the structure

c. Show the products expected from S_N1 solvolysis of these halides in ethanol.

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Textbook Question

Chlorocyclohexane reacts with sodium cyanide (NaCN) in ethanol to give cyanocyclohexane. The rate of formation of cyanocyclohexane increases when a small amount of sodium iodide is added to the solution. Explain this acceleration in the rate.

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Textbook Question

Give the substitution products expected from solvolysis of each compound by heating in ethanol.

(c)

(d)

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Textbook Question

Give the substitution products expected from solvolysis of each compound by heating in ethanol.

(a)

(b)