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 56

Chapter 6, Problem 56

The following reaction takes place under second-order conditions (strong nucleophile), yet the structure of the product shows rearrangement. Also, the rate of this reaction is several thousand times faster than the rate of substitution of hydroxide ion on 2-chlorobutane under similar conditions. Propose a mechanism to explain the enhanced rate and rearrangement observed in this unusual reaction. (“Et” is the abbreviation for ethyl.)

Verified step by step guidance

Step 1: Analyze the reaction conditions. The reaction occurs under second-order conditions with a strong nucleophile (Et₂N:), which suggests an SN2 mechanism. However, the product shows rearrangement, indicating that the reaction may involve additional steps beyond a simple SN2 substitution.

Step 2: Consider the structure of the substrate. The substrate contains a β-chlorine atom attached to a secondary carbon, which is adjacent to a tertiary carbon. This arrangement makes the molecule prone to rearrangement due to the formation of a more stable carbocation intermediate.

Step 3: Propose the mechanism. The reaction likely begins with the nucleophile (Et₂N:) attacking the β-carbon, displacing the chloride ion (Cl⁻). This step forms a carbocation intermediate at the tertiary carbon, which is stabilized by hyperconjugation and inductive effects.

Step 4: Explain the rearrangement. The carbocation intermediate undergoes a hydride shift or alkyl shift to form a more stable carbocation. This rearrangement accounts for the structural change observed in the product.

Step 5: Final substitution step. The hydroxide ion (OH⁻) then attacks the rearranged carbocation, leading to the formation of the final product with the hydroxyl group (-OH) attached to the tertiary carbon. The chloride ion (Cl⁻) is released as a byproduct.

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Key Concepts

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

Nucleophilic Substitution Mechanisms

Nucleophilic substitution reactions involve the replacement of a leaving group (like Cl) by a nucleophile (like ethyl amine). In this case, the reaction proceeds under second-order kinetics, indicating that both the nucleophile and the substrate are involved in the rate-determining step. Understanding the mechanism, whether it follows an SN1 or SN2 pathway, is crucial for explaining the rearrangement and the reaction's rate.

Carbocation Rearrangement

Carbocation rearrangement occurs when a carbocation intermediate can shift to a more stable configuration during a reaction. In this case, the formation of a more stable carbocation from 2-chlorobutane can lead to the observed rearrangement in the product. Recognizing the stability of different carbocation structures (primary, secondary, tertiary) is essential for predicting the outcome of the reaction.

Reaction Rate Factors

The rate of a chemical reaction can be influenced by several factors, including the nature of the nucleophile, the leaving group, and the solvent. In this reaction, the strong nucleophile (ethyl amine) and the good leaving group (Cl) contribute to the significantly enhanced reaction rate compared to hydroxide ion substitution. Understanding these factors helps explain why this reaction occurs much faster than typical nucleophilic substitutions.

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Related Practice

Textbook Question

Triethyloxonium tetrafluoroborate, (CH₃CH₂)₃O⁺ BF₄^–, is a solid with melting point 91–92°C. Show how this reagent can transfer an ethyl group to a nucleophile (Nuc:⁻) in an S_N2 reaction. What is the leaving group? Why might this reagent be preferred to using an ethyl halide? (Consult Table 6-2)

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Textbook Question

Propose mechanisms to account for the observed products in the following reactions.

(b)

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Textbook Question

Show the products you expect when each compound reacts with NBS with light shining on the reaction.

(a)

(b)

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Textbook Question

Propose mechanisms to account for the observed products in the following reactions.

(a)

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Textbook Question

Because the S_N1 reaction goes through a flat carbocation, we might expect an optically active starting material to give a completely racemized product. In most cases, however, S_N1 reactions actually give more of the inversion product. In general, as the stability of the carbocation increases, the excess inversion product decreases. Extremely stable carbocations give completely racemic products. Explain these observations.

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Textbook Question

Furfuryl chloride can undergo substitution by both S_N2 and S_N1 mechanisms. Because it is a 1° alkyl halide, we expect S_N2 but not S_N1 reactions. Draw a mechanism for the S_N1 reaction shown below, paying careful attention to the structure of the intermediate. How can this primary halide undergo S_N1 reactions?

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