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Ch.6 - Alkyl Halides; Nucleophilic Substitution
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh.6 - Alkyl Halides; Nucleophilic SubstitutionProblem 39
Chapter 6, Problem 39

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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Step 1: Recognize the type of reaction taking place. The reaction between chlorocyclohexane and sodium cyanide (NaCN) in ethanol is an example of an SN2 (bimolecular nucleophilic substitution) reaction. In this mechanism, the nucleophile (CN⁻) attacks the carbon bonded to the leaving group (Cl) in a single concerted step.
Step 2: Understand the role of the leaving group. The leaving group in this reaction is chloride (Cl⁻). The efficiency of an SN2 reaction depends significantly on the ability of the leaving group to depart. A better leaving group facilitates the reaction by stabilizing the negative charge after departure.
Step 3: Analyze the effect of sodium iodide (NaI). Sodium iodide dissociates in ethanol to produce iodide ions (I⁻). Iodide is a much better nucleophile and leaving group compared to chloride due to its larger size and ability to stabilize the negative charge. When iodide ions are present, they can replace the chloride ion on chlorocyclohexane via an SN2 reaction, forming iodo-cyclohexane as an intermediate.
Step 4: Explain the acceleration in the rate. The intermediate iodo-cyclohexane reacts with cyanide ions (CN⁻) much faster than chlorocyclohexane because iodide is a superior leaving group compared to chloride. This substitution process increases the overall rate of formation of cyanocyclohexane.
Step 5: Summarize the catalytic role of sodium iodide. Sodium iodide acts as a catalyst by facilitating the formation of iodo-cyclohexane, which accelerates the subsequent reaction with cyanide ions to produce cyanocyclohexane. This demonstrates how the presence of a better leaving group can enhance the rate of an SN2 reaction.

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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 this case, sodium cyanide acts as a nucleophile that attacks the carbon atom bonded to the chlorine in chlorocyclohexane, leading to the formation of cyanocyclohexane. Understanding the mechanism of these reactions, particularly whether they proceed via an SN1 or SN2 pathway, is crucial for analyzing reaction rates.
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Role of Sodium Iodide

Sodium iodide can enhance the reaction rate by providing a better leaving group compared to chloride. The presence of iodide ions can facilitate the formation of a more reactive intermediate or transition state, thereby increasing the overall rate of the nucleophilic substitution. This phenomenon is often referred to as the 'iodide effect' in organic chemistry.
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Solvent Effects on Reaction Rates

The choice of solvent can significantly influence the rate of chemical reactions. In this scenario, ethanol serves as a polar protic solvent, which can stabilize charged intermediates and transition states. The solvent's ability to solvate ions and facilitate nucleophilic attack is essential for understanding how the addition of sodium iodide accelerates the formation of cyanocyclohexane.
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Related Practice
Textbook Question

When tert-butyl bromide is heated with an equal amount of ethanol in an inert solvent, one of the products is ethyl tert-butyl ether.

a. What happens to the reaction rate if the concentration of ethanol is doubled?

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Textbook Question

When ethyl bromide is added to potassium tert-butoxide, the product is ethyl tert-butyl ether.

CH3CH2–Br + (CH3)3C–O–K+ → (CH3)3C–O–CH2CH3 ethyl bromide potassium tert-butoxide ethyl tert-butyl ether

b. What happens to the rate if the concentration of potassium tert-butoxide is tripled and the concentration of ethyl ­bromide is doubled?

c. What happens to the rate if the temperature is raised?

Textbook Question

Allylic halides have the structure

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

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Textbook Question

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

(c)

(d)

Textbook Question

When tert-butyl bromide is heated with an equal amount of ethanol in an inert solvent, one of the products is ethyl tert-butyl ether.

b. What happens to the rate if the concentration of tert-butyl bromide is tripled and the concentration of ethanol is doubled?

c. What happens to the rate if the temperature is raised?

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Textbook Question

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

(a)

(b)

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