Ch. 14 - Ethers, Epoxides, and Thioethers

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

Wade 9th Edition

Ch. 14 - Ethers, Epoxides, and Thioethers

Problem 12

Chapter 14, Problem 12

Propose a mechanism for the acid-catalyzed condensation of n-propyl alcohol to n-propyl ether, as shown above. When the temperature is allowed to rise too high, propene is formed. Propose a mechanism for the formation of propene, and explain why it is favored at higher temperatures.

Verified step by step guidance

Step 1: Begin by understanding the acid-catalyzed condensation mechanism for the formation of n-propyl ether from n-propyl alcohol. The process typically involves protonation of the alcohol to form a good leaving group.

Step 2: Protonation of n-propyl alcohol occurs when the alcohol oxygen atom donates a pair of electrons to a hydrogen ion (H⁺) from the acid catalyst, forming an oxonium ion. This is represented as:

{CH}_{3} {CH}_{2} {CH}_{2} - OH + H + \to {CH}_{3} {CH}_{2} {CH}_{2} - OH +

Step 3: The oxonium ion can then undergo nucleophilic attack by another molecule of n-propyl alcohol, leading to the formation of n-propyl ether. This step involves the displacement of water as a leaving group, forming the ether linkage.

Step 4: For the formation of propene at higher temperatures, consider the elimination mechanism. The oxonium ion can lose a water molecule, forming a carbocation intermediate. This is followed by a deprotonation step, where a hydrogen ion is removed from the adjacent carbon, resulting in the formation of propene. This is represented as:

{CH}_{3} {CH}_{2} {CH}_{2} - OH + \to {CH}_{3} {CH}_{2} - CH + \to {CH}_{3} CH = CH 2

Step 5: Explain why propene formation is favored at higher temperatures. The elimination reaction leading to propene is an endothermic process, which means it requires heat to proceed. At higher temperatures, the energy barrier for elimination is overcome more easily, favoring the formation of propene over the condensation reaction.

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Key Concepts

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

Acid-Catalyzed Condensation

Acid-catalyzed condensation is a reaction where an acid facilitates the formation of an ether from alcohols by promoting the nucleophilic attack of an alcohol on another alcohol molecule. The acid protonates the hydroxyl group, making it a better leaving group, which allows for the formation of a new C-O bond and the release of water. This process is crucial for understanding how n-propyl alcohol can be converted into n-propyl ether.

Elimination Reaction

An elimination reaction involves the removal of a small molecule from a larger one, resulting in the formation of a double bond. In the context of n-propyl alcohol, when the temperature increases, the reaction can shift towards elimination, leading to the formation of propene. This occurs because higher temperatures favor the formation of products with greater entropy, such as alkenes, over the more stable ether.

Le Chatelier's Principle

Le Chatelier's Principle states that if a system at equilibrium is disturbed, the system will adjust to counteract the disturbance and restore a new equilibrium. In the case of the acid-catalyzed condensation and subsequent elimination reactions, increasing the temperature shifts the equilibrium towards the formation of propene, as the system seeks to minimize the effect of the added heat by favoring the endothermic elimination process.

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Textbook Question

Explain why bimolecular condensation is a poor method for making unsymmetrical ethers such as ethyl methyl ether.

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Textbook Question

Which of the following ethers can be formed in good yield by condensation of the corresponding alcohols? For those that cannot be formed by condensation, suggest an alternative method that will work.

(a) dibutyl ether

(b) ethyl n-propyl ether

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Textbook Question

Show how the following ethers might be synthesized using (1) alkoxymercuration–demercuration and (2) the Williamson synthesis. (When one of these methods cannot be used for the given ether, point out why it will not work.)

f. tert-butyl phenyl ether

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Textbook Question

Show how the following ethers might be synthesized using (1) alkoxymercuration– demercuration and (2) the Williamson synthesis. (When one of these methods cannot be used for the given ether, point out why it will not work.)

(e) 1-isopropoxy-1-methylcyclopentane

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Textbook Question

Propose a mechanism for the following reaction.

Predict the products of the following reactions. An excess of acid is available in each case.

(a) ethoxycyclohexane + HBr

(b) tetrahydropyran + HI

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