Textbook Question
Alcohols combine with ketones and aldehydes to form interesting derivatives, which we will discuss in Chapter 18. The following reactions show the hydrolysis of two such derivatives. Propose mechanisms for these reactions.
(a)
21. Aldehydes and Ketones: Nucleophilic Addition
Acetal
5:16 minutesProblem 61b
Textbook Question
There are three dioxane isomers: 1,2-dioxane, 1,3-dioxane, and 1,4-dioxane. One of these acts like an ether and is an excellent solvent for Grignard reactions. Another one is potentially explosive when heated. The third one quickly hydrolyzes in dilute acid. Propose a mechanism for the acid hydrolysis of the third isomer.
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Identify the structure of the three dioxane isomers: 1,2-dioxane, 1,3-dioxane, and 1,4-dioxane. These are cyclic ethers with two oxygen atoms in the ring. The position of the oxygen atoms in the ring will affect their chemical properties.
Determine which isomer is likely to undergo acid hydrolysis. Acid hydrolysis typically involves the cleavage of ether bonds, and the isomer with oxygen atoms in adjacent positions (1,2-dioxane) is more prone to this reaction due to the strain and reactivity of the ring.
Propose the mechanism for the acid hydrolysis of 1,2-dioxane. The first step involves protonation of one of the oxygen atoms by the acid, increasing the electrophilicity of the carbon atoms adjacent to the oxygen.
Following protonation, the ring opens as the bond between the protonated oxygen and the adjacent carbon breaks, forming a carbocation intermediate. This step is facilitated by the strain in the 1,2-dioxane ring.
Finally, water acts as a nucleophile, attacking the carbocation and leading to the formation of a diol. This completes the hydrolysis process, resulting in the cleavage of the ether bond and formation of two alcohol groups.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Dioxane Isomers
Dioxane isomers refer to the structural variations of dioxane, which is a six-membered cyclic ether containing two oxygen atoms. The isomers 1,2-dioxane, 1,3-dioxane, and 1,4-dioxane differ in the positions of the oxygen atoms within the ring. These structural differences significantly influence their chemical properties, including reactivity and solubility, which are crucial for understanding their behavior in reactions such as hydrolysis.
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Grignard Reactions
Grignard reactions involve the use of Grignard reagents, which are organomagnesium compounds that react with electrophiles to form carbon-carbon bonds. These reactions are highly sensitive to moisture and require anhydrous conditions, often utilizing ethers as solvents. Understanding the role of solvents in Grignard reactions is essential, as the choice of solvent can affect the reaction's efficiency and the stability of the Grignard reagent.
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Acid Hydrolysis Mechanism
Acid hydrolysis is a chemical reaction where a compound reacts with water in the presence of an acid, leading to the breakdown of the compound. In the case of dioxane isomers, the mechanism typically involves protonation of the ether oxygen, followed by nucleophilic attack by water, resulting in the cleavage of the C-O bond. This process is important for understanding the stability and reactivity of the isomers under acidic conditions, particularly for the isomer that hydrolyzes rapidly.
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