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Chapter 16: Problem 32

Propose a mechanism to account for the formation of a cyclic acetal from 4-hydroxypentanal and one equivalent of methanol. If the carbonyl oxygen of 4-hydroxypentanal is enriched with oxygen-18, do you predict that the oxygen label appears in the cyclic acetal or in the water?

Short Answer

Expert verified
Answer: The oxygen-18 label will appear in the cyclic acetal and not in the water.

Step by step solution

01

Identify the reactants and products

The reactants in this reaction are 4-hydroxypentanal and one equivalent of methanol. The cyclic acetal is formed as the product, along with water as a side product.
02

Understand the cyclic acetal formation

Cyclic acetals are formed by the nucleophilic attack of an alcohol on a carbonyl compound. The carbonyl oxygen gets protonated, making it a good leaving group. A nucleophilic attack takes place, and a hemiacetal intermediate is formed. The intermediate gets protonated, and a second nucleophilic attack occurs, forming the cyclic acetal.
03

Write the mechanism for the given reactants

1. Protonation of the carbonyl oxygen in 4-hydroxypentanal by methanol. 2. Nucleophilic attack of methanol on protonated 4-hydroxypentanal to form a tetrahedral intermediate. 3. Deprotonation of the intermediate to form a hemiacetal. 4. Protonation of the hydroxy group in the hemiacetal by another equivalent of methanol. 5. Intramolecular nucleophilic attack by the methoxy oxygen on the carbonyl carbon, forming a five-membered ring. 6. Deprotonation of the resulting intermediate to give the cyclic acetal and a water molecule.
04

Determine the location of oxygen-18

By following the mechanism outlined in steps 1-6, we can trace the path of the carbonyl oxygen. In step 1, the carbonyl oxygen of 4-hydroxypentanal gets protonated. Throughout the mechanism, this oxygen atom remains attached to the carbonyl carbon, eventually becoming a part of the cyclic acetal structure. Therefore, the oxygen-18 will appear in the cyclic acetal and not the water.

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Most popular questions from this chapter

In dilute aqueous base, \((R)\)-glyceraldehyde is converted into an equilibrium mixture of \((R, S)\)-glyceraldehyde and dihydroxyacetone. Propose a mechanism for this isomerization.

In Section \(11.5\) we saw that ethers, such as diethyl ether and tetrahydrofuran, are quite resistant to the action of dilute acids and require hot concentrated HI or HBr for cleavage. However, acetals in which two ether groups are linked to the same carbon undergo hydrolysis readily, even in dilute aqueous acid. How do you account for this marked difference in chemical reactivity toward dilute aqueous acid between ethers and acetals?

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The base-promoted rearrangement of an \(\alpha\)-haloketone to a carboxylic acid, known as the Favorskii rearrangement, is illustrated by the conversion of 2 -chlorocyclohexanone to cyclopentanecarboxylic acid. (a) Propose a mechanism for base-promoted conversion of 2 -chlorocyclohexanone to the proposed intermediate. (b) Propose a mechanism for base-promoted conversion of the proposed intermediate to sodium cyclopentanecarboxylate.

( \(R\) )-Pulegone is converted to \((R)\)-citronellic acid by addition of HCl followed by treatment with \(\mathrm{NaOH}\).
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