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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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