Question

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

Question: Describe the mechanism for the formation of a cyclic acetal from 4-hydroxypentanal and methanol, and predict the position of the oxygen-18 label in the final product. Answer: The mechanism involves the formation of a hemiacetal intermediate followed by cyclization to form the cyclic acetal. The oxygen-18 label ends up in the water molecule and not in the cyclic acetal.

Step-by-step solution

Draw the structure of 4-hydroxypentanal

To start, we will draw the structure of 4-hydroxypentanal. It has a total of five carbon atoms, with a hydroxyl group on carbon 4 and an aldehyde group on carbon 1. O || CH3-CH2-CH-CH2-CH

Add methanol to the carbonyl group

Next, we will add methanol to the carbonyl group of the aldehyde. Methanol has a strong nucleophilic oxygen atom, so it can attack the electrophilic carbonyl carbon to form a new C-O bond. This step results in the formation of a hemiacetal intermediate with the displacement of the oxygen-18 labeled oxygen atom. CH3-CH2-CH-CH2-CH O | H O-CH3

Formation of cyclic acetal

Now, we will form the cyclic acetal. The oxygen atom of the hydroxyl group on carbon 4 forms a bond with the carbonyl carbon atom, which is now part of the hemiacetal. A water molecule (with the oxygen-18 label) is eliminated in this process, resulting in the formation of a cyclic acetal. O | H O CH3-CH2-CH | CH2-CH

Predict the position of oxygen-18 label

As we formed the cyclic acetal, the oxygen-18 labeled atom was displaced during the formation of the hemiacetal intermediate in step 2. Therefore, the oxygen-18 label ends up in the water molecule that was eliminated during the cyclic acetal formation. In conclusion, the mechanism for the formation of a cyclic acetal from 4-hydroxypentanal and methanol involves the formation of a hemiacetal intermediate and the subsequent cyclization to form the cyclic acetal. The oxygen-18 label ends up in the water molecule and not in the cyclic acetal.