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

Answer 1

Answer:

If carbonyl oxygen of 4-hydroxypentanal is enriched with $O^{18}$, then the oxygen label appears in the water .

Explanation:

• In the first step, -OH group at C-4 gives intramolecular nucleophilic addition reaction at carbonyl center to produce a cyclic hemiacetal.
• Then, one equivalent of methanol gives nucleophilic substitiution reaction by substituting -OH group in cyclic hemiacetal to produce cyclic acetal.
• If carbonyl oxygen of 4-hydroxypentanal is enriched with $O^{18}$, then the oxygen label appears in the water produced at the end of reaction.
• Full reaction mechanism has been shown below.