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Acetal Formation
Major concepts
 A hemiacetal is a functional group in which a carbon is bonded to a hydroxyl group and an -OR
group
 The acetal is a functional group in which a carbon atom is bonded to two –OR groups
 Acetal formation is a condensation reaction between two hydroxyl groups and a ketone or
aldehyde in which water is lost.
Vocabulary
 Acetal
 Hemiacetal
 Diol
Students should be able to:
 Identify the acetal and hemiacetal groups in a molecule
 Explain the mechanism of acetal formation
 Predict the products of acetal formation given starting materials
 Given an acetal, do a retrosynthesis to the corresponding carbonyl and alcohols or diol
 Be able to compare/contrast acetal formation (a condensation reaction ) with aldol
condensation and imine formation.
Daily Problems
1. Which of these molecules can serve as an electrophile in acetal formation? Explain.
O
O
O
HO
O
O
O
O
Cl
H
H2N
In order to do an addition reaction, there should be no good leaving groups; therefore, aldehydes and
ketone are the only good electrophiles for acetal formation.
2. Which of these molecules can serve as the nucleophile in acetal formation?
3. Propose a mechanism for the formation of this hemiacetal and then acetal. (You may use your notes
to anwer this question.) What type of reaction is the formation of the hemiacetal? How could you
describe the second half of the reaction? How could you describe the overall reaction?
O
CH3OH solvent
H
HO
OCH3
H3CO
OCH3
catalytic H+
H
H
4. Identify any hemiacetal and acetal functional groups in this molecule:
acetal
hemiacetal
5. Draw the acetal products that form when acetaldehyde is condensed with each of these alcohols
with catalytic acid.
6. Draw the acetal products that form when ethanol is condensed with each of these
aldehydes/ketones and catalytic acid.
7. Retrosynthesis: Give the structure of the alcohol(s) and the carbonyl compound necessary to make
these acetals under acidic conditions.
Cumulative Problems:
8. What is a condensation reaction? What are three types of condensation reactions that we have
learned? Compare and contrast them.
A condensation reaction is a reaction that results in the loss of a small molecule such as water after the
addition of a nucleophile under certain conditions. Two types of condensation reaction that we have
learned are 1) imine formation, and 2) acetal formation. They both result in the loss of water when a
nucleophile attacks an aldehyde or ketone, but they all differ in the nucleophile –imines are formed by
attack of an amine, acetals are formed by the attack of alcohols.
9. Draw the product of these condensation reactions and label the type of condensation reaction.
Extension problem:
10. Find the hemiacetal. Do the retrosynthesis to get back to the alcohol and the aldehyde. (Hint:
Don’t try to move the carbon chain—keep it in the same orientation.