Addition of Alcohols to form Hemiacetals and Acetals
In this organic chemistry topic, we shall see how alcohols (R-OH) add to carbonyl groups. Carbonyl groups are characterized by a carbon-oxygen double bond. The two main functional groups that consist of this carbon-oxygen double bond are Aldehydes and Ketones.
Aldehydes and ketones react with Alcohol to give adducts called hemiacetals (hemi, Greek, half). These hemiacetals are intermediates on the way to acetals. Both acids and bases catalyze this process. Noteworthy is the fact that only acids transform hemiacetals to acetals by replacement of the hydroxy group with an alkoxy substituent. Bases fail to move beyond the hemiacetal stage.
ALDEHYDE AlCOHOL HEMIACETAL
KETONE ALCOHOL HEMIACETAL
The addition reactions above are governed by equilibria that usually favor the starting materials. Therefore, Hemiacetals are usually not isolable. However, the exceptions are those formed from reactive carbonyl compounds such as formaldehyde or 2,2,2-trichloroacetaldehyde.
Furthermore, hemiacetals are also isolable if cyclization occurs in hydroxy aldehydes and ketones to form relatively strain-free five- and six-membered rings. This is also known as intramolecular hemiacetal formation.
Intramolecular Hemiacetal Formation
5-Hydroxypentanal A cyclic, hemiacteal, stable
Intramolecular Hemiacetal formation is common in sugar chemistry. For example, the common sugar glucose exists in the cylcic manner more than 99% of the time in a mixture of aqueous solution.
Acids-Catalyzed Acetal Formation
In the presence of excess alcohol, the acid-catlayzed reaction of the carbonyl group in aldehydes and ketones proceeds beyond the hemiacetal stage. Under these circumstances, the hydroxy function of the initial adduct is replaced by another alkoxy unit derived from the alcohol. As mentioned and depicted before, the resulting compounds are called acetals. Note: Ketal is an older term for an acetal derived from a ketone.
KETONE 2 moles of Alcohol Acetal H2O
The overall/net change is the replacement of the carbonyl oxygen by two alkoxy groups. The formation of an equivalent of water is noteworthy as well.
STEP 1. Hemiacetal Generation
STEP 2. Acetal Generation
Every step in the Mechanism of Acetal Formation is reversible. The entire sequence is an equilibreum process. In the equilibreum, the entire result can be either the acetal, which is done by using excess alcohol, or it can be the carbonyl group, which is done by adding excess water; a process called acetal hydrolysis. However, in contrast with hydrates and hemiacetals, acetals may be isolated as pure substances by neutralizing the acid catalyst used in their formation.
1. (via Basic Condiitions)
3. Can an Acetal be isolated?
4. Can Hemiacetals be isolated?
5. Can Hydrates be isolated?
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