How does the formation of acetals work

Aldehydes and ketones react with alcohols. In the case of aldehydes, hemiacetals and acetals (also called full acetals) are formed. Half-ketals and ketals are formed with ketones. But in both cases one speaks of acetals.


A reaction of great importance
The (semi-) acetal formation is of great importance, be it for nature or for industry. Both have learned to use this class of compounds for themselves. In nature, the hemiacetal bond is found in carbohydrates, for example: the two acetal formation reactions enable the typical ring formation. Full acetals are the combination of monosaccharides to oligo- or polysaccharides.

In technical and scientific chemistry, hemiacetals and full acetals (especially cyclic ones) are used in syntheses as protective groups for the carbonyl group. With the analogous ketal formation by acetone it was even possible to elucidate the real configuration of sugar molecules.


Formation of hemiacetals
The reaction of aldehydes or ketones with alcohols is one of the most important nucleophilic additions. It produces half and full acetals.

Schematic representation of the reaction mechanism of hemiacetal formation
using the example of the reaction of acetone with ethanol

The reaction to the hemiacetal takes place in an acidic environment. The protons of the acid only serve as a catalyst here. At the beginning a proton reacts with the oxygen of the acetone; the carbon atom of the carbonyl group receives a positive charge. If this compound is then reacted with ethanol, the partially negatively charged oxygen atom of the ethanol binds to the positively charged C atom of the carbonyl group. The hemiacetal is formed by splitting off the proton bound to the oxygen atom.

Hemiacetal formation and cleavage are catalyzed by acids and especially by bases; the reaction is completely reversible.


Formation of full acetals
If there is an excess of alcohol, this reaction goes beyond the hemiacetal stage and a full acetal is formed. Acid catalysis is a necessary prerequisite for this.

Schematic representation of the reaction mechanism of acetal formation

Acetal formation and cleavage are only catalyzed by acids (better: by protons). The proton of the acid adds to the hydroxyl group of the hemiacetal, which creates a positive charge at this point. In the next step, a water molecule splits off. Energetic reasons for this lie in the increase in entropy (disorder) due to the splitting off of water. The resulting compound has a positive charge on the carbon atom. If you now add ethanol, its partially negatively charged oxygen can attach to the positively charged carbon atom. By splitting off a proton, one finally obtains the full acetal.

The full acetal is relatively stable compared to the hemiacetal. However, the full acetal can also be split - but only by acids and with the expenditure of a lot of energy. This plays a role in the breakdown of cellulose by hot hydrochloric acid. This "wood saccharification" is the basis for the formation of "biofuel" through fermentation of glucose obtained from waste wood.


Further texts on the subject of aldehydes and ketones