Formation of Acetals
Aldehydes can undergo reactions with alcohols under acidic conditions. The product of this nucleophilic addition reaction is called hemiacetal. Hemiacetal is formed by the nucleophilic addition of alcohol to the carbonyl carbon. Hemiacetal can react with one more mole of alcohol to form an acetal.
In monosaccharides (e.g., glucose, fructose), the cyclic ring structure is formed by the intramolecular reaction between the carbonyl and the hydroxyl groups within the same molecule. Glucose (an aldohexose) forms a cyclic hemiacetal by the intramolecular reaction between the aldehyde group of carbon 1 and the hydroxyl group of carbon 5.