14.4 Hemiacetals and Acetals Maltose, a disaccharide, is produced from the hydrolysis of the starches in grains, such as barley. In maltose, an acetal bond links two glucose molecules. Learning Goal Draw the condensed or line-angle structural formulas for the products of the addition of alcohols to aldehydes and ketones.

Hemiacetal and Acetal When one alcohol adds to an aldehyde or ketone in the presence of an acid catalyst, the product is a hemiacetal. Hemiacetals are generally unstable and react with a second molecule of the alcohol to form a stable acetal and water. Hemiacetals contain two functional groups on the same carbon atom, a — OH group and an — OR group. Acetals contain two alkoxy (— OR) groups on the same carbon atom.

Hemiacetal and Acetal Formation Commercially, compounds that are acetals are used to produce vitamins, dyes, pharmaceuticals, and perfumes. Aldehydes are often more reactive than ketones because the carbonyl carbon is more positive. The presence of two alkyl groups in ketones makes it more difficult for an alcohol to form a bond with the carbon in the carbonyl group.

Hemiacetal and Acetal Formation The reactions in the formation of hemiacetals and acetals are reversible.

Cyclic Hemiacetals One very important type of hemiacetal that can be isolated is a cyclic hemiacetal that forms when the carbonyl group and the — OH group are in the same molecule.

Formation of Cyclic Hemiacetals Five- and six-atom cyclic hemiacetals and acetals are more stable than their open-chain isomers. Glucose forms a six-carbon cyclic hemiacetal when the hydroxyl group on carbon 5 bonds with the carbonyl group on carbon 1. The hemiacetal of glucose is so stable that almost all the glucose exists as the cyclic hemiacetal in aqueous solution.

Glucose Forms a Cyclic Hemiacetal

Alcohol Adds to a Cyclic Hemiacetal An alcohol can add to the cyclic hemiacetal and form a cyclic acetal. This reaction explains how sugar molecules can link together and form disaccharides and polysaccharides.

Glucose + Glucose = Maltose Maltose is a disaccharide consisting of two glucose molecules. an acetal bond (shown in red) that links two glucose molecules. One carbon atom in glucose retains the cyclic hemiacetal bond (shown in green).

Study Check Identify each as a hemiacetal or an acetal. A. B.

Study Check Identify each as a hemiacetal or an acetal. A. B.

Study Check Draw the condensed structural formula of the acetal formed by adding CH3OH to butanal.

Solution Draw the condensed structural formula of the acetal formed by adding CH3OH to butanal. CH3—CH2—CH2—CHO + CH3—OH 