14.4 Hemiacetals and Acetals Maltose, a disaccharide, is

14.4 Hemiacetals and Acetals Maltose, a disaccharide, is

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. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake

2016 Pearson Education, Inc. 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 contain two functional groups on the same carbon atom, a OH group and an OR group. Hemiacetals are generally unstable and react with a second molecule of the alcohol to form a stable acetal and water. Acetals contain two alkoxy ( OR) groups on the same carbon atom.

General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake 2016 Pearson Education, Inc. 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.

General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake 2016 Pearson Education, Inc. Hemiacetal and Acetal Formation The reactions in the formation of hemiacetals and acetals are reversible. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake 2016 Pearson Education, Inc.

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. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake 2016 Pearson Education, Inc. 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. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake 2016 Pearson Education, Inc.

Glucose Forms a Cyclic Hemiacetal General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake 2016 Pearson Education, Inc. 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.

General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake 2016 Pearson Education, Inc. 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).

General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake 2016 Pearson Education, Inc. Study Check Identify each as a hemiacetal or an acetal. A. B. General, Organic, and Biological Chemistry: Structures of Life, 5/e

Karen C. Timberlake 2016 Pearson Education, Inc. Study Check Identify each as a hemiacetal or an acetal. A. hemiacetal B. acetal

General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake 2016 Pearson Education, Inc. Study Check Draw the condensed structural formula of the acetal formed by adding CH3OH to butanal. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake

2016 Pearson Education, Inc. Solution Draw the condensed structural formula of the acetal formed by adding CH3OH to butanal. CH3CH2CH2CHO + CH3OH OCH3 CH3CH2CH2C H OCH3

General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake 2016 Pearson Education, Inc.

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