14.1 Aldehydes and Ketones Formaldehyde is the simplest aldehyde. It is a colorless gas with a pungent odor. An aqueous solution called formalin, which contains 40% formaldehyde, is used as a germicide and to preserve biological specimens.

Carbonyl Group A carbonyl group in an aldehyde is bonded to at least one H atom. a ketone is bonded to two carbon groups. Aldehydes and ketones contain a carbonyl group that consists of a carbon–oxygen double bond with two groups of atoms attached to the carbon at angles of 120°.

Carbonyl Polarity The oxygen atom with two lone pairs of electrons is much more electronegative than the carbon atom. The carbonyl group has a strong dipole with a partial negative charge on the oxygen and a partial positive charge on the carbon. The polarity of the carbonyl group strongly influences the physical and chemical properties of aldehydes and ketones.

Naming Aldehydes an aldehyde is named by replacing the e of the corresponding alkane name with al.

Naming Aldehydes

Naming Ketones Ketones are often given common names Acetone, another name for propanone, has been retained by the IUPAC system.

Naming Ketones the name of a ketone is obtained by replacing the e in the alkane name with one.

Naming Cyclic Ketones For cyclic ketones, the prefix cyclo is used in front of the ketone name.

Chemistry Link to Health: Important Aldehydes and Ketones Formalin, a 40% formaldehyde aqueous solution, is used as a germicide and to preserve biological specimens. is used in industry as a reactant in the synthesis of polymers used to make fabrics, insulation materials, carpeting, pressed wood products such as plywood, and plastics for kitchen counters. Exposure to fumes can irritate the eyes, nose, and upper respiratory tract and cause skin rashes, headaches, dizziness, and general fatigue.

Chemistry Link to Health: Important Aldehydes and Ketones The simplest ketone, known as acetone or propanone (dimethyl ketone), is a colorless liquid with a mild odor that has wide use as a solvent in cleaning fluids, paint and nail polish removers, and rubber cement. In the body, acetone may be produced in uncontrolled diabetes, fasting, and high-protein diets when large amounts of fats are metabolized for energy.

Chemistry Link to Health: Important Aldehydes and Ketones Several naturally occurring aromatic aldehydes are used to flavor food and as fragrances in perfumes. Benzaldehyde is found in almonds. Vanillin is found in vanilla beans. Cinnamaldehyde is found in cinnamon.

Study Check (3 of 7) Identify the following compounds as either an aldehyde or a ketone: A. B. C. D.

Study Check (4 of 7) Identify the following compounds as either an aldehyde or a ketone: A. B. C. D.

14.2 Physical Properties of Aldehydes and Ketones Aldehydes and ketones contain a polar carbonyl group with a partial negative charge on the carbonyl oxygen atom and a partial positive charge on the carbon atom.

Polarity of Carbonyl Group At room temperature, methanal (formaldehyde), and ethanal, (acetaldehyde) are gases. Aldehydes and ketones containing 3 to 10 carbon atoms are liquids. The polar carbonyl group with a partially negative oxygen atom and a partially positive carbon atom has an influence on the boiling points and the solubility of aldehydes and ketones in water.

Boiling Points of Aldehydes and Ketones Aldehydes and ketones have dipole–dipole interactions between polar groups. higher boiling points than alkanes and ethers of similar mass as a result of dipole–dipole interactions. lower boiling points than alcohols of similar mass because they do not form hydrogen bonds.

Boiling Points of Aldehydes and Ketones (2 of 3) For aldehydes and ketones, the boiling points of both aldehydes and ketones increase as the number of carbon atoms and dispersion forces increase, raising their higher boiling points.

Boiling Points of Aldehydes and Ketones Table 14.2 Boiling Points and Solubility of Selected Aldehydes and Ketones Compound Condensed Structural Formula Number of Carbon Atoms Boiling Point (degree Celsius) Solubility in Water Methanal (formaldehyde) Chain of single-bonded molecules: H, C H O 1 −21 Soluble Ethanal (acetaldehyde) Chain of single-bonded molecules: C H 3, C H O 2 21 Propanal (propionaldehyde) Chain of single-bonded molecules: C H 3, C H 2, C H O 3 49 Propanone (acetone) Chain of single-bonded molecules: C H 3, C O, C H 3 56 Butanal (butyraldehyde) Chain of single-bonded molecules: C H 3, C H 2, C H 2, C H O 4 75 Butanone Chain of single-bonded molecules: C H 3, C O, C H 2, C H 3 80 Pentanal Chain of single-bonded molecules: C H 3, C H 2, C H 2, C H 2, C H O 5 103 Slightly soluble 2-Pentanone Chain of single-bonded molecules: C H 3, C O, C H 2, C H 2, C H 3 102 Hexanal Chain of single-bonded molecules: C H 3, C H 2, C H 2, C H 2, C H 2, C H O 6 129 Not soluble 2-Hexanone Chain of single-bonded molecules: C H 3, C O, C H 2, C H 2, C H 2, C H 3 127

Solubility of Aldehydes and Ketones with one to four carbons are soluble in water. with five or more carbons are not very soluble in water. form hydrogen bonds with water between the carbonyl oxygen and hydrogen atoms of water.

Study Check (1 of 3) Select the compound in each pair that would have the higher boiling point. Explain. A. B. C.

Solution (1 of 2) Select the compound in each pair that would have the higher boiling point. Explain. A. The aldehyde has a lower boiling point than the alcohol, as alcohols can form hydrogen bonds with each other. B. The ketone has a higher boiling than the alkane because of dipole–dipole interactions between the carbonyl groups.

Solution (2 of 2) Select the compound in each pair that would have the higher boiling point. Explain. C. Ketones have a higher boiling point than alkanes because of increased dipole–dipole interactions between the carbonyl groups.

Study Check (2 of 3) Indicate if each of the following is or is not soluble in water: A. B. C. D.

Study Check (3 of 3) Indicate if each of the following is or is not soluble in water: Not soluble in water A. B. Forms H-bonds; soluble in water Not soluble; aldehyde has more than five carbon atoms. C. Soluble; ketones with four or fewer carbon atoms are soluble in water. D.

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.

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 Hemiacetals are generally unstable and react with a second molecule of the alcohol to form a stable acetal and water. Acetals contain two alkoxy groups on the same carbon atom.

Hemiacetal and Acetal Formation Commercially, compounds that are acetals are used to produce vitamins, dyes, pharmaceuticals, and perfumes.

Cyclic Hemiacetals One very important type of hemiacetal a cyclic hemiacetal that forms when the carbonyl group and the are in the same molecule.

Glucose Forms a Cyclic Hemiacetal

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 (1 of 3) Identify each as a hemiacetal or an acetal. A. B.

Study Check (2 of 3) Identify each as a hemiacetal or an acetal. A. B.

14.5 Chiral Molecules An object whose mirror image is identical to the original and can be superimposed on it is achiral. Objects such as hands that have nonsuperimposable mirror images are chiral.

Stereoisomers Molecules are structural isomers when they have the same molecular formula but different bonding arrangements. In stereoisomers, the atoms are bonded in the same sequence but differ in the way they are arranged in space.

Chirality Left and right hands are chiral because they have mirror images that cannot be superimposed on each other. When the mirror images cannot be completely matched, they are nonsuperimposable.

Chiral or Achiral

Chiral Carbon Atoms Carbon atoms are chiral if they have four different atoms or groups. When stereoisomers cannot be superimposed, they are called enantiomers.

Achiral Carbon Atoms If a molecule with two or more identical atoms bonded to the same atom is rotated, the atoms can be superimposed and the mirror images represent the same structure.

Study Check (1 of 5) For each of the following molecules, indicate whether the carbon in red is chiral or achiral. A. B.

Solution (1 of 5) For each of the following molecules, indicate whether the carbon in red is chiral or achiral. A. B.

Chemistry Link to Health: Enantiomers in Biological Systems Molecules in nature also have mirror images, and often one stereoisomer has a different biological effect than the other one. One enantiomer of carvone, produced by the spearmint plant, smells and tastes like spearmint, whereas its mirror image, produced by the caraway plant, has the odor and taste of caraway in rye bread. Our senses of smell and taste are responsive to the chirality of molecules.

Chemistry Link to Health: Enantiomers in Biological Systems

Chemistry Link to Health: Enantiomers in Biological Systems Molecules in nature also have mirror images, and often one stereoisomer has a different biological effect than the other one. One enantiomer of carvone, produced by the spearmint plant, smells and tastes like spearmint, whereas its mirror image, produced by the caraway plant, has the odor and taste of caraway in rye bread. Our senses of smell and taste are responsive to the chirality of molecules.

Chemistry Link to Health: Enantiomers in Biological Systems A substance used to treat Parkinson’s disease is L-dopa, which is converted to dopamine in the brain, where it raises the serotonin level. The D-dopa enantiomer is not effective for the treatment of Parkinson’s disease.

Study Check (2 of 5) Identify each as a chiral or an achiral compound.

Solution (2 of 5) Identify each as a chiral or an achiral compound.