Chapter Sixteen Aldehydes and Ketones James E. Mayhugh Oklahoma City University  2007 Prentice Hall, Inc. Fundamentals of General, Organic, and Biological Chemistry 5th Edition

Prentice Hall © 2007 Chapter Sixteen 2 Outline ►16.1 The Carbonyl Group ►16.2 Naming Aldehydes and Ketones ►16.3 Properties of Aldehydes and Ketones ►16.4 Some Common Aldehydes and Ketones ►16.5 Oxidation of Aldehydes ►16.6 Reduction of Aldehydes and Ketones ►16.7 Addition of Alcohols: Hemiacetals and Acetals

Prentice Hall © 2007 Chapter Sixteen The Carbonyl Group ►Carbonyl compound: Any compound that contains a carbonyl group, C=O. ►Carbonyl group: A functional group that has a C atom joined to an O atom by a double bond. ►The bond angles between the three substituents on the carbonyl carbon atom are 120°, or close to it.

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Prentice Hall © 2007 Chapter Sixteen 5 ►Aldehyde: A compound that has a carbonyl group bonded to at least one hydrogen. ►Ketone: A compound that has a carbonyl group bonded to two carbons in organic groups that can be the same or different.

Prentice Hall © 2007 Chapter Sixteen Naming Aldehydes and Ketones ►The simplest aldehydes are known by their common names, which end in -aldehyde, for example, formaldehyde, acetaldehyde, and benzaldehyde. ►In the IUPAC system, the -e ending of the alkane with the same number of C’s is replaced by -al. ►When substituents are present, the chain is numbered beginning with 1 for the carbonyl carbon. For example, 3-methylbutanal.

Prentice Hall © 2007 Chapter Sixteen 7 ►Common names for ketones give the names of the two alkyl groups bonded to the carbonyl carbon followed by the word ketone. ►Ketones are named systematically by replacing the final -e of the alkane name with -one. The numbering of the chain begins at the end nearest the carbonyl group. The location of the carbonyl group is indicated by placing the number of the carbonyl carbon in front of the name. Using this nomenclature scheme, acetone would be named 2-propanone.

Prentice Hall © 2007 Chapter Sixteen Properties of Aldehydes and Ketones ►Aldehydes and ketones cannot hydrogen-bond with one another, so they are lower boiling than alcohols. ►Aldehydes and ketones are higher boiling than alkanes because of the polarity of the carbonyl group. Common aldehydes and ketones are liquids.

Prentice Hall © 2007 Chapter Sixteen 9 ►Simple aldehydes and ketones are water-soluble due to hydrogen bonding with water molecules, and ketones are good solvents. ►Simple ketones are less toxic than simple aldehydes. ►Many aldehydes and ketones have distinctive odors. ► ►A perfumer sits at a mixing table testing new combinations of fragrances, many of which are aldehydes and ketones.

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Prentice Hall © 2007 Chapter Sixteen Some Common Aldehydes and Ketones ►Formaldehyde is a colorless gas. Low concentrations in air cause eye, throat, and bronchial irritation, and high concentrations trigger asthma attacks. Skin contact produces dermatitis. CH 2 O is very toxic by ingestion, causing serious kidney damage, coma, and sometimes even death. ►CH 2 O kills viruses, fungi, and bacteria by reaction with the groups in proteins, allowing for its use in disinfecting and sterilizing equipment and as a preservative for biological specimens.

Prentice Hall © 2007 Chapter Sixteen 12 ►The first successful synthetic plastic was a polymer of phenol and formaldehyde known as Bakelite, once widely used for such items as pot handles, fountain pens, and cameras. Urea–formaldehyde polymers are now more widely used than Bakelite. ► ►CH 2 O polymers release CH 2 O into the air. Concerns over the toxicity and possible carcinogenicity of CH 2 O limits most household applications.

Prentice Hall © 2007 Chapter Sixteen 13 ►Acetaldehyde is a sweet-smelling, flammable liquid present in ripe fruits, and produced in the normal breakdown of carbohydrates. ►Acetaldehyde is a general narcotic, and large doses can cause respiratory failure. Chronic exposure produces symptoms like those of alcoholism.

Prentice Hall © 2007 Chapter Sixteen 14 ►Acetone is one of the most widely used of all organic solvents. It dissolves most organic compounds and is also miscible with water. Acetone is the solvent in many varnishes, lacquers, and nail polish removers. ►When the metabolism of fats and carbohydrates is out of balance (for example, in starvation or diabetes mellitus), acetone is produced in the liver, a condition known as ketosis that in severe cases leaves the odor of acetone on a patient’s breath.

Prentice Hall © 2007 Chapter Sixteen Oxidation of Aldehydes ►In aldehyde oxidation, the hydrogen bonded to the carbonyl carbon is replaced by an –OH group. ►Ketones, because they do not have this hydrogen, do not react with mild oxidizing agents.

Prentice Hall © 2007 Chapter Sixteen 16 ►(a) In the Tollens’ test, colorless silver ion is reduced to metallic silver. ►(b) In the Benedict’s test, the blue copper(II) ion is reduced to copper(I) in brick-red copper(I) oxide ►In both tests, the aldehyde is oxidized to the carboxylic acid.

Prentice Hall © 2007 Chapter Sixteen Reduction of Aldehydes and Ketones ►The reduction of a carbonyl group occurs with the addition of hydrogen across the double bond to produce an –OH group, a reaction that is the reverse of the oxidation of an alcohol. ►Aldehydes are reduced to primary alcohols, and ketones are reduced to secondary alcohols.

Prentice Hall © 2007 Chapter Sixteen Addition of Alcohols: Hemiacetals and Acetals The initial product of addition reactions of aldehydes and ketones with alcohols are known as hemiacetals. Compounds with both an -OH group and an -OR group bonded to the same carbon atom.

Prentice Hall © 2007 Chapter Sixteen 19 ►Ethanol forms hemiacetals with acetaldehyde and acetone. ►Hemiacetals rapidly revert back to aldehydes or ketones by loss of alcohol and establish an equilibrium with the aldehyde or ketone. ►When equilibrium is reached, very little hemiacetal is present.

Prentice Hall © 2007 Chapter Sixteen 20 ►A major exception occurs when the C=O and -OH functional groups that react are part of the same molecule. The resulting cyclic hemiacetal is more stable than a noncyclic hemiacetal. ►Most simple sugars exist mainly in the cyclic hemiacetal form, as shown below for glucose, rather than in the open-chain form.

Prentice Hall © 2007 Chapter Sixteen 21 ►If a small amount of acid catalyst is added to the reaction of an alcohol with an aldehyde or ketone, the hemiacetal initially formed is converted into an acetal in a substitution reaction. ►An acetal is a compound that has two -OR groups bonded to what was once the carbonyl carbon atom.

Prentice Hall © 2007 Chapter Sixteen 22 The aldehyde or ketone from which an acetal is formed can be regenerated by reversing the reaction. Reversal requires an acid catalyst and a large quantity of water. Hydrolysis: A reaction in which a bond or bonds are broken and the -H and -OH of water add to the atoms of the broken bond or bonds.

Prentice Hall © 2007 Chapter Sixteen 23 Chapter Summary ►The carbonyl group is a C=O. The group is polar, with a partial (-) charge on O and a partial (+) charge on C. The O and the two substituents on the carbonyl- group C atom form a planar triangle. ►The simplest aldehydes and ketones are known by common names. Aldehydes are named systematically by replacing the final -e in an alkane name with -al. ►Ketones are named systematically by replacing the final -e in an alkane name with -one and numbering starting with 1 at the end nearer the group. The location of the carbonyl group is indicated by placing the number of its carbon before the name.

Prentice Hall © 2007 Chapter Sixteen 24 Chapter Summary Cont. ►Aldehyde and ketone molecules are polar, do not hydrogen-bond with each other, but can hydrogen- bond with water. Small ones are water-soluble. Aldehydes and ketones are higher boiling than alkanes but lower boiling than alcohols. ►Aldehydes and ketones are present in many plants, where they contribute distinctive, pleasant odors. Such natural aldehydes and ketones are widely used in perfumes and flavorings. ►Formaldehyde (an irritating and toxic substance) is used in polymers, is present in smog-laden air, and is produced biochemically from ingested methanol.

Prentice Hall © 2007 Chapter Sixteen 25 Chapter Summary Cont. ►Acetone is a widely used solvent and is a by-product of food breakdown during diabetes and starvation. Many sugars are aldehydes or ketones. ►Mild oxidizing agents (Tollens’ and Benedict’s reagents) convert aldehydes to carboxylic acids but have no effect on ketones. ►With reducing agents, hydride ion adds to the C of the group in an aldehyde or ketone and hydrogen ion adds to the O to produce primary or secondary alcohols, respectively.

Prentice Hall © 2007 Chapter Sixteen 26 Chapter Summary Cont. ►Aldehydes and ketones establish equilibria with alcohols to form hemiacetals or acetals. ►Hemiacetals, which have an -OH and an -OR on what was the carbonyl carbon, result from addition of one alcohol molecule to the C=O bond. ►The more stable acetals, which have two -OR groups on what was the carbonyl carbon, form by addition of a second alcohol molecule to a hemiacetal. ►The aldehyde or ketone can be regenerated from an acetal by treatment with an acid catalyst and a large quantity of water, which is an example of a hydrolysis reaction.

Prentice Hall © 2007 Chapter Sixteen 27 End of Chapter 16