1. Prentice Hall;Copyright 20071 16.6 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.

2. Prentice Hall;Copyright 20072 16.7 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.

3. Prentice Hall;Copyright 20073 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.

4. Prentice Hall;Copyright 20074 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.

5. Prentice Hall;Copyright 20075 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.

6. Prentice Hall;Copyright 20076 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.

7. Prentice Hall;Copyright 20077 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.

8. Prentice Hall;Copyright 20078 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.

9. Prentice Hall;Copyright 20079 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.

10. Prentice Hall;Copyright 200710 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.