1 Chapter 15 Aldehydes and Ketones and Chiral Molecules 15.6 Oxidation and Reduction 15.7 Addition Reactions

2 Oxidation Aldehydes are easily oxidized to carboxylic acids. O O || [O] || CH 3 —C—H CH 3 —C—OH AcetaldehydeAcetic acid

3 Hydrate formation Hydrate formation is the reason that aldehydes are oxidized ([O]=-2H=oxidation) to carboxylic acids in aqueous media. O OH O || H 2 O | [O] || RCH ↔ RC – OH → RCOH | H

4 Tollens’ Test Tollens’ reagent, which contains Ag +, oxidizes aldehydes, but not ketones. Ag + is reduced to metallic Ag, which appears as a “mirror” in the test tube.

5 Tollens’ reagent detects an aldehyde Tollens’ reagent (a solution of Ag + in aqueous NH 3 ) can be used to detect the presence of the aldehyde group in the unknown sample. As the oxidation of the aldehyde proceeds, silver metal is deposited on the walls of the reaction flask as a shiny mirrow. If upon addition of the Tollens’ reagent to the unknown, the test tube becomes silvery, the unknown is an aldehyde. RCHO + 2Ag(NH 3 ) OH - → RCOO - + NH Ag + 3NH 3 + H 2 O anTollens’ salt of a silver aldehydereagentcarboxylic acid mirrow

6 Benedict’s Test Benedict’s reagent, which contains Cu 2+, reacts with aldehydes that have an adjacent OH group. When an aldehyde is oxidized to a carboxylic acid, Cu 2+ is reduced to give Cu 2 O(s).

7 Test for glucose (Benedict’s test)

8 Addition Reactions Polar molecules can add to the carbonyl groups of aldehydes and ketones. The negative part of the added molecule bonds to the positive carbonyl carbon. The positive part of the added molecule bonds to the negative carbonyl oxygen. |  +  -  +  - | —C=O + X—Y — C—O—X | Y

9 Acetal Formation Alcohols add to the carbonyl group of aldehydes and ketones. The addition of two alcohols forms acetals.

10 Hemiacetal Formation The addition of one alcohol to an aldehyde or ketone forms an intermediate called a hemiacetal. Usually, hemiacetals are unstable and difficult to isolate.

11 Cyclic Hemiacetals Stable hemiacetals form when the C=O group and the —OH are both part of five- or six-atom carbon compounds.