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14.3 Oxidation and Reduction of Aldehydes and Ketones In Tollens’ test, a “silver mirror” forms when the oxidation of an aldehyde reduces silver ions to metallic silver. The silvery surface of a mirror is formed in a similar way. Learning Goal Draw the condensed or line-angle structural formulas for the reactants and products in the oxidation or reduction of aldehydes and ketones. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Oxidation of Aldehydes • Aldehydes oxidize readily to form carboxylic acids. • Ketones do not undergo oxidation. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Tollens’ Test: Silver Mirror Tollens’ test • may be used to distinguish between aldehydes and ketones. • utilizes Tollens’ reagent, which is a solution of Ag+ (AgNO3) and ammonia. • oxidizes aldehydes but not ketones. • reduces Ag+, as the aldehyde is oxidized and forms a layer called a “silver mirror” on the inside of the container. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Benedict’s Test Benedict’s test • gives a positive result with compounds that have an aldehyde functional group and an adjacent hydroxyl group. • utilizes Benedict’s solution, which contains Cu2+ (CuSO4). When the solution is added to this type of aldehyde and heated, a brick-red solid of Cu2O forms • is negative with simple aldehydes and ketones. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Benedict’s Test: 2-Hydroxy Aldehyde Because sugars such as glucose contain this type of aldehyde grouping, Benedict’s reagent can be used to determine the presence of glucose in blood or urine. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Reduction of Aldehydes and Ketones Aldehydes and ketones • are reduced by hydrogen (H2) or sodium borohydride (NaBH4) and a catalyst such as nickel, platinum, or palladium. • are reduced to alcohols by decreasing the number of carbon–oxygen bonds. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Reduction of Aldehydes and Ketones Aldehydes are reduced to primary alcohols, and ketones are reduced to secondary alcohols. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Study Check Write the condensed structural formula and the name of the oxidized product when each of the following is mixed with Tollens’ reagent. A. Butanal B. Acetaldehyde C. Ethyl methyl ketone General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Solution Write the condensed structural formula and the name of the oxidized product when each of the following is mixed with Tollens’ reagent. A. Butanal butanoic acid B. Acetaldehyde ethanoic acid General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Solution Write the condensed structural formula and the name of the oxidized product when each of the following is mixed with Tollens’ reagent. C. Ethyl methyl ketone None; ketones are not oxidized by Tollens’ reagent. No reaction General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Study Check Predict the product for each of the following reactions: Pt + H2 A. Ni B. + H2 General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Solution Predict the product for each of the following reactions: Pt + H2 A. Aldehydes are reduced to primary alcohols. Ni B. + H2 Keytones are reduced to secondary alcohols. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.
