CHAPTER 13: Unit 6. Chemical reactions of Aldehyde & Ketone

Learning Objectives

1. Name the typical reactions take place with aldehydes and ketones.
2. Describe the reactions of aldehyde and ketones.

A.) Oxidation : Oxidation means either loss of hydrogen or gain of oxygen. Similarly, reduction means gain of hydrogen or loss of oxygen.Aldehydes are easily oxidized to carboxylic acids, ketones do not undergo oxidation. During oxidation, oxygen is added between C-H bond. Since ketone have C-C relatively stronger single bond, oxygen cannot be inserted.Reduction of aldehydes and ketones produces primary and secondary alcohols respectively. Hydrogen is added to carbonyl group carbon in both aldehyde and ketone.

B.) Reduction: Aldehydes and ketones are easily reduced by hydrogen gas (H2), in presence of a catalyst (Ni, Pt or Cu)  to form alcohols. the reduction, via hydrogenation, of aldehydes produces primary alcohols andreduction of ketone yields secondary alcohols.

The distinction between Aldehydes and Ketones: Tollen’s test and Benedicts testTollens’ reagent, for example, is an alkaline solution of silver (Ag+) ion complexed with ammonia (NH3), which keeps the Ag+ ion in solution.H3N—Ag+—NH3When Tollens’ reagent oxidizes an aldehyde, the Ag+ ion is reduced to free silver (Ag).Deposited on a clean glass surface, the silver produces a mirror (Figure 3.1 “Aldehyde Reactions”). Ordinary ketones do not react with Tollens’ reagent.Positive Tollen’s test: Solid Silver depositedBenedict’s Test: The blue copper solution turns to brick red Cu2O solid precipitate.Because Ketones cannot be oxidized, these treatments with a mild oxidizing agent is used as a test to distinguish between aldehydes and ketones.

C.) Hemiacetals and Acetals: A characteristics reactions of aldehydes and ketones is the addition of an alcohol across the carbonyl bod to produce hemiacetals. The reaction of a second alcohol molecule with a hemiacetal produces an acetal. Hemiacetals contain an OH group and an OR group bonded to the same carbon. Acetals contain two OR groups bonded to the same carbon. Treatment of an aldehyde or ketone with an alcohol ( ROH) first forms an unstable hemiacetal, which reacts with more alcohol to form an acetal. There are several examples of cyclic hemiacetals and some of them are very stable compounds present in carbohydrates and some drugs.Acetals can be converted back to aldehydes and ketones by hydrolysis with water and acid.

D.) Cyclic hemiacetals: most common examples are carbohydrates.

IN CLASS PRACTICE PROBLEMS

Homework ExerciseWhich compound in each pair has the higher boiling point?A. acetone or 2-propanolB. dimethyl ether or acetaldehyde2. Which compound in each pair has the higher boiling point?A.  butanal or 1-butanolB. acetone or isobutane3. Draw the structure of the alcohol that could be oxidized to each compound.A. cyclohexanoneB. 2-methyl-1-propanal4. Draw the structure of the alcohol that could be oxidized to each compound.A. 2-pentanoneB. o-methylbenzaldehyde5. Acetaldehyde is treated with each substance.A. Ag+(aq)—What inorganic product, if any, is formed?B. K2Cr2O7in an acid solution—What organic product, if any, is formed?6. Acetone is treated with each substance.A. Ag+(aq) —What inorganic product, if any, is formed?B. K2Cr2O7in an acid solution—What organic product, if any, is formed?Answers1.2-propanolacetaldehyde 3.a. b.  5.silver metal (Ag)acetic acid (CH3COOH)Additional Worksheet

Nomenclature of hemiacetal and Acetal: A descriptive method is used to name the hemiacetal or acetal. methyl hemiacetal of propanal or diethyl acetal of propanone are used to name these compounds.