Reactions Involving Aldehydes and Ketones Chemical Reactions - Oxidation When aldehydes are prepared by oxidizing primary alcohols with KMnO 4 or K 2 Cr 2 O 7, the reaction may continue and produce carboxylic acids Ketones are less susceptible to further oxidation Which of the following compounds would you start with if you wanted to form 3-pentanone through an oxidation reaction? 1. 3-pentanal 2. 3-pentanol 3. Cyclopentanol 4. It can't be done Aldehydes and ketones are reduced to alcohols by the addition of H 2 in the presence of catalysts such as nickel and palladium Examples: The reduction of a ketone will result in the formation of a(n) 1. alkene 2. primary alcohol 3. secondary alcohol 4. tertiary alcohol Notice, no prediction rule needed. Why? 1
Chemical Reactions - Oxidation Difference in reactivity toward oxidation is the chief reason why aldehydes and ketones are classified in separate families Examples Attempted Oxidation of Acetone and Oxidation of Benzaldehyde Chemical Reactions - Oxidation Effectiveness of the reagents used to test for presence of aldehydes depends on the ease of oxidation of aldehydes Tollens reagent: Mild oxidizing solution containing Ag + ions The Tollens Test All aldehydes react with Tollens reagent Ketones do not oxidize with Tollens reagent Produces metallic silver, often used for mirroring Benedict s reagent: Mild oxidizing solution containing Cu 2+ ions Ketones fail to react with these reagents (one exception) The Benedict s Test Only certain aldehydes and one specific type of ketone oxidize in Benedict s reagent: Aldehydes and ketones are reduced to alcohols by the addition of H 2 in the presence of catalysts such as nickel and palladium These structural features are commonly found in sugars Benedict s test for glucose 2
Examples: The reduction of a ketone will result in the formation of a(n) 1. alkene 2. primary alcohol 3. secondary alcohol 4. tertiary alcohol Notice, no prediction rule needed. Why? Chemical Reactions Formation of Hemiacetals and Acetals Reaction of aldehydes with alcohols: Forms hemiacetals that are not very stable and are difficult to isolate Chemical Reactions Formation of Hemiacetals and Acetals General reaction Forms a stable product called acetal in the presence of an acid catalyst and excess alcohol Example Chemical Reactions Formation of Hemiacetals and Acetals Hemiacetals Contain an OH group, hydrogen, and an OR group on the same carbon Chemical Reactions Formation of Hemiacetals and Acetals Hemiacetal formation Acetals Contain a carbon that has a hydrogen and two OR groups Resemble diethers Acetal formation 3
Practice Write the reaction for the hemiacetal formation that occurs when 4-methyl-hexanal reacts with n-propyl alcohol (propanol). Then, show the substitution of the hemiacetal intermediate to form the resulting acetal Ketone Reactions Ketones react with alcohols to form: Hemiketals Ketals Ketone Reactions General reaction Identify each of the following as a hemiacetal, hemiketal, acetal, ketal, or none of the above: a. b. Example c. d. Cyclic Hemiacetals and Hemiketals Contain both an OH and a C=O group on different carbon atoms More stable than open-chain compounds Identify each of the following as cyclic hemiacetal, hemiketal, acetal, or ketal: a. b. Important in carbohydrate chemistry, where cyclic acetals and ketals may form from the hemistructures c. d. 4
Draw the structural formula for the products of the following reaction: Chemical Reactions - Aldehyde and Ketone Hydrolysis Acetals and ketals are stable but may be converted back to aldehydes and ketones through acid catalyzed hydrolysis Hydrolysis: Bond breakage by reaction with water General reaction for acetal hydrolysis Chemical Reactions - Aldehyde and Ketone Hydrolysis General reaction for ketal hydrolysis Aldehyde and Ketone Reaction Map Specific example Practice Write the reaction for the acid catalyzed hydrolysis of the ketal produced by the acid catalyzed addition of ethanol to 2- pentanone 5