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BU CHEM 102 - Preparation of Aldehydes and Ketones
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Chem 102 1st Edition Lecture 14 Outline of Last Lecture I. Chapter 15: Lessons 4.5-4.8 : Nomenclature for Ketones, Isomerism for Aldehydes and Ketones, Selected common Aldehydes and Ketones, Physical Properties of Aldehydes and KetonesOutline of Current Lecture II. 4.9 - Preparation of Aldehydes and KetonesIII. 4.10 - Oxidation and Reduction of Aldehydes and KetonesIV. 4.11 - Reaction of Aldehydes and Ketones with AlcoholsCurrent Lecture4.9 Preparation of Aldehydes and Ketones• Aldehydes produced by the mild oxidation of primary alcohols • Ketones are produced by the mild oxidation of secondary alcohols • Oxidizing agents (mild): KMnO4 or K2Cr2O7 4.10 Oxidation and Reduction of Aldehydes and Ketones• Oxidation of Aldehydes and Ketones: – Aldehydes readily undergo oxidation to carboxylic acids – Ketones are resistant to oxidation Tollens test • Tollens test (silver mirror test): Tollens reagent [aqueous silver nitrate (AgNO3) and ammonia (NH3)] when added to an aldehyde, Ag+ ion is reduced to silver metal, which deposits on the inside of the test tube, forming a silver mirror. – The appearance of this silver mirror is a positive test for the presence of the aldehyde group. – The Ag ion will not oxidize ketones. Benedict’s Test • Similar to the Tollens test except that the metal ion oxidizing agent is Cu2+• Cu 2+ ion (bluish green) is reduced to Cu+ , which precipitates from solution as Cu2O (brick-red coloredsolid) • Ketones are not oxidized Reduction of Aldehydes and Ketones • Aldehydes and ketones are easily reduced by hydrogen gas (H2), in the presence of a catalyst (Ni, Pt, or Cu), to form alcohols. – Aldehydes produces primary alcohols. – Ketones yield secondary alcohols. 4.11 Reaction of Aldehydes and Ketones with Alcohols• Aldehydes and ketones react with alcohols to form hemiacetals and then to form acetals. • Hemiacetal is an organic compound in which a carbon atom is bonded to both a hydroxyl group (—OH) and an alkoxy group (—OR). • A hemiacetal can also be formed due to the reaction between an —OH and C=O functional groups within the same molecule. - Such a reaction produces a cyclic hemiacetal - Cyclic hemiacetals are more stable than noncyclic hemiacetals • Acetal is an organic compound in which a carbon atom is bonded to two alkoxy groups (-OR) Acetal Hydrolysis • Hydrolysis – Reaction of a compound with water, in which the compound splits into two or more fragments as the elements of water ( H, and OH) are added to the compound. • Acetals undergo hydrolysis in acidic solution to form the aldehyde or ketone and alcohols that originally reacted to form the


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