Aldehydes and Ketones 2024-2025 PDF

Aldehydes and ketones Aldehydes and ketones constitute an important class of organic compounds containing the carbonyl group. Aldehyde has the structure RCH(=O) while a ketone has the structure of R2C(=O). Where R may be an alkyl, alkenyl, alkynyl or aryl group. Aim: To identify the presence of al...

Aldehydes and ketones Aldehydes and ketones constitute an important class of organic compounds containing the carbonyl group. Aldehyde has the structure RCH(=O) while a ketone has the structure of R2C(=O). Where R may be an alkyl, alkenyl, alkynyl or aryl group. Aim: To identify the presence of aldehydes or ketones functional group in the given organic compound and to distinguished between them. Theory: Aldehydes and ketones of low molecular weights are volatile compounds. Identification of aldehydes and ketones is based on two types of reactions: addition reaction to the double bond and oxidation reaction. In aldehydes, the carbonyl group is attached to a hydrogen atom and an aliphatic or aromatic radical. Formaldehyde is an exceptional case in which the carbonyl present in formaldehyde is attached to two hydrogen atoms. In ketones, the carbonyl group is attached to two aliphatic or aromatic group. Acetone is an exceptional case in which the carbonyl present in acetone attached to two methyl molecule. The following tests are used to identify the presence of aldehydes and ketones: a. 2,4-dinitrophenyl hydrazine test: Aldehydes and ketones react with 2,4-dinitrophenyl hydrazine give a yellow to orange precipitate. b. Sodium bisulfite test: Aldehydes and ketones combine with sodium bisulfite to for well-crystallized water-soluble products known as “aldehyde bisulfite” and “ketone bisulfite”. Note: Formation of crystalline precipitate confirms carbonyl group. The difference between ketone and aldehyde is the carbonyl group present in aldehydes can be easily oxidized to carboxylic acid whereas the carbonyl group in ketones are not oxidized easily. This difference in reactivity is the basis for the distinction of aldehydes and ketones. They are generally distinguished by the following tests: a. Schiff’s test b. Fehling’s test c. Tollen’s test d. Test with chromic acid e. Sodium nitroprusside test Distinguishing tests between Aldehydes and Ketones Tollen’s Test: (Silver Mirror Test) This test is also called the silver mirror test. Tollen’s reagent consists of silver ammonia complex in ammonia solution. Aldehydes reacts with Tollen’s reagent gives a grey-black precipitate or a silver mirror. Always a freshly prepared Tollen’s reagent should be used. Aldehydes are oxidized to the corresponding acid and silver in Tollen’s reagent is reduced from +1 oxidation state to its elemental form. Generally ketones do not respond to this test. RCHO + 2[Ag(NH3)2]OH → R-COONH4 + 3NH3 + H2O + 2Ag↓(silver mirror) Note: The appearance of shiny silver mirror confirms the presence of aldehydes. Tollen's test: Reagents: 1- 10% Silver nitrate(AgNO3 ). 2- 10% Sodium hydroxide (NaOH). 3- 30% Ammonium hydroxide(NH4OH). Procedure: In clean two test tube add for each one tube (3 drops) of AgNO3 , then add (3drops) of NaOH , then will appear brown ppt. , then add (3 drops) of NH4OH with mix until the brown ppt. disappear. On the test tube no. (1) add drop by drop of acetaldehyde to the inner side of the test tube. The silver mirror will appears (presence of H ). On the test tube no. (2) add drop by drop of acetone. We will observed no reaction (Absence of H ).  Fehling’s Test: +2 Feling’s solution is a complex compound of Cu. When aldehyde compound is +2 +1 treated with Fehling’s solution Cu is reduced to Cu and the aldehyde is reduced to acids. During the reaction, a red precipitate is formed. Homework: Aromatic aldehydes do not respond to Fehling’s test. Why? What is Fehling’s solution? The solution of Fehling is prepared by combining two separate solutions, known as Fehling’s (1) and Fehling’s (2). Fehling’s (1) is aqueous solution of deep blue copper (II) sulfate. Fehling’s (2) is a colorless solution of sodium - potassium tartrate. (also known as Rochelle salt) made from strong alkali, commonly made from sodium hydroxide. Fehling’s solution is prepared by mixing equal amounts of Fehling’s (1) and Fehling’s (2) solution. principle of fehling test : Aldehyde act as reducing agent it can reduce the deep blue colour of fehling solution into red ppt. of cuprous oxide. Fahling test reaction mechanics  O O +2 - O - R C H + Cu + OH R C O + Cu2O + H2 Cupric ion Carboxyl ion red ppt. Ald. of cuprous oxide O +2 - O R C R + Cu + OH no reaction acetone Cupric ion Procedure: In clean two test tubes add for each one tube (5 drops) of Fehling’s solution (1) , and (5 drops) of Fehling’s solution (2). Then : Add to the first test tube (5 drops) of acetaldehyde. Add to the second test tube (5 drops) of acetone. Then put the two test tubes into a boiling water bath for (3-5minutes). If a brick red precipitate appears indication the presence of H. Notes: 1. The reagents should be freshly prepared to perform the test. 2. Not to heat the reaction mixture directly on the flame. 3. After performing the Tollen’s test wash the test tube with nitric acid to destroy the silver mirror, because it’s an explosive substance. - Ve + Ve  Test Observation Conclusion Tollen's test Aldehyde +Ve + Presence of H Acetaldehyde Silver mirror Ketone -Ve + Absence of H Acetone No reaction Fehling test + Aldehyde +Ve Presence of H Acetaldehyde Red ppt. -Ve + Ketone Absence of H Blue color Acetone (No reaction )