Question

What are Tollen's reagent and Fehling's reagent? How do they help in identification of carbonyl compounds?

Short answer

Answer: In Tollen's test, the formation of a silver mirror or a black precipitate of elemental silver indicates the presence of an aldehyde. In Fehling's test, the formation of a brick-red precipitate of copper(I) oxide suggests the presence of an aldehyde or a reducing sugar.

Step-by-step solution

Tollen's Reagent

Tollen's reagent, also known as ammoniacal silver nitrate, is a chemical reagent used to test for the presence of aldehydes in a sample. It consists of a silver complex with ammonia [Ag(NH3)2]+ in the presence of aqueous ammonia and aqueous silver nitrate. The primary reaction occurring is the reduction of the silver complex to metallic silver by the aldehyde, indicating a positive result for the test.

Fehling's Reagent

Fehling's reagent is a chemical reagent used to test for the presence of reducing sugars and aldehydes in a sample. It consists of two solutions: Fehling's solution A, which is an aqueous solution of copper(II) sulfate, and Fehling's solution B, which is an aqueous solution of potassium tartrate and sodium hydroxide. When these solutions are mixed together, a copper(II)-tartrate complex forms in an alkaline solution. The primary reaction occurring is the reduction of the copper(II) complex to copper(I) oxide, which precipitates as a brick-red solid, indicating a positive result for the test.

Identification of Carbonyl Compounds

Both Tollen's reagent and Fehling's reagent help in identifying carbonyl compounds (specifically aldehydes) by exploiting their reducing properties. When a carbonyl compound reacts with either of these reagents, a visible color change or precipitation occurs, signaling the presence of an aldehyde in the sample.

Tollen's Test Procedure

To perform the Tollen's test: 1. Prepare the Tollen's reagent by mixing aqueous ammonia and aqueous silver nitrate. 2. Add a few drops of the sample to be tested to the Tollen's reagent. 3. Observe the reaction mixture. If a silver mirror (metallic silver) forms on the inside of the glass container or if a black precipitate (elemental silver) is observed, the test is positive, indicating the presence of an aldehyde.

Fehling's Test Procedure

To perform the Fehling's test: 1. Prepare the Fehling's reagent by mixing equal volumes of Fehling's solution A and Fehling's solution B. 2. Add a few drops of the sample to be tested to the Fehling's reagent. 3. Heat the reaction mixture gently in a hot water bath. 4. Observe the reaction mixture. If a brick-red precipitate of copper(I) oxide forms, the test is positive, indicating the presence of an aldehyde or a reducing sugar.