Test for Aldehydes
22nd Nov 2024
Experiment 171
Objective: Test for Aldehydes
Introduction:
The Tollens' test is a classical method for detecting aldehydes. In this reaction, Ag⁺ ions from silver nitrate (AgNO₃) react with hydroxide ions (OH⁻) from aqueous ammonia to produce a brown precipitate of silver oxide (Ag₂O). Upon the addition of excess aqueous ammonia, the silver oxide dissolves to form a silver diamine complex, [Ag(NH₃)₂]⁺. When glucose is present, it reduces this complex to metallic silver, which forms a reflective silver coating on the inner surface of the flask. Glucose, in this process, is oxidized to gluconic acid.
This test is significant in carbohydrate chemistry, particularly for detecting reducing sugars. Fructose, although a ketose, can also reduce Tollens' reagent due to its ability to isomerize to glucose under basic conditions. The silver mirror formed during the reaction can be readily dissolved with a small amount of concentrated nitric acid.
Chemicals Required:
· Glucose
· 0.1 M Silver Nitrate (AgNO₃)
· 25% Aqueous Ammonia
Apparatus and Glassware:
Hot plateCrystallizing dish (95 mm diameter, 55 mm height) for water bath500 mL Florence flask100 mL Measuring cylinderThermometer2 Beakers (40 mL each)1 Beaker (200 mL)Volumetric pipettes (5 mL and 10 mL)2 Pipette bulbsGlass stirring rod
Experimental Procedure:
1. While stirring, add 5 mL of ammonia to 150 mL of aqueous 0.1 M silver nitrate solution. A brown precipitate of silver oxide will form.
2. Add another 5 mL of ammonia to dissolve the precipitate, forming the silver diamine complex.
3. Dissolve 4 g of glucose in 10 mL of distilled water.
4. Combine the glucose solution with the prepared silver diamine solution in a round-bottom flask.
5. Place the flask in a water bath heated to 70°C.
6. Stir gently and observe the reaction.
Observations:
Within 4 minutes, the inside surface of the flask will become coated with a reflective silver mirror, indicating the presence of an aldehyde.
Discussion Questions:
1. What Test is Required to Identify an Aldehyde? The Tollens' test is used to identify the presence of aldehydes through the reduction of silver ions to metallic silver.
2. What Evidence Confirms the Presence of an Aldehyde? The formation of a reflective silver mirror on the inside of the flask confirms the reduction of the silver diamine complex by an aldehyde, indicating the presence of an aldehyde in the sample.
3. Is Glucose an Aldehyde? Yes, glucose is an aldose (a type of sugar containing an aldehyde group), and its reaction with Tollens' reagent supports this classification.