Identifying Aldehydes Flashcards
What is Tollens’ reagent composed of?
Silver(1) ions, Ag* (aq), in the presence of ammonia
What role do silver ions play in the reaction with aldehydes?
Oxidising agent
What is produced when silver ions are reduced during the reaction with aldehydes?
Silver
What happens to the aldehyde when it reacts with Tollens’ reagent?
Oxidised to a carboxylic acid
What is the indication of a positive Tollens’ test?
Formation of a silver mirror
What precipitate forms during the 2,4-DNP test?
2,4-dinitrophenylhydrazone precipitate
How is the solid precipitate from the 2,4-DNP test processed?
Recrystallised to produce a pure sample of crystals
What is the next step after obtaining the pure sample of 2,4-DNP?
Measure and record the melting point
What should the melting point be compared to for identification purposes?
A database or data table of melting points
How can you distinguish between aldehydes and ketones after identifying a carbonyl group?
Using Tollens’ reagent
What is the visual result of an aldehyde reacting with Tollens’ reagent?
Silver mirror forms
What happens if a ketone is tested with Tollens’ reagent?
No reaction is observed
Describe the preparation method for Tollens’ reagent.
Add silver nitrate to sodium hydroxide until brown precipitate forms, then add dilute ammonia until it dissolves
Fill in the blank: Tollens’ reagent should be prepared immediately before carrying out the test because it has a _______.
Short shelf-life
What is the first step in testing an unknown solution with Tollens’ reagent?
Pour 2cm depth of the unknown solution into a clean test tube
What temperature should the test tube with Tollens’ reagent be placed in for the reaction?
About 50°C
Which carbonyl compound would produce a silver mirror with Tollens’ reagent?
Aldehydes
Write the general equation for the reaction of an aldehyde with Tollens’ reagent.
RCHO + 2Ag^+ + 2OH^- → RCOO^- + 2Ag + 2H2O
