2, 4- DNP: A solution of 2,4- dinitrophenylhydrazine is used to detect presence of carbonyl
functional group in aldehydes and ketones. If present, a yellow/ orange precipitate called 2,4-
dinitrophenylhydrazone produced.
Brady’s Reagent: In practical’s, 2,4- DNP dissolved in methanol and sulfuric acid to form orange
solution called Brady’s Reagent.
Tollens’ Reagent: Tollens’ Reagent- a solution of silver nitrate in aqueous ammonia.
Making Tollens’ Reagent: Add aqueous silver nitrate and then aqueous sodium hydroxide until
brown precipitate formed. Add dilute ammonia solution until brown precipitate dissolves to for
colourless solution.
Method for Distinguishing Ketone/ Aldehyde: Add Tollens Reagent. Warm in water bath to
60°C. Aldehyde present means a silver mirror will form as silver reduced Ag+ (aq) + e- -> Ag(s).
Ketone present means no visible observation/ no reaction.
Equation for Tollens’ Reagent: Silver ions Ag+ act as oxidising agent in presence of ammonia.
Silver ions reduced to silver and only aldehyde oxidised (O added) to form carboxylic acid. Goes
from CHO to COOH. [O] represents oxidising agent.
Only works with aldehydes and not ketones because only aldehydes can be oxidised to a
carboxylic acid.
A Chemical Test to Detect Carbonyl AND Determine Its Identity: Write this when talks about
being ‘identified/ using product’ and at least 2/3 marks. Even if aldehyde, don’t talk about
Tollens’s Reagent.
– Add 2,4-Dinitrophenylhydrazine (2,4-DNP).
– Orange precipitate hydrazone will form if a carbonyl is present (Aldehyde or Ketone).
– Purify the orange hydrazone precipitate by recrystallization.
– Measure the melting point of the crystals formed. Compare melting point with known data
table values to identify the aldehyde or ketone.
Here benzaldehyde forms two products- one is reduced to form alcohol and other oxidised to form
carboxylic acid. Since in KOH, forms COO- or COOK instead of COOH.