Main Notes

All reactions of NH3 derivatives with C=O are nucleophilic addition-elimination (condensation) reactions:

• The -NH2 nitrogen attacks C=O (addition)
• Water is eliminated (elimination)
• A C=N double bond forms

Four Key Reagents and Products:

Key Point about 2,4-DNP:

• Reacts with both aldehydes and ketones (detects the C=O group)
• Produces orange or yellow precipitate — the intensity of orange color is diagnostic
• More conjugated carbonyls give deeper orange
• Used as the first test in carbonyl identification hierarchy

Condensation Reaction Pattern: $\text{R}_2\text{C=O} + \text{H}_2\text{N}-\text{X} \xrightarrow{-H_2O} \text{R}_2\text{C=N}-\text{X}$

Cue Column

• Which reagent gives orange ppt with ketones?
• What is the functional group in an oxime?
• Why are these called condensation reactions?

Summary

Hydroxylamine → oximes (C=NOH); phenylhydrazine → phenylhydrazones (C=NNHPh); 2,4-DNP → orange ppt (detects all C=O); semicarbazide → semicarbazones (C=NNHCONH2). All involve loss of water.