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Fehling's Test
Fehling's solution gives brick-red precipitate with aliphatic aldehydes. Aromatic aldehydes don't respond.
How It Works
1
Three Unknown Compounds
Tubes A, B, C contain aliphatic aldehyde, ketone, and aromatic aldehyde.
2
Add Fehling's + Heat
Fehling's A (CuSO₄) + B (alkaline tartrate) = blue. Aldehydes reduce Cu²⁺ to Cu₂O.
3
Observe Precipitate
Brick-red ppt = Aliphatic aldehyde. No ppt = Ketone or Aromatic aldehyde.
Reaction Key
Aliphatic Ald.:
Brick-red precipitate (Cu₂O)
Ketone:
No brick-red precipitate
Aromatic Ald.:
No precipitate (doesn't reduce Fehling's)