Phloroglucinol Hydrate in Tollens' and Borntrager's Tests: Differentiating Aldehydes and Ketones

Phloroglucinol (1,3,5-trihydroxybenzene) hydrate serves as a chromogenic reagent in two classic qualitative organic tests.

Tollens' Test (Silver Mirror Test) — For Aldehydes

• Reagent: Ag(NH₃)₂⁺ (ammoniacal silver nitrate) + phloroglucinol.
• Mechanism: Aldehyde reduces Ag⁺ → metallic silver (mirror); phloroglucinol forms red-violet condensation product with aldehyde.
• Positive: Silver mirror + red-violet color (aldehyde).
• Negative: No mirror, no color (ketone).

Borntrager's Test — For Ketones

• Reagent: Phloroglucinol in acidic/ethanolic solution (sometimes with HCl).
• Mechanism: Acid-catalyzed condensation of phloroglucinol with ketone → yellow-orange chromophore.
• Positive: Yellow-orange color (ketone).
• Negative: No color (aldehyde).

Procedural Summary

Limitations

• α-Hydroxyketones may give false-positive Tollens'.
• Enolizable aldehydes may react in Borntrager's.
• Modern spectroscopy (IR, NMR) has largely replaced these for structure elucidation.

These tests remain valuable teaching tools for carbonyl reactivity and condensation chemistry.

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Step	Tollens'	Borntrager's
1. Add sample	Few drops aldehyde/ketone	Few drops aldehyde/ketone
2. Add reagent	2–3 drops Tollens'	2–3 drops Borntrager's
3. Heat	Gentle water bath	Gentle water bath
4. Observe	Red-violet (aldehyde)	Yellow-orange (ketone)