Silver Mirror and Fehling's Tests

Also called

Tollens' test · silver mirror test · Fehling's test · brick red precipitate · aldehyde test · aldehydes and ketones · ammoniacal silver nitrate · cuprous oxide · carbonyl group · nucleophilic addition · ethanal · acetaldehyde · propanone · acetone · oxidation of aldehydes · ethanoic acid · रजत दर्पण परीक्षण · ऐल्डिहाइड · कीटोन

Topics

• Tollens' reagent is ammoniacal silver nitrate: diamminesilver(I) ions, [Ag(NH₃)₂]⁺, in alkaline solution.
• Warmed with an aldehyde, Tollens' reagent deposits a bright silver mirror on the inside of the tube.
• RCHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → RCOO⁻ + 2Ag + 2H₂O + 4NH₃: the aldehyde is oxidised, silver ions reduced.
• Ketones give no silver mirror: their carbonyl carbon has no hydrogen, so mild oxidants cannot oxidise them.
• Fehling's solution (alkaline Cu²⁺ tartrate, deep blue) gives a brick-red precipitate of Cu₂O with aliphatic aldehydes.
• Aromatic aldehydes such as benzaldehyde do not respond to Fehling's test.
• The C=O bond is polar: the carbon is partly positive, so nucleophiles such as OH⁻ attack it (nucleophilic addition).
• Oxidation ladder: a primary alcohol loses two H to give an aldehyde, which gains an O to give a carboxylic acid.

About this model

A GLB file of about 743 KB, shown in Lably's 3D viewer and, on a phone, placed in your room with AR. Credit: Model, science and animation: Lably (built procedurally with three.js; no downloaded meshes). | CC BY 4.0. Original geometry and animation made for Lably.

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