Aldehyde To Carboxylic Acid Silver

Aldehyde → Carboxylic Acid with Silver (Ag₂O/H₂O)

Aldehyde → Carboxylic Acid with Silver (Ag₂O/H₂O)

Silver(I) oxidants such as Ag₂O/H₂O or Tollens reagent convert aldehydes to carboxylates (or carboxylic acids after a quick acidification). In water the aldehyde hydrates to its gem-diol, which then delivers two electrons to Ag(I); silver is reduced to Ag(0) (the classic silver mirror) while the carbonyl carbon is oxidized to the carboxylate. This guide presents a single RDKit-backed mechanism and narrative that supports both neutral Ag₂O/H₂O and basic Tollens conditions via overlay swaps.


Quick Summary

  • Reagents/conditions: Ag₂O/H₂O (neutral to mildly basic, 0–25 °C) or Tollens reagent ([Ag(NH₃)₂]⁺, basic); acidify Tollens mixtures during workup.
  • Outcome: Aldehyde → carboxylate (basic medium) → carboxylic acid after H₃O⁺; neutral Ag₂O/H₂O can deliver the acid directly.
  • Mechanistic spine: Hydrate the aldehyde → coordinate/deprotonate with Ag(I) → two-electron oxidation to carboxylate with Ag(I) → Ag(0) → protonate (optional) to the acid.
  • Selectivity: Aldehydes react readily; simple ketones remain unoxidized (a hallmark of the Tollens test).
  • Observation: Metallic silver deposits (mirror or black film) as Ag(I) is reduced to Ag(0).

Mechanism — Radical-Assisted Silver Oxidation (7 Steps)


Step 1 polarization of the aldehyde carbonyl toward oxygen.
**Step 1 – Carbonyl polarization:** Electron density shifts from C=O toward oxygen, priming the aldehyde for single-electron transfer.
Step 2 single-electron transfer from oxygen to Ag(I) creating an oxygen-centered radical.
**Step 2 – Single-electron transfer to Ag(I):** The polarized carbonyl oxygen donates one electron into Ag(I), generating an oxygen-centered radical and initiating silver reduction.
Step 3 hydroxide adds to the radical cation carbon.
**Step 3 – Hydroxide capture:** Hydroxide approaches the radical cation and adds into the carbonyl carbon, installing the second O–H on the hydrate.
Step 4 hydrogen atom transfer re-forms a neutral oxygen and creates a carbon-centered radical.
**Step 4 – Hydrogen transfer shuffle:** Homolytic C–H cleavage transfers H• to the oxygen radical, giving a carbon-centered radical while the O–H bond is regenerated.
Step 5 carbon radical engages Ag(I) while both oxygens remain protonated.
**Step 5 – Radical diol engages silver:** The carbon-centered radical donates electron density toward Ag(I), depicting the reduction step that deposits Ag(0).
Step 6 hydroxide-mediated proton transfer collapses the hydrate to the carbonyl.
**Step 6 – Proton transfer rebuilds the carbonyl:** Hydroxide abstracts the gem-diol proton and the O–H bond collapses into a C=O, revealing the carboxylate.
Step 7 final carboxylic acid product and metallic silver by-product.
**Step 7 – Product frame:** The mechanism concludes with the carboxylic acid (or carboxylate in basic Tollens runs) plus metallic silver, matching the Mechanism Solver output.

Mechanistic Checklist (Exam Focus)

  • Hydrate-first logic: oxidation proceeds from the gem-diol, not directly from the aldehyde.
  • Track redox: aldehyde carbon is oxidized as Ag(I) is reduced to Ag(0) (mirror test).
  • Medium controls the observed product: Tollens (basic) yields carboxylate until you add acid; neutral Ag₂O/H₂O can show the acid directly.
  • Ketones are negative — a key diagnostic for Tollens-type reagents.
  • Stoichiometry cue: each aldehyde typically reduces 2 Ag⁺ to 2 Ag(0).

Worked Examples


Propanal → Propanoic acid (Ag₂O/H₂O)

Propanal reactant Ag₂O/H₂O reagent button Propanoic acid product

Neutral Ag₂O/H₂O oxidizes propanal smoothly to propanoic acid; the silver mirror reveals reduction to Ag(0).

Benzaldehyde → Benzoic acid (Tollens, then H₃O⁺)

Benzaldehyde reactant Silver oxidant icon Benzoic acid product

Tollens reagent (basic) first gives benzoate; a quick acidification furnishes benzoic acid after silver deposition.

Furfural → Furoic acid (Ag₂O/H₂O)

Furfural reactant Ag₂O/H₂O icon Furoic acid product

Heteroaryl aldehydes also oxidize efficiently; the furan ring survives while Ag(I) is reduced to Ag(0).


Scope & Limitations

  • Great substrates: Aliphatic, benzylic, and heteroaryl aldehydes; formaldehyde reacts fastest (fully hydrated).
  • Unreactive: Simple ketones are Tollens-negative; α-hydroxy aldehydes still oxidize.
  • Medium effects: Tollens is basic — protect base-sensitive groups; Ag₂O/H₂O is milder but still aqueous.
  • Silver salts: Halides can precipitate AgX and quench the oxidant; minimize competing ligands (thiols, phosphines).

Practical Tips

  • Prepare Tollens reagent fresh; do not store dry residues (avoid shock-sensitive silver azides).
  • Swirl reaction vessels to observe the silver mirror or black Ag(0) coating.
  • Filter off metallic silver, rinse glassware thoroughly, and collect Ag waste for recovery.
  • Acidify basic mixtures after oxidation to isolate the neutral carboxylic acid cleanly.

Exam-Style Summary

In water, aldehydes hydrate to gem-diols. Silver(I) accepts two electrons from the gem-diol, oxidizing the carbonyl carbon to a carboxylate while Ag(I) is reduced to metallic Ag(0) (silver mirror). Neutral Ag₂O/H₂O often shows the acid directly; Tollens reagent produces the carboxylate, which is protonated on workup. Ketones remain inert — a classic diagnostic.


Interactive Toolbox

  • Mechanism Solver — Use Mechanism Solver to see each step of the silver oxidation mechanism along with descriptions of each step!
  • Reaction Solver — Quickly find the product of any aldehyde reacted with Ag₂O/H₂O or Tollens reagent!
  • IUPAC Namer — Learn the naming ins and outs of aldehyde starting materials and carboxylic acid products.

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