Felhing is a common misspelling used online for Fehling’s test, a classic qualitative redox test in aqueous basic medium. A deep-blue copper(II) complex is reduced during heating, forming a characteristic brick-red precipitate of cuprous oxide, \(\mathrm{Cu_2O(s)}\), when an aldehyde or another reducing agent is present.
Reagent composition and observable result
Fehling’s solution is prepared by combining two components immediately before use:
- Copper(II) source: aqueous \(\mathrm{CuSO_4}\) (often written as \(\mathrm{CuSO_4 \cdot 5H_2O}\) as the solid).
- Alkaline complexing solution: \(\mathrm{NaOH}\) together with potassium sodium tartrate (Rochelle salt), which complexes \(\mathrm{Cu^{2+}}\) and stabilizes it in strongly basic solution.
A positive felhing/Fehling test is indicated by loss of the blue color and formation of a red/orange solid. The solid is mainly \(\mathrm{Cu_2O(s)}\), containing copper in the +1 oxidation state.
Redox chemistry behind the color change
The key electron-transfer event is the reduction of \(\mathrm{Cu^{2+}}\) to \(\mathrm{Cu^+}\) (captured as \(\mathrm{Cu_2O}\)), coupled to oxidation of an aldehyde (\(\mathrm{RCHO}\)) to a carboxylate (\(\mathrm{RCOO^-}\)) in basic solution.
Complexation in alkaline solution
In strongly basic water, free \(\mathrm{Cu^{2+}}\) tends to form insoluble \(\mathrm{Cu(OH)_2(s)}\). Tartrate ligands coordinate \(\mathrm{Cu^{2+}}\), keeping copper dissolved and available as an oxidizing agent until a suitable reductant is present.
Balanced net ionic equation in basic medium
A rigorous general-chemistry treatment uses half-reactions written in basic solution.
Oxidation half-reaction (aldehyde to carboxylate)
\[ \mathrm{RCHO + 3OH^- \rightarrow RCOO^- + 2H_2O + 2e^-} \]
Reduction half-reaction (copper(II) to cuprous oxide)
\[ \mathrm{2Cu^{2+} + 2OH^- + 2e^- \rightarrow Cu_2O(s) + H_2O} \]
Overall net ionic equation (felhing / Fehling reaction)
Adding the half-reactions and canceling electrons yields:
\[ \mathrm{RCHO + 2Cu^{2+} + 5OH^- \rightarrow RCOO^- + Cu_2O(s) + 3H_2O} \]
Reducing sugar example (glucose)
For glucose (a reducing sugar), the aldehyde form is present in equilibrium and can reduce \(\mathrm{Cu^{2+}}\). A representative net ionic form is:
\[ \mathrm{C_6H_{12}O_6 + 2Cu^{2+} + 5OH^- \rightarrow C_6H_{11}O_7^- + Cu_2O(s) + 3H_2O} \]
Interpretation and selectivity
| Compound class | Typical felhing/Fehling outcome | Chemical reason (general) |
|---|---|---|
| Aldehydes (\(\mathrm{RCHO}\)) | Positive: \(\mathrm{Cu_2O(s)}\) red precipitate | Oxidation to carboxylate is favorable under alkaline conditions |
| Ketones (\(\mathrm{RCOR'}\)) | Often negative | Oxidation requires C–C bond changes; typical conditions are insufficient |
| Reducing sugars (glucose, lactose, maltose) | Positive | Open-chain aldehyde (or equivalent reducing form) reduces \(\mathrm{Cu^{2+}}\) |
| Non-reducing sugars (sucrose, many acetals) | Often negative | No readily available aldehyde/hemiacetal reducing form |
Visualization of the observation and electron transfer
Common pitfalls in general-chemistry interpretation
- Alkalinity requirement: insufficient \(\mathrm{OH^-}\) reduces the rate and can shift copper speciation away from the intended complex.
- Complexation requirement: absence of tartrate promotes \(\mathrm{Cu(OH)_2(s)}\) formation, producing misleading blue precipitates unrelated to \(\mathrm{Cu_2O}\).
- Heating dependence: many substrates react slowly at room temperature; gentle heating often controls the observation.
- Oxidizable interferents: other reducing agents can also reduce \(\mathrm{Cu^{2+}}\), giving a positive result that is not exclusive to aldehydes.