How to Use Mild Oxidizing Reagents to Distinguish among a Set of Compounds – Part 1

Tollens’ reagent, Fehling’s solution and Benedict’s reagent are known as mild oxidizers.


Tollens’ reagent is the strongest among the three.

Aldehydes test positive and ketones test negative to Tollens’ reagent, Fehling’s solution and Benedict’s reagent, in general. However, compounds having some other functional groups test positive as well.

False positives are possible due to contaminations. All reagents are freshly prepared due to their short shelf lives.

Tollens’ reagent: AgNO3 (Silver nitrate) + NH4OH (Ammonium hydroxide) + NaOH (Sodium hydroxide)

2AgNO3(aq) + 2NaOH(aq) → Ag2O(s) + 2NaNO3(aq) + H2O(l)

Silver oxide

Ag2O(s) + 4NH3(aq) + 2NaNO3(aq) + H2O(l) → 2[Ag(NH3)2]NO3(aq) + 2NaOH(aq)

Overall equation: 2AgNO3(aq) + 4NH3(aq) → 2[Ag(NH3)2]NO3(aq)

Fehling’s solution: A. CuSO4 (Copper(II) sulfate, blue) B. Potassium sodium tartrate ((C4H4O6)22─Na+K+) + NaOH

Benedict’s reagent: CuSO4 + Sodium citrate ((C6H5O7)─Na+) + Na2CO3 (Sodium carbonate)

Question:

Which of the following compounds would show positive test to Tollens’ reagent, Fehling’s solution and/or Benedict’s reagent and negative test to them?

Acetaldehyde, Acetone, Benzaldehyde, Hydroxyacetone, Formic acid, Hydrazine, Acetaldehyde phenylhydrazone, Glyoxal, Methylglyoxal, Dimethylglyoxal, Benzil, Furil, 3-Hydroxybutan-2-one, Benzoin, Furoin, Vitamin C, D-Glucose, D-Fructose, Sucrose, Acetylene

Solution:




Click/tap to enlarge the images

P.S. Reducing sugars, hemiacetals/acetals +ve to all; β-diketones & P-H bond containing compounds like Hypophosphorous acid (H3PO2), Phosphorous acid (H3PO3), Pyrophosphorous acid (H4P2O5) +ve to Tollens’ reagent


Click/tap to enlarge the image

Interested in the reaction mechanisms of these reagents with some other functional groups? Stay tuned for the part 2 (HERE)!

Magical mild oxidizers❓ Let Chemaficionado know in the comments below or at mychemistryhomework@gmail.com❗

NOTE: Post updated since published

References:

(1)    Wikipedia Contributors. Analytical chemistry. Wikipedia. https://en.wikipedia.org/wiki/Analytical_chemistry.

(2)    Wikipedia Contributors. Tollens’ reagent. Wikipedia. https://en.wikipedia.org/wiki/Tollens%27_reagent.‌

(3)    Wikipedia Contributors. Benedict’s reagent. Wikipedia. https://en.wikipedia.org/wiki/Benedict%27s_reagent.‌

(4)    Wikipedia Contributors. Fehling’s solution. Wikipedia. https://en.wikipedia.org/wiki/Fehling%27s_solution.

(‌5)    Graham, T. W.; Fryhle, C. B.; Snyder, S. A. Organic Chemistry.; Hoboken, Nj John Wiley Et Sons, Inc, 2016.

(6)    Brown, C.; Ford, M. Higher Level Chemistry, 2nd ed.; Pearson Education: Harlow, Essex, 2014.

(7)    Ncert. Chemistry : Textbook for Class XII - Part.II; National Council Of Educational Research And Training: New Delhi.

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