How to Use Mild Oxidizing Reagents to Distinguish among a Set of Compounds – Part 2

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


Tollens’ reagent is the strongest among the three.

Aldehydes (─CHO) test positive and ketones (>C=O) 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)

What are the reaction mechanisms of these reagents with the other functional groups than aldehydes? For a table of summary of the reactions and the reactions with aldehydes (balanced equations), refer to part 1 (HERE). For the reaction mechanisms, go through the infographics below.





Click/tap to enlarge the images

MO = Molecular orbital, RedOx = Reduction-Oxidation, SET = Single electron transfer

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

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.

(8)    Weiss, M.; Appel, M. The Catalytic Oxidation of Benzoin to Benzil. Journal of the American Chemical Society 1948, 70 (11), 3666–3667. https://doi.org/10.1021/ja01191a036.

(9)    Chemistry Stack Exchange

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