# How tartaric acid gives Tollen's test?

I wanted to know how Tollen's test is given by tartaric acid. Google searches say that it belongs to some "Hydroxyl" group. I know that it oxidises aliphatic and aromatic aldehydes along with α-hydroxy ketones and some other compounds such as alkynes and it's mechanism. Can someone help me out in the mechanism of oxidation or provide some more detail on this reaction?

• – Mathew Mahindaratne Mar 6 at 23:46
• I know how α - hydroxy ketones are oxidised. But, this is α - hydroxy acid. The answer doesn't explain. – Sarthak Rout Mar 7 at 6:09

Maxted[1] reported in 1926 that the reaction of tartaric acid with silver produces oxalic acid, formic acid, CO2 and water as products and that the stoichiometry is 6 moles of silver reduced per tartartic acid. He proposed the following possible net reactions:

$$\ce{C4H6O6 + 6 Ag+ + 2H2O -> 2(C2H2O4) + 6 Ag + 6 H+}$$

$$\ce{C4H6O6 + 6 Ag+ + 2H2O -> C2H2O4 + CO2 + HCO2H + 6 Ag + 6 H+}$$

$$\ce{C4H6O6 + 6 Ag+ + 2H2O -> 2(HCO2H) + 2CO2 + 6 Ag + 6 H+}$$

For the mechanism, he concluded that initial decarboxylation to 2-hydroxy-3-oxo-propionate or splitting into two glyoxylic acid molecules were both possibilities. The glyoxylate could be oxidized to oxalic acid or oxidatively decarboxylated to formic acid, while the 2-hydroxy-3-oxo-propionate could split into glyoxylic acid and formic acid.

I'm sure someone must have done more recent mechanistic work on this to pin it down further, but that gives a pretty good idea of the possibilities.

[1] Maxted, D.R. (1926) CCLXXXVIII-The Oxidation of Tartartic Acid by Solutions of Silver Salts; J. Chem Soc. 129:2178.

I outright do not "know" the answer, if there is one. It could just be put down to the old "for every rule there's an exception, except in chemistry where there seems to be hundreds".

I will hazard a guess though.

In solution, aldehydes and alpha-hydroxy ketones can tautomerise through an enediol state. Tartaric acid can also tautomerise into something similar- a "hydroxy-enediol", if you will. Perhaps this is enough outright to allow the reduction reaction to occur, or perhaps enough of the hydroxy-enediol can then tautomerise into aldehyde (however unfavourable this may be) to react with Tollen's and then be driven by equilibrium.

This is speculation, perhaps someone else knows of any literature that confirms the mechanism of Tollen's and tartaric acid.

• Well, you surely are not tautomerising the carbonyl of acid group? – Sarthak Rout Mar 7 at 13:51
• Yes I am. The following paper describes the potential for the tartaric acid tautomer described. The tautomer most probably is very unfavourable, especially so when taken out of the context of nano-particle treatments, however if it could be formed a little, this may simply mean a slower reaction to Tollen's driven by equilibrium. researchgate.net/publication/… – Curt P Mar 7 at 14:11
• Hmm, that is interesting. – Sarthak Rout Mar 7 at 15:02
• I think the relevant reference is Maxted, journal of the chemical society 1926 p 2178, but I don’t have access to the full article right now. Decarboxylation is definitely involved. – Andrew Mar 8 at 1:47