How does Tollens' test works?

Question

Paisley Boney · Accepted Answer

Tollens' test exploits the ability of silver ion to oxidize aldehydes and primary alcohols to give carboxylic acids.

Half equation for reduction: #Ag^(+) + e^(-) rarr Ag(s) darr# #(i)# Half equation for oxidation: #RC(=O)H + H_2O rarr RC(=O)OH + 2H^(+) + 2e^-# #(ii)# Overall, #2xx(i)+(ii)# #2Ag^(+) + RC(=O)H + H_2O rarr 2Ag(s) +RC(=O)OH + 2H^(+) # Both primary alcohols, and aldehydes can be oxidized to carboxylic acids using #Ag^+#, and this test is commonly used to differentiate aldehydes from most ketones (the test does give a positive for cyclohexanone). Using scrupulously clean glassware, you can get a beautiful silver mirror lining the inside of your glass. In fact the best mirrors (i.e. looking glasses) are still made with this reaction.

Isaac Aronson · Answer

undefined Tollens' test involves the reaction of an aldehyde with Tollens' reagent (silver mirror test). Tollens' reagent consists of silver ions complexed with ammonia. In the presence of an aldehyde, the silver ions are reduced to silver metal, forming a silver mirror on the test tube. The aldehyde is oxidized to a carboxylic acid. The reaction proceeds in an alkaline medium. The overall reaction can be represented as follows:

$$ \text{3RCHO + 4Ag(NH}_3\text{)_2^+ + 5OH}^- \rightarrow \text{3RCOOH + 2H}_2\text{O + 4Ag + 8NH}_3 $$

Tollens' test is commonly used to distinguish aldehydes from ketones.