Question

How does Tollens' test works?

Answer 1

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

Explanation:

Reduction half equation:

`Ag^(+) + e^(-) rarr Ag(s) darr` `(i)`

Oxidation half equation:

`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.