Question

If you preform the tollens test with the following, which ones will give a positive reaction and create a silver mirror? a) Acetaldehyde b)Propanone c)Ethanol d)Benzaldehyde e)Glucose ?

Answer 1

The presence of a hydrogen atom attached to the carbon-oxygen double bond in the aldehyde makes them easily oxidizable(or strong reducing agent) and hence are readily oxidized even by mild oxidizing agents. Since, ketones don't have that particular hydrogen atom, they are resistant to oxidation. Only very strong oxidising agents oxidise ketones.

Tollens reagent is a solution of the diamminesilver(I) complex ion, [Ag(NH₃)₂]⁺.It is a weak oxidizing agent and it can oxidize only aliphatic aldehydes, but is not capable of oxidizing aromatic aldehydes.

Sugars in which the cyclic hemiacetal or hemiketal forms are in equilibrium with their straight-chain form gives a positive Tollen's reagent test.Sugars do not give positive tests with Tollens' reagent if the cyclic and aldehyde forms do not exist in equilibrium with the aldehyde form.

a) Acetaldehyde --- it will give a positive Tollen's reagent test since it is an aliphatic aldehyde.

b) Propanone --- It will not give Tollen's reagent test since it is a ketone.

c) Ethanol --- It will not give Tollen's reagent test.

d) Benzaldehyde --  It will not give Tollen's reagent test since it ia an aromatic aldehyde.

e) Glucose - It will give Tollens reagent test.