Question

The reaction of acetaldehyde with Tollens’ reagent gives:
a.) Silver mirror
b.) Acetic acid
c.) Both A and B
d.) Red ppt of $C{u}_{2}O$

Answer 1

Hint: Tollens test or silver mirror test uses tollens reagent and it is basically oxidation of aldehyde group to its corresponding acid.
- Tollens reagent is a solution of silver nitrate and ammonia.
- Acetaldehyde when reacted with tollens test it gets oxidized to acetic acid.


Complete Solution :
Tollens Reagent refers to the chemical reagent which is used in the detection of an aldehyde functional group, an aromatic aldehyde functional group, or an alpha hydroxy ketone functional group in a given compound.
- The compounds in the tollens reagent gets converted to a coordinate complex Diamine silver (I) complex in the mixture is an oxidizing agent and is the essential reactant in Tollens' reagent.
- The diamine silver(I) complex oxidizes the aldehyde to a carboxylate ion and in the process is reduced to elemental silver and aqueous ammonia. The elemental silver precipitates out of solution onto the inner surface of the reaction vessel, giving a characteristic "silver mirror”. This is what gives the Tollens test named as the silver mirror test.
- So, when acetaldehyde reacts with the above reagent, elemental silver precipitates out and acetaldehyde gets oxidized to acetic acid.
And the reaction is as follows:
$2{\left[Ag{\left(N{H}_3\right)}_2\right]}^{+}+C{H}_{3}CHO+H_{2}O\to 2Ag\left(s\right)\downarrow +4N{H}_3+C{H}_{3}COOH+2H^{+}$

So, the correct answer is “Option C”.

Note: Another test used for aldehyde and ketones is Fehling’s test.it is used to distinguish between both these groups.
- It is detected by reduction of deep blue solution of copper to a red precipitate of copper oxide.
- It is commonly used for reducing sugars.