Question

Which of the following compounds exhibit silver mirror test?
This question has multiple correct options.
A ) Acetaldehyde
B ) Acetone
C ) Formaldehyde
D ) Benzophenone

Answer 1

Hint: Compounds having an aldehyde group form a silver mirror when treated with Tollen’s reagent (ammoniacal silver nitrate solution). This test is not given by ketones. Thus, silver mirror test can be used to distinguish between aldehydes and ketones.

Complete answer:
Silver mirror test is used to distinguish between aldehydes and ketones. If an unknown compound contains a carbonyl group, then it is possible to say if this compound is an aldehyde or ketone, based on the results of a silver mirror test. Thus, silver mirror test helps in the structure determination (functional group determination). Aldehydes give silver mirror tests but ketones do not give silver mirror tests.
Aldehydes (acetaldehyde and formaldehyde) reacts with Tollen’s reagent (ammoniacal silver nitrate solution) to form a silver mirror. Aldehydes themselves are oxidised to carboxylate ions and silver nitrate is reduced to black silver metal.

\[{\text{R}} - {\text{CHO + }}\left[ {{\text{Ag}}\left( {{\text{N}}{{\text{H}}_3}} \right)_2^ + } \right]{\text{O}}{{\text{H}}^ - }{\text{ }} \to {\text{ RCO}}{{\text{O}}^ - }{\text{ + Ag}} \downarrow {\text{ + NH}}_4^ + \]

Acetaldehyde \[\left( {{\text{C}}{{\text{H}}_3} - {\text{CHO }}} \right)\] and formaldehyde \[\left( {{\text{H}} - {\text{CHO }}} \right)\] exhibit silver mirror test as they contain an aldehyde group.
Acetone \[\left( {{\text{C}}{{\text{H}}_3} - {\text{CHO}} - {\text{C}}{{\text{H}}_3}} \right)\] and benzophenone \[\left( {{{\text{C}}_6}{{\text{H}}_5} - {\text{CHO}} - {{\text{C}}_6}{{\text{H}}_5}} \right)\] does not exhibit silver mirror test as they do not contain an aldehyde group.

Thus the correct options are A) and C).

Note: Aldehyde contains \[ - {\text{CHO }}\] functional group. One hydrogen atom is joined to the carbonyl group. Aldehydes can be oxidised to carboxylate ions. Hence, aldehydes give silver mirror test. Ketones contain the \[{\text{ > CO }}\] functional group. No hydrogen atom is joined to the carbonyl group. As we know that the Ketones cannot be oxidised to carboxylate ions. Hence, ketones do not give silver mirror test.