Question

Which positive test of following gives Tollens reagent.
A) $HCH{O_{(aq)}}$
B) $C{H_2}COCH$
C) $phCHO$
D) $C{H_2}CHO$

Answer 1

Tollens Reagent is the chemical reagent which is used in for detection of an aldehyde functional group, an aromatic aldehyde functional group, or an alpha hydroxy ketone functional group in a given test substance.

Complete answer:
Silver nitrate solution with ammonium hydroxide in the presence of NaOH solution combine to form Tollens Reagent. Tollens reagent oxidizes an aldehyde into the corresponding carboxylic acid.
Formula of Tollens Reagent is ${\left( {Ag{{\left( {NH} \right)}_2}} \right)_3}$ $$
In option no (A) HCHO will show positive test with Tollens Reagent because Tollens Reagent oxidizes aldehyde to its corresponding carboxylic acid and HCHO is also an aldehyde.
$HCH{O_{(aq)}}$ $+ {\left( {Ag{{(NH)}_3}} \right)_2}^ + + 3O{H^ - } \to HCO{O^ - }_{(aq)} + 2A{g_{(s)}} + 4N{H_{3(aq)}}$
In option number (B) CH2CHO is also an aldehyde and after reaction with the tollens reagent it will give the same result as an option number (A) so option number (B) will also test post you with tollens reagent.
In option number C$phCHO$ is also an aldehyde and after reaction with the tollens reagent it will give the same result as in upper cases.
In option number (D) $C{H_2}CHO$ is ketone and after reaction with Ketone tollens reagent doesn't produce Carboxylic acid so this option will test negative with tollens reagent.
Hence the correct answer is Option (D).

Note: The precipitation of elemental silver indicates a positive test with Tollens' reagent often producing a characteristic "silver mirror" on the inner surface of the reaction vessel. Brady's reagent is also used to distinguish between aldehyde and ketone.