Question

Which of the following pairs cannot be differentiated by Tollen’s reagent?
A. benzaldehyde and benzyl alcohol
B. hexanal and 2 – hexanone
C. 2 – hexanol and 2 – hexanone
D. pentanal and diethyl ether

Answer 1

Hint: Tollen’s reagent is the ammonical solution of silver nitrate. The Tollen’s reagent test is also known as the silver mirror test. A metallic silver with bright shining is obtained when any aldehyde is subjected to heat in the presence of tollen’s reagent. This reagent is also a mild oxidizing agent.

Complete answer:
Tollen’s reagent is the ammoniacal silver complex with the formula, \[AgN{{O}_{3}}+N{{H}_{4}}OH\]. This ammoniacal silver complex is a mild oxidizing agent and is used in the identification of aldehydes.
When an aldehyde is warmed in a test tube in the presence of tollen’s reagent, then a metallic bright shining silver mirror is obtained at the end of the test tube. This test is given by aldehydes and is known as silver mirror test. The reaction of the aldehyde with tollens reagent is as follows:
$\begin{align}
  & C{{H}_{3}}CHO+2{{[Ag{{(N{{H}_{3}})}_{2}}]}^{+}}+3O{{H}^{-}}\to C{{H}_{3}}CO{{O}^{-}}+2Ag\downarrow +4N{{H}_{3}}+2{{H}_{2}}O \\
 & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,silver\,mirror \\
\end{align}$
The tollen’s reagent oxidizes the aldehyde into acetate ion, while the silver of the tollen’s reagent is converted to a metallic silver deposit.
Since this test is used to distinguish between an aldehyde and another organic compound, the pair 2 – hexanol and 2 – hexanone cannot be differentiated using tollen’s reagent, as these are alcohol and ketone respectively.

Hence, the pair that cannot be differentiated from tollen’s reagent is 2 – hexanol and 2 – hexanone, so option C is correct.

Note:
Organic compounds like alcohol, hydrocarbons, ketones, or ethers, do not form a silver mirror in tollen’s reagent test. Tollen’s reagent test is used to distinguish between glucose and fructose. Glucose is an aldohexose, containing aldehyde, hence it gives positive tollen’s test, while fructose is a ketohexose, having a ketone, gives a negative tollen’s test.