Question

Dilute \[{\rm{KMn}}{{\rm{O}}_{\rm{4}}}\] at room temperature reacts with \[{\rm{R}} - {\rm{CH}} = {\rm{CH}} - {\rm{R}}\] to give
A) R-CHO
B) R-COOH
C) RCHOH-CHOH-R
D) \[{\rm{C}}{{\rm{O}}_{\rm{2}}} + {{\rm{H}}_{\rm{2}}}{\rm{O}}\]

Answer 1

Hint: Dilute potassium permanganate when undergoes a reaction with an unsaturated compound gives the oxidised product because of the oxidising nature of \[{\rm{KMn}}{{\rm{O}}_{\rm{4}}}\] .

Complete Step by Step Answer:
Let’s understand Baeyer’s test in detail. Baeyer’s reagent is cold, acidified, and dilutes potassium permanganate. The reaction of Baeyer's reagent with an unsaturated compound makes the potassium permanganate solution colourless and produces a brown-coloured precipitate. Therefore, this test is called the unsaturation test. That means this test helps to distinguish between saturated and unsaturated compounds.

Now, we will understand the given reaction. Here, an alkene is given and it undergoes a reaction with Baeyer’s reagent. We know, when an alkene undergoes a Baeyer’s test, the product obtained is a diol. Therefore, the reaction is,
\[{\rm{R}} - {\rm{CH}} = {\rm{CH}} - {\rm{R}} \overset{Dilute KMnO_{4}}{\rightarrow} {\rm{R - CH(OH) - CH(OH) - R}} + \mathop {{\rm{Mn}}{{\rm{O}}_2}}\limits_{{\rm{Manganese}}\,{\rm{dioxide}}} \]
The manganese dioxide obtained in the reaction is a brown-colored precipitate. Therefore, the product obtained in the reaction is RCHOH-CHOH-R.
Hence, the answer is option C.

Additional Information: Let’s understand the reaction of an alkyne with dilute potassium permanganate. This reaction gives oxalic acid and manganese dioxide as products. Also, this reaction turns the purple-colored potassium permanganate into colourless. The reaction is,
\[{\rm{C}}{{\rm{H}}_{\rm{2}}} = {\rm{C}}{{\rm{H}}_{\rm{2}}} + {\rm{KMn}}{{\rm{O}}_{\rm{4}}} \overset{Alkaline KMnO_{4}}{\rightarrow} \mathop {{\rm{COOH}} - {\rm{COOH}}}\limits_{{\rm{Oxalic}}\,{\rm{acid}}} + \mathop {{\rm{Mn}}{{\rm{O}}_{\rm{2}}}}\limits_{{\rm{Brown}}} \]

Note: It is to be noted that, the reactions of an alkene with dilute and concentrated potassium permanganate give two different products. When an alkene undergoes a reaction with concentrated potassium permanganate and is then heated, two molecules of carboxylic acids are obtained. So, the nature of the reagent plays an important role in chemical reactions.