Question

Mention the reagent used for the hydroxylation of alkenes.
A.Cold dil. alkaline $KMn{O_4}$
B.Cold dil. acidified $KMn{O_4}$
C.Cold dil. acidified ${K_2}C{r_2}{O_7}$
D.Cold dil. alkaline ${K_2}C{r_2}{O_7}$

Answer 1

Hint:During hydroxylation of alkenes, the carbon-hydrogen bond in the alkene oxidizes to a carbon-hydroxyl bond. Hydroxylation of alkenes is an oxidation reaction. A reagent that increases the oxidation number is called an oxidizing agent. Both potassium permanganate and potassium dichromate are oxidizing agents but the hydroxylation of alkenes takes place in particular conditions.

Complete step by step answer:
First, let us know about the hydroxylation reactions of alkenes.
Hydroxylation of alkenes is an oxidation reaction where a carbon-carbon double bond converts to a carbon-hydroxyl bond.
In the hydroxylation of alkenes the reagent used is cold dilute alkaline $KMn{O_4}$.
In this reaction the permanganate reacts with the carbon-carbon double bond and the product formed is a vicinal diol.
Cold dilute alkaline $KMn{O_4}$ is used as a reagent to test the presence of carbon-carbon double bonds in an organic compound.
Let us take ethene, as an example for showing the reaction of hydroxylation of alkenes,

Ethene ethane-1, 2-diol
$KMn{O_4}$ is a strong oxidizing agent. It oxidizes the carbon-carbon double bond in ethene to form ethane-1, 2-diol. The $KMn{O_4}$ is used under alkaline conditions. Under alkaline conditions, the manganate (VII) ions in $KMn{O_4}$ is reduced to manganate (VI) ions which are green colored.
Thus the purple-colored $KMn{O_4}$ changes to green in alkaline conditions.
This green colored reduced manganate (VI) ions react with the ethene (alkene) and then we get a dark brown manganese (IV) oxide precipitate (manganese oxide) along with the product (diol).
Thus cold dilute alkaline $KMn{O_4}$ is a reagent used to test unsaturation in organic compounds.
If an organic compound reacts with cold alkaline $KMn{O_4}$ and gives a green solution followed by a dark brown precipitate, then we can confirm the presence of a carbon-carbon double bond in that compound.
But acidified $KMn{O_4}$ is rarely used since it has a destructive nature. If we used acidified $KMn{O_4}$, it can lead to the breaking of the carbon-carbon bonds and thus lead to the formation of carbonyl compounds.
Acidic potassium permanganate or acidic potassium dichromate is used to oxidize alkenes to ketones or carboxylic acid depending on the nature of the alkene.
Therefore cold dilute alkaline $KMn{O_4}$ is used in the hydroxylation of alkenes.
Thus the answer is an option (A) cold dilute alkaline $KMn{O_4}$ is the reagent used for the hydroxylation of alkenes.


Note:
Potassium manganate (VII) solution is a strong oxidizing agent. So if we are not using cold dilute conditions then product formation will not stop at the diol. The manganate (VII) ions can further oxidize the diol if we are using concentrated acidic conditions. Therefore dilute cold alkaline $KMn{O_4}$ should be used for the hydroxylation of alkenes.