Question

What reagent can be used to test for unsaturation of hydrocarbons.

Answer 1

Hint: In order to deal with this question first we will elaborate the term hydrocarbon further we will discuss its methods for detecting the unsaturation.
Complete step-by-step answer:

Organic compounds containing carbon, and hydrogen are known as hydrocarbons. Most organic compounds are derived from hydrocarbons by displacing one or more hydrogen atoms or a group of atoms such as acid, aldehyde, reactive hydroxide and halogen groups etc. by a single atom. The hydrocarbons are graded as saturated and unsaturated hydrocarbons.

Unsaturated Hydrocarbons
The presence of double and triple bonds in organic compounds is causing insaturation. The aliphatic hydrocarbons that have a double bond between the two carbon atoms are termed alkenes, and the triple-bonded hydrocarbons are termed alkynes.

There are two approaches in an organic compound to measure the insaturation.
Alkaline potassium permanganate test (Baeyer’s test)
Bromine test

1. Alkaline potassium permanganate test (Baeyer’s test):
In this test the hue of pink potassium permanganate disappears by applying an unsaturated hydrocarbon to an alkaline potassium permanganate. The pink color can go away, with or without the formation of brown manganese oxide.

Note: Decolourization of pink colour of $KMn{O_4}$ indicates unsaturation.
The chemical reaction is given below:
$
  KMn{O_4} + {H_2}O \to 2KOH + 2Mn{O_2} + 3\left[ O \right] \\
   - C = C - + {H_2}O \to - C\left( {OH} \right) - C\left( {OH} \right) - + KOH\left( {{\text{colourless}}} \right) \\
 $

2. Bromine Test
The bromine solution is brown in color. If the hydrocarbon is unsaturated the brown colour will vanish in this test when applied to the unsaturated hydrocarbon bromine solution. Bromine produces an extra component with the unsaturated hydrocarbon.

Note: Decolorization of orange-red bromine solution shows unsaturation.
The chemical reaction is given below
\[ - C = C - + B{r_2} \to - C\left( {Br} \right) - C\left( {Br} \right) - \left( {{\text{colourless}}} \right)\]

Note- Saturated hydrocarbon molecules which have no double bonds in them are called saturated or generally referred to as alkanes. That literally means the molecule contains as many hydrogen atoms as possible, and cannot be added any more. They are also classified, and are least sensitive, as paraffins.
Alkenes and alkynes form unsaturated carbohydrates. Aliphatic hydrocarbon is a dual bond called alkenes which contains one sigma bond and one pi bond. The hydrocarbon comprises 1 sigma bond and 2 pi bond, and is a triple bond or alkyne. Any sources for alkenes and alkynes include propene, propine, butene, and butyne.