Question

Give a test that can be used to differentiate between saturated and unsaturated hydrocarbons.

Answer 1

Saturated hydrocarbons:-

• A hydrocarbon where all the carbon-carbon bonds are single bonds are called saturated hydrocarbons.
• This means that no double or triple bond between the carbons atoms is present in a saturated hydrocarbon.
• They are generally called as Alkanes
• The general formula for saturated hydrocarbons is ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}+2}$
• Examples: Propane$\left({\mathrm{C}}_3{\mathrm{H}}_8\right){\mathrm{CH}}_3-{\mathrm{CH}}_2-{\mathrm{CH}}_3$, Butane$\left({\mathrm{C}}_4{\mathrm{H}}_{10}\right)$${\mathrm{CH}}_3-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{CH}}_3$.

Unsaturated hydrocarbons:-

• These are hydrocarbons that have double or triple bonds present between the carbon atoms.
• These types of hydrocarbon have at least one carbon-carbon double bond.
• Hydrocarbons that have a double bond between the carbon atoms are called Alkenes, and the carbons which have triple bonds between themselves are called Alkynes.
• The general formula of alkenes are ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}}$, and of alkyne are ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}-2}$
• Examples of alkenes are: Propene$\left({\mathrm{C}}_3{\mathrm{H}}_6\right){\mathrm{CH}}_2=\mathrm{CH}-{\mathrm{CH}}_3$, Ethene$\left({\mathrm{C}}_2{\mathrm{H}}_4\right){\mathrm{CH}}_2={\mathrm{CH}}_2$.
• Examples of alkynes are: Ethyne$\left({\mathrm{C}}_2{\mathrm{H}}_2\right)\mathrm{CH}\equiv \mathrm{CH}$, Propyne$\left({\mathrm{C}}_3{\mathrm{H}}_4\right)\mathrm{CH}\equiv \mathrm{C}-{\mathrm{CH}}_3$.

Test used to differentiate between saturated and unsaturated hydrocarbons:-

• Bromine water test is used to distinguish between saturated and unsaturated hydrocarbons.
• Bromine water is nothing but bromine in water.
• When an unsaturated hydrocarbon is reacted with bromine water, the colour of bromine water changes from red-brown to colourless.
• For example:

• But when a saturated hydrocarbon is treated with bromine water, it does not discharge the colour of bromine water.
• For example: When ethane is treated with bromine water, no reaction takes place and the bromine water does not get decolourised.

Therefore, by the bromine water test we can differentiate between saturated and unsaturated hydrocarbons.