Question

Give a chemical test to distinguish between saturated and unsaturated compounds.

Answer 1

Saturated compounds are the ones that only contain single bonds whereas unsaturated compounds are the ones that contain multiple bonds.

So, in order to distinguish between them, we can use Bromine ($\mathrm{Br}$) water test. When ${\mathrm{Br}}_2$ water is added to a saturated compound no reaction occurs since there is no double or triple bond present for ${\mathrm{Br}}_2$ to reaction and the reddish-brown colour of ${\mathrm{Br}}_2$ remains intact in the solution. When an unsaturated compound reacts with ${\mathrm{Br}}_2$ water the following reactions take place:

$\underset{\underset{\left(\mathrm{colourless}\right)}{\mathrm{Ethene}}}{{\mathrm{CH}}_2={\mathrm{CH}}_2\left(\mathrm{g}\right)}+\underset{\underset{(\mathrm{Reddish}\mathrm{brown})}{\mathrm{Bromine}}}{{\mathrm{Br}}_2\left(\mathrm{l}\right)}\to \underset{\underset{\left(\mathrm{colourless}\right)}{1,2-\mathrm{dibromoethane}}}{{\mathrm{BrCH}}_2-{\mathrm{CH}}_2\mathrm{Br}\left(\mathrm{aq}\right)}$

$\underset{\underset{\left(\mathrm{colourless}\right)}{\mathrm{Ethyne}}}{\mathrm{CH}\equiv \mathrm{CH}\left(\mathrm{g}\right)}+\underset{\underset{(\mathrm{Reddish}\mathrm{brown})}{\mathrm{Bromine}}}{{\mathrm{Br}}_2\left(\mathrm{l}\right)}\to \underset{1,2-\mathrm{Dibromoethene}}{\mathrm{BrCH}=\mathrm{CHBr}\left(\mathrm{aq}\right)}\to \underset{\underset{\left(\mathrm{colourless}\right)}{1,1,2,2-\mathrm{Tetrabromoethane}}}{{\mathrm{Br}}_2\mathrm{CH}-{\mathrm{CHBr}}_2\left(\mathrm{aq}\right)}$

So, in both the cases ${\mathrm{Br}}_2$ water is decolourises and by using this method we can distinguish between saturated and unsaturated compounds.