Question

Which of the following aliphatic compounds will discharge red colour of bromine
A. ${{C}_{2}}{{H}_{4}}$
B. ${{C}_{3}}{{H}_{6}}$
C. ${{C}_{4}}{{H}_{8}}$
D. All of these

Answer 1

Hint: An aliphatic compound in which carbon and hydrogen are attached via single, double and triple bonds. Alkenes are aliphatic compounds that will discharge the red colour of bromine. To solve this problem first we will have to identify the alkenes from the given option.

Complete Step by Step Answer:
Alkanes are saturated hydrocarbons having a single bond whereas alkenes and alkynes are unsaturated hydrocarbons containing a carbon-carbon double bond and triple bond.

Unsaturated compounds discharge the red colour of bromine. But alkanes that are saturated aliphatic compounds do not discharge the red colour of bromine water solution. Double and triple bonds contain pi bonds that are relatively weak as compared to sigma bonds.

Alkenes react with bromine water, a bromine atom becomes bonded to each carbon of the double atom. There is a loss of the original red-brown colour from bromine water and the solution becomes colourless.

 Alkane, alkene and alkyne are the aliphatic hydrocarbons containing carbon and hydrogen atoms. Alkanes have a general formula ${{C}_{2}}{{H}_{2n+2}}$, alkenes have general formula ${{C}_{n}}{{H}_{2n}}$and alkynes have general formula ${{C}_{n}}{{H}_{2n-2}}$. Here n is the number of carbon atoms.

In (A) We have ${{C}_{2}}{{H}_{4}}$(here $n=2$) that is an alkene. Both in (B) and (C) we have ${{C}_{3}}{{H}_{6}}$ and ${{C}_{4}}{{H}_{8}}$both are alkenes (where $n=3$ in case of B and $n=4$ for C). That is all the compounds are alkene and will discharge the red colour of bromine.
Thus, option (D) is correct.

Note: Unsaturated compounds are more reactive than saturated compounds. This is the main principle behind the unsaturation test of alkenes and alkynes. A few drops of liquid bromine are added to the unsaturated compound under a test tube and after shaking, the solution of bromine becomes colourless. This is called the unsaturated test.