An organic compound which decolorizes bromine water is likely to be?

Analysis

The question asks us to identify an organic compound that decolorizes bromine water. This is a classic test for unsaturation in organic chemistry.

Concept Explanation

• Bromine Water Test: Bromine water ($Br_2$ in water) is reddish-brown in color. When it reacts with an unsaturated hydrocarbon (a compound containing a carbon-carbon double bond $C=C$ or triple bond $C \equiv C$), the bromine adds across the bond, forming a colorless dibromo-compound.
• Unsaturated Hydrocarbons: These are alkenes ($C_nH_{2n}$) and alkynes ($C_nH_{2n-2}$).
• Saturated Hydrocarbons: These are alkanes ($C_nH_{2n+2}$). Alkanes contain only single bonds and do not react with bromine water under normal conditions; therefore, they do not decolorize it.

Step-by-Step Solution

1. Analyze the options based on the general formula for alkanes ($C_nH_{2n+2}$): • A. $C_3H_8$: $n=3$, $2(3)+2 = 8$. This fits the formula $C_nH_{2n+2}$. It is an alkane (Propane). • B. $C_2H_6$: $n=2$, $2(2)+2 = 6$. This fits the formula $C_nH_{2n+2}$. It is an alkane (Ethane). • C. $C_4H_{10}$: $n=4$, $2(4)+2 = 10$. This fits the formula $C_nH_{2n+2}$. It is an alkane (Butane). • D. $C_2H_4$: $n=2$, $2(2) = 4$. This fits the formula $C_nH_{2n}$. It is an alkene (Ethene).

2. Evaluate: Since options A, B, and C are alkanes, they are saturated and will not react with bromine water. Option D ($C_2H_4$) is an alkene, which is unsaturated. It will undergo an addition reaction with bromine to form 1,2-dibromoethane, causing the reddish-brown color of the bromine water to disappear.

Conclusion

The compound that decolorizes bromine water must be an unsaturated hydrocarbon. Among the choices provided, only $C_2H_4$ is an alkene.

Exam Tip: Always check the ratio of Carbon to Hydrogen. If the number of Hydrogens is exactly double the number of Carbons ($C_nH_{2n}$), it is an alkene and will decolorize bromine water. If the Hydrogens are $2n+2$, it is an alkane and will not react.