Question

Which statement(s) is/are correct?
(1) Benzene decolourises bromine water
(2) Benzene decolourises alkaline permanganate solution.
(3) Acetylene decolourises alkaline permanganate solution.
(4) Acetylene decolourises bromine water

(A) $3$ and $4$
(B) $2$and $3$
(C) $1$and $4$
(D) $1$and $2$

Answer 1

Bromine water test is given by the compounds having unsaturation in them such as alkyne and alkenes. The bromine molecule adds across the double bond to form an alkane.
Alkaline permanganate solution is also called Baeyer’s reagent It is used as a test for unsaturation for testing the presence of double-bonded or triple bonded compounds.

Complete step by step answer:
We know that bromine water is used to test the presence of unsaturation in a compound. Bromine water adds across double bonds in the following manner
$C{H_2} = C{H_2} + B{r_2} \to C{H_2}Br - C{H_2}Br$
Benzene does not have alternate double bonds and single bonds. It has a pi bond that is delocalized over the entire ring formed from the interaction of p orbital with the benzene carbon atoms. Benzene doesn’t undergo the bromine water test as it destroys the delocalization of the system and the aromaticity of the system. Therefore this statement is wrong.

For the decolorization of alkaline permanganate solution the aromatic compound having a double or triple bond in the side chain decolorizes the alkaline permanganate solution. Benzene does not decolorize the alkaline permanganate solution due to the delocalization of pi electrons. Also, it is resonance stabilized and doesn’t react with an alkaline permanganate solution. Therefore this statement is wrong.

Acetylene is known as ethyne. It is an unsaturated compound and it decolorizes bromine water as triple bonds are present in it which is a condition for additional reaction of bromine water. Therefore this statement is correct.

Acetylene being unsaturated decolorizes alkaline permanganate solution as well as it is a linear molecule with a carbon-carbon triple bond. Therefore this statement holds correct.

So, the correct answer is Option A .

Note: The double bonds in benzene are not stationary and delocalized and this very fact prevents it from additional reactions. For most of the organic compounds which are not benzene derivatives, bromine water and permanganate solution acts as differentiating reagents. Alkanes and other such saturated compounds also don’t give bromine water reaction due to the presence of single bonds in it.