Question

Which alkene on heating with alkaline KMnO$_4$ solution gives acetone and a gas, which turns lime water milky:
(a) 2-Methyl-2-butene
(b) Isobutylene
(c) 1-Butene
(d) 2-Butene

Answer 1

Hint: In this reaction, alkaline potassium permanganate is used. As we know it is used for the displacement of carbon-carbon double bonds. It is mainly used to identify whether hydrocarbon is alkane, or alkene. So, we can identify the alkene in this reaction.

Complete step by step solution:
> First, let us know about the alkenes. These are categorized under hydrocarbons having at least one carbon – carbon double bond.
> Now, if we talk about the given options. The mentioned options belong to the category of alkenes.
> As we know, when an alkene having two hydrogens at one end of the bond, reacts with alkaline potassium permanganate; the product obtained will be a ketone, carbon-dioxide, and water.
> So, from all the given options, when 2-methyl-2-butene, 1-butene, and 2-butene does not show reaction with alkaline potassium permanganate.
> They represent the dihydroxylation of alkenes with cold, dilute potassium permanganate to give diols.
> Thus, we can say that isobutylene reacts with alkaline KMnO$_4$. The chemical reaction is
(CH$_3$)$_2$C=CH$_2$ $\overset{alkalineKMnO_4}{\rightarrow}$ (CH$_3$)$_2$C=O + CO$_2$
So, in the end we can conclude that isobutylene alkene reacts with a solution that gives acetone, and a gas carbon-dioxide which turns lime water milky.
Hence, the correct option is (B).

Note: Don’t get confused while writing the chemical reaction. We will not consider acidified potassium permanganate solution, as it reacts too rapidly to break the carbon-carbon double bond, it is rarely used in organic chemistry. So, we have used the alkaline potassium permanganate solution for the reaction with alkene.