Question: An organic liquid A, containing C,H and O, has a pleasant odour with the boiling point of \({78^ \ci...

Question

An organic liquid A, containing C,H and O, has a pleasant odour with the boiling point of ${78^ \circ }C$ . Upon boiling A with conc. ${H_2}S{O_4}$ , a colourless gas B is produced which decolourise bromine water and alkaline $KMn{O_4}$ . The organic liquid A is:
(A) $C{H_3}OH$
(B) ${C_2}{H_5}CHO$
(C) ${C_2}{H_6}$
(D) ${C_2}{H_5}OH$

Answer 1

Solution

The compounds that have alcohol group (-OH) may have pleasant odour. All the compounds that have unsaturation in their structure decolourised both bromine water and alkaline potassium permanganate solution.

Complete step by step solution:
We will see all the characteristics of the compound given in the question and find which compound gives that.
- It is given that the organic liquid having C, H and O atoms has a pleasant odour. So, the functional group present in the compound can be alcohol group (-OH).
- It is given that the compound gives a colourless gas upon boiling it with concentrated sulphuric acid. Out of all the given compounds, the one which gives a gas upon an easy reaction with concentrated sulphuric acid is ethanol. The reaction is,
${C_2}{H_5}OH\xrightarrow[{ - {H_2}O}]{{{H_2}S{O_4}}}{H_2}C = C{H_2}$
Thus, we can see that ethene is produced by the dehydration reaction of ethanol. We know that concentrated sulphuric acid is a strong dehydrating agent. Ethene is a colourless gas.
- More importantly ethene decolourise the bromine water as well as alkaline $KMn{O_4}$ solution. It gives this reaction because unsaturation is present in ethene as a C-C double bond is there. So, any compound that has unsaturation gives a decolourization reaction with bromine water and potassium permanganate.
- The boiling point of ethanol is also ${78^ \circ }C$ .
Thus, we can say that the compound A is ethanol ( ${C_2}{H_5}OH$ ).

So, the correct answer is (D).

Note: Any alcohol when allowed to react with concentrated sulphuric acid will lose a molecule of water and form a C-C double bond. This reaction involves loss of a water molecule from the substrate so it is also called dehydration reaction.