Question

A compound X on heating with excess conc. Sulphuric acid at 443 K gives an unsaturated compound Y. X also reacts with sodium metal to evolve a colourless gas Z. Identify X, Y and Z. Write the equation of the chemical reaction of formation of Y and also write the role of sulphuric acid in the reaction.

Answer 1

Hint: The dehydration of alcohol occurs in the presence of the concentrated mineral acids. Reaction of metals with alcohol occurs to release the lightest gas. The unsaturated hydrocarbon is an alkene.

Complete answer:
The given compound X is ethanol. When ethanol is reacted with the concentrated sulphuric acid then it forms an unsaturated hydrocarbon that is ethene. The above reaction is also known as dehydration of alcohol. Water molecules leave from the alcohol and a double bond is formed. The reaction occurs as:
\[{{\text{C}}_2}{{\text{H}}_5}{\text{OH}}\xrightarrow{{{{\text{H}}_2}{\text{S}}{{\text{O}}_4}\left( {{\text{Conc}}{\text{.}}} \right)}}{{\text{C}}_2}{{\text{H}}_4} + {{\text{H}}_2}{\text{O}}\]
Hence the unsaturated compound having double bond formed is ethene. The molecular formula of ethene is \[{{\text{H}}_2}{\text{C}} = {\text{C}}{{\text{H}}_2}\] .
The active metals such as sodium and potassium reacts with the alcohol to dorm the respective salts and the colourless gas that is hydrogen is evolved during the process. The reaction occurs as:
\[{{\text{C}}_2}{{\text{H}}_5}{\text{OH}} + {\text{Na}} \to {{\text{C}}_2}{{\text{H}}_5}{\text{ONa}} + {{\text{H}}_2}\]
The sulphuric acid acts as a very good dehydrating agent. Being an acid it releases the hydrogen ion. As we know oxygen has a lone pair of electrons and it grabs the hydrogen ion released by the acid. So this makes the release of water molecules easy from the alcohol and hence the ethene is formed.

Note:
Ethanol is volatile in nature and is extensively used in organic synthesis and as a solvent too. It is used as an antifreeze mixture with water. It decreases the freezing point of water so that the engine of the automobile does not block away in cooler temperatures. The liberation of hydrogen gas is a common practice when we react metals with the acid. Hence the liberation of the hydrogen gas when we react with alcohol with metal indicates that alcohol is acidic in nature.