A sweet-smelling organic liquid (A) consisting of C,H and O boils at `78^@C`. (A) on heating with conc. `H_2SO_4`, formula of (B) is `CH_2.` (B) decolourises brominewater and alkaline `KMnO_4` solution. Again, each mole of (B) absorbs one mole of `H_2` at high temperature in the presence of Ni catalyst. Identify A and B .

Explanation

Since the gaseous substance (B) decolourises bromine water and alkaline `KMnO_(4)` solution and consumes one mole of `H_(2)` per mole in catalytic hydroenation, it must be an alkene. Since the empirical formula of (B) is `CH_(2)`, it may be the gaseous alkene ethylene, `(CH_(2))_(2) or C_(2)H_(4)`. Ethylene may be obtained when the sweet-smelling organic liquid ethyl alcohol `(CH_(3)CH_(2)OH)` having boiling point `78^(@)C` is dehydrated by heating with concentrated `H_(2)SO_(4)`. Therefore, the organic liquid (A) is ethyl alcohol.
Reaction: `underset("Ethyl alcohol (A)")(CH_(93)CH_(2)OH) underset("Heat")overset(conc. H_(2)SO_(2) ("excess"))rarr underset("Ethylene(B)")(CH_(2) = CH_(2) + H_(2)O)`
`underset("Ethylene(B)")(CH_(2) = CH_(2)) + underset("(red)")(Br_(2)) overset(H_(2)O)rarr underset("2-bromoethanol (colourless)")(BrCH_(2)CH_(2)OH + HBr)`
`underset("Ethylene(B)")(CH_(2) = CH_(2)) + H_(2)O + [O] underset(("reddish -violet"))overset("Alkaline " KMnO_(4))rarr underset(underset(("colourless"))("Ethylene glycol"))(HOCH_(2)CH_(2)OH)`
`underset("Ethylene(B)")(CH_(2) = CH_(2)) + H_(2) underset(Delta)overset(Ni)rarr underset("Ethane")(CH_(3) - CH_(3))`