Two isomers 'A' and 'B' with molecular formula ${\mathrm{C}}_4{\mathrm{H}}_8$ give different products on oxidation with ${\mathrm{KMnO}}_4$ in acidic medium. Isomer ‘A’ on reaction with ${\mathrm{KMnO}}_4/{\mathrm{H}}^{+}$results in effervescence of a gas and gives ketone. The compound ‘A’ is