butane and isobutane are structural isomers. Isobutane is produced by the isomerization reaction of butane.
CH3CH2CH2CH3--(t°,AlCl3)-›CH3CH(CH3)-CH3.
They have the same chemical formula C4H10.But structural formulas are different
Butane CH3CH2CH2CH3
Isobutane CH3CH(CH3)CH3
Isobutane has methyl radical.