Here's why:
* Constitutional isomers are molecules with the same molecular formula but different connectivity of atoms.
* Alkanes are hydrocarbons with only single bonds.
* Methane (CH4), ethane (C2H6), and propane (C3H8) have only one possible arrangement of atoms, so they cannot have constitutional isomers.
* Butane (C4H10) can exist as two isomers:
* n-butane: A straight chain of four carbon atoms.
* Isobutane: A branched chain with three carbons in a row and one carbon attached to the middle carbon.
Therefore, butane is the simplest alkane with the ability to form constitutional isomers.
