Final answer:
Butanal is the correct structural isomer of butan-2-one, as it has the same molecular formula C4H8O but features a different structure, with the carbonyl group at the end of the chain rather than the second position.
Step-by-step explanation:
The structural isomer of butan-2-one has to have the same molecular formula, which is C4H8O, but a different arrangement of atoms. Considering the choices given, butanal is the correct answer. Butanal has the molecular formula C4H8O as well, and like butan-2-one, it is a four-carbon compound. However, in butanal, the carbonyl group is at the end of the carbon chain (making it an aldehyde), whereas in butan-2-one, the carbonyl group is positioned at the second carbon (making it a ketone). The compound 2-methylpropane is mentioned to illustrate the concept of structural isomers. 2-methylpropane and n-butane are also structural isomers with the same molecular formula, C4H10, but differing structures.