2–Pentanone and 3–methylbutan-2-one are

2–Pentanone and 3–methylbutan-2-one, as shown in the image, are functional isomers.

• Both compounds have the same molecular formula: C₅H₁₀O
• Both are ketones (carbonyl group at the 2-position)
• Their carbon skeletons differ:
  • 2-pentanone: a straight-chain ketone
  • 3-methylbutan-2-one: a branched-chain ketone

Functional isomers are compounds with the same molecular formula but different connectivity or arrangement of atoms. In this case, they also belong to a special class called chain isomers (a subclass of structural isomerism), because both are ketones but differ in the arrangement of carbon atoms.