constitutional (structural) isomers

Meaning: Constitutional isomers share a molecular formula but differ in which atoms are bonded to which. Common types are chain isomers (butane and isobutane, both C₄H₁₀), position isomers (1-propanol and 2-propanol), and functional group isomers (ethanol and dimethyl ether, both C₂H₆O). Because the bonds differ, polarity, intermolecular forces and reactivity differ. Ethanol hydrogen bonds and boils well above dimethyl ether. Among alkane isomers, more branching means less surface contact and usually a lower boiling point.

On the MCAT: You may be shown two structures or formulas and asked to classify the pair as identical, constitutional isomers, or stereoisomers. A typical task is counting isomers, for example C₄H₁₀ has two and C₅H₁₂ has three. Another is ranking boiling points or solubility of isomers by their functional groups. Compare molecular formulas first, then trace the connectivity. Keto and enol tautomers are constitutional isomers that interconvert quickly.

Common confusion: Students call conformers (rotation about a single bond) or enantiomers and diastereomers "structural" differences. These all have the same connectivity and differ only in spatial arrangement, so they are not constitutional isomers. Also, molecules with different formulas are never isomers, however similar they look.