constitutional isomer
/ con-stih-TOO-shun-ul /
Take the same set of atoms — say four carbons and ten hydrogens — and you can snap them together in more than one way, like building two different toys from the identical pile of bricks. Constitutional isomers are molecules that share a molecular formula but differ in how the atoms are connected. Same ingredients, different blueprint, genuinely different substances with different boiling points and chemistry.
Constitutional isomers (also called structural isomers) differ in their connectivity: which atom is bonded to which. C4H10 can be butane, a straight chain of four carbons, or isobutane (2-methylpropane), a three-carbon chain with a methyl branch — different boiling points, same formula. The differences come in flavours: chain isomers (straight vs branched skeleton), positional isomers (a functional group in a different spot, like 1-propanol vs 2-propanol), and functional-group isomers (the atoms form an entirely different group, like ethanol the alcohol vs dimethyl ether). What constitutional isomers do not differ in is the spatial arrangement of a fixed connectivity — that is the realm of stereoisomers, which is a separate idea.
Constitutional isomerism is why a single molecular formula explodes into a vast number of possible molecules — and why organic chemistry needs a careful naming system to tell them apart. The count grows fast: there is one C1 alkane, but already 75 different C10 alkanes and millions by C20. Recognizing that a formula hides many isomers is a first big mental shift for the new organic chemist.
Ethanol (CH3CH2OH) and dimethyl ether (CH3OCH3) are both C2H6O but are constitutional isomers: in ethanol the oxygen sits at the end carrying an H, in the ether the oxygen bridges two carbons. One is a drinkable liquid, the other a gas.
Same C2H6O formula, different connectivity — a functional-group constitutional isomer pair.
Do not confuse constitutional isomers (different connectivity) with stereoisomers (same connectivity, different 3D arrangement). They are two distinct branches of isomerism.