Definitions: Constitutional Isomers
Constitutional isomers are compounds that have the same molecular formula and different connectivity. To determine whether two molecules are constitutional isomers, just count the number of each atom in both molecules and see how the atoms are arranged. If both molecules have the same count for all of the different atoms, and the atoms are arranged in different ways (their connectivity is different), the molecules will be constitutional isomers.
(Recall that connectivity means how the atoms are attached to one another. For example, an ether has a connectivity of C-O-C, and an alcohol has a connectivity of C-O-H.)
For example, consider the following molecules.
In the previous section, it was determined that these compounds were isomers. To be more precise, these are constitutional isomers
because the connectivity of the atoms is different.
For more examples, reconsider the third and fourth examples
in the previous section.