Structural isomer
[10] For example, replacing one of the 12 hydrogen atoms –H by a hydroxyl group –OH on the n-pentane parent molecule can give any of three different position isomers: Another example of regioisomers are α-linolenic and γ-linolenic acids, both octadecatrienoic acids, each of which has three double bonds, but on different positions along the chain.Thus 1-propanol and 2-propanol have relatively similar infrared spectra because of the hydroxyl group, which are fairly different from that of methyl ethyl ether.In the second case, two molecules with the same number of atoms of each isotope but distinct bonding schemes are said to be structural isotopomers.[12] Thus, for example, all four hydrogen atoms of methane are structurally equivalent, because any permutation of them will preserve all the bonds of the molecule.That is the case also for the hydrogen atoms in cyclopentane, allene, 2-butyne, hexamethylenetetramine, prismane, cubane, dodecahedrane, etc.The eight hydrogens of propane C3H8 are partitioned into two structural equivalence classes (the six on the methyl groups, and the two on the central carbon); therefore there are only two positional isomers of propanol (1-propanol and 2-propanol).Once a substitution is made on a parent molecule, its structural symmetry is usually reduced, meaning that atoms that were formerly equivalent may no longer be so.Thus a second substitution of hydrogen by chlorine can yield three positional isomers: 1,2- or ortho-, 1,3- or meta-, and 1,4- or para-dichlorobenzene.Xylenol, which is benzene with one hydroxyl substituent and two methyl substituents, has a total of 6 isomers: Enumerating or counting structural isomers in general is a difficult problem, since one must take into account several bond types (including delocalized ones), cyclic structures, and structures that cannot possibly be realized due to valence or geometric constraints, and non-separable tautomers.