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Inchemistry, astructural isomer (orconstitutional isomer in theIUPAC nomenclature[1]) of acompound is a compound that contains the same number and type of atoms, but with a different connectivity (i.e. arrangement ofbonds) between them.[2][3] The termmetamer was formerly used for the same concept.[4]
For example,butanolH3C−(CH2)3−OH,methyl propyl etherH3C−(CH2)2−O−CH3, anddiethyl ether(H3CCH2−)2O have the samemolecular formulaC4H10O but are three distinct structural isomers.
The concept applies also to polyatomic ions with the same total charge. A classical example is thecyanate ionO=C=N− and thefulminate ionC−≡N+−O−. It is also extended to ionic compounds, so that (for example)ammonium cyanate[NH4]+[O=C=N]− andurea(H2N−)2C=O are considered structural isomers,[4] and so aremethylammonium formate[H3C−NH3]+[HCO2]− andammonium acetate[NH4]+[H3C−CO2]−.
Structural isomerism is the most radical type ofisomerism. It is opposed tostereoisomerism, in which the atoms and bonding scheme are the same, but only the relative spatial arrangement of the atoms is different.[5][6] Examples of the latter are theenantiomers, whose molecules are mirror images of each other, and thecis andtrans versions of2-butene.
Among the structural isomers, one can distinguish several classes includingskeletal isomers,positional isomers (orregioisomers),functional isomers,tautomers,[7] andstructural isotopomers.[8]
Askeletal isomer of a compound is a structural isomer that differs from it in the atoms and bonds that are considered to comprise the "skeleton" of the molecule. Fororganic compounds, such asalkanes, that usually means the carbon atoms and the bonds between them.
For example, there are three skeletal isomers ofpentane:n-pentane (often called simply "pentane"),isopentane (2-methylbutane) andneopentane (dimethylpropane).[9]
 |  |  |
| n-Pentane | Isopentane | Neopentane |
If the skeleton isacyclic, as in the above example, one may use the termchain isomerism.
Position isomers (alsopositional isomers orregioisomers) are structural isomers that can be viewed as differing only on the position of afunctional group,substituent, or some other feature on the same "parent" structure.[10]
For example, replacing one of the 12 hydrogen atoms –H by ahydroxyl group –OH on then-pentane parent molecule can give any of three different position isomers:
 |  |  |
| Pentan-1-ol | Pentan-2-ol | Pentan-3-ol |
Another example of regioisomers areα-linolenic andγ-linolenic acids, bothoctadecatrienoic acids, each of which has three double bonds, but on different positions along the chain.
Functional isomers are structural isomers which have differentfunctional groups, resulting in significantly different chemical and physical properties.[11]
An example is the pairpropanal H3C–CH2–C(=O)-H andacetone H3C–C(=O)–CH3: the first has a –C(=O)H functional group, which makes it analdehyde, whereas the second has a C–C(=O)–C group, that makes it aketone.
Another example is the pairethanol H3C–CH2–OH (analcohol) anddimethyl ether H3C–O–CH2H (anether). In contrast,1-propanol and2-propanol are structural isomers, but not functional isomers, since they have the same significant functional group (thehydroxyl –OH) and are both alcohols.
Besides the different chemistry, functional isomers typically have very differentinfrared spectra. The infrared spectrum is largely determined by the vibration modes of the molecule, and functional groups like hydroxyl and esters have very different vibration modes. 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.[citation needed]
In chemistry, one usually ignores distinctions betweenisotopes of the same element. However, in some situations (for instance inRaman,NMR, ormicrowave spectroscopy) one may treat different isotopes of the same element as different elements. In the second case, two molecules with the same number of atoms of each isotope but distinct bonding schemes are said to bestructural isotopomers.
Thus, for example,ethene would have no structural isomers under the first interpretation; but replacing two of the hydrogen atoms (1H) bydeuterium atoms (2H) may yield any of two structural isotopomers (1,1-dideuteroethene and 1,2-dideuteroethene), if both carbon atoms are the same isotope. If, in addition, the two carbons are different isotopes (say,12C and13C), there would be three distinct structural isotopomers, since 1-13C-1,1-dideuteroethene would be different from 1-13C-2,2-dideuteroethene. And, in both cases, the 1,2-dideutero structural isotopomer would occur as two stereoisotopomers,cis andtrans.
Two molecules (including polyatomic ions) A and Bhave the same structure if each atom of A can be paired with an atom of B of the same element, in a one-to-one way, so that for every bond in A there is a bond in B, of the same type, between corresponding atoms; and vice versa.[3] This requirement applies also to complex bonds that involve three or more atoms, such as thedelocalized bonding in the benzene molecule and other aromatic compounds.
Depending on the context, one may require that each atom be paired with an atom of the same isotope, not just of the same element.
Two molecules then can be said to be structural isomers (or, if isotopes matter, structural isotopomers) if they have the same molecular formula but do not have the same structure.
Structural symmetry of a molecule can be defined mathematically as apermutation of the atoms that exchanges at least two atoms but does not change the molecule's structure. Two atoms then can be said to be structurallyequivalent if there is a structural symmetry that takes one to the other.[12]
Thus, for example, all four hydrogen atoms ofmethane are structurally equivalent, because any permutation of them will preserve all the bonds of the molecule.
Likewise, all six hydrogens ofethane (C
2H
6) are structurally equivalent to each other, as are the two carbons; because any hydrogen can be switched with any other, either by a permutation that swaps just those two atoms, or by a permutation that swaps the two carbons and each hydrogen in one methyl group with a different hydrogen on the other methyl. Either operation preserves the structure of the molecule. That is the case also for the hydrogen atoms incyclopentane,allene,2-butyne,hexamethylenetetramine,prismane,cubane,dodecahedrane, etc.
On the other hand, the hydrogen atoms ofpropane are not all structurally equivalent. The six hydrogens attached to the first and third carbons are equivalent, as in ethane, and the two attached to the middle carbon are equivalent to each other; but there is no equivalence between these twoequivalence classes.
Structural equivalences between atoms of a parent molecule reduce the number of positional isomers that can be obtained by replacing those atoms for a different element or group. Thus, for example, the structural equivalence between the six hydrogens ofethaneC
2H
6 means that there is just one structural isomer ofethanolC
2H
5OH, not 6. The eight hydrogens ofpropaneC
3H
8 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 and2-propanol). Likewise there are only two positional isomers ofbutanol, and three ofpentanol orhexanol.
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 substitution of two or more equivalent atoms by the same element may generate more than one positional isomer.
The classical example is the derivatives ofbenzene. Its six hydrogens are all structurally equivalent, and so are the six carbons; because the structure is not changed if the atoms are permuted in ways that correspond to flipping the molecule over or rotating it by multiples of 60 degrees. Therefore, replacing any hydrogen by chlorine yields only onechlorobenzene. However, with that replacement, the atom permutations that moved that hydrogen are no longer valid. Only one permutation remains, that corresponds to flipping the molecule over while keeping the chlorine fixed. The five remaining hydrogens then fall into three different equivalence classes: the one opposite to the chlorine is a class by itself (called thepara position), the two closest to the chlorine form another class (ortho), and the remaining two are the third class (meta). Thus a second substitution of hydrogen by chlorine can yield three positional isomers:1,2- orortho-,1,3- ormeta-, and1,4- orpara-dichlorobenzene.
 |  |  |
| ortho-Dichlorobenzene | meta-Dichlorobenzene | para-Dichlorobenzene |
| 1,2-Dichlorobenzene | 1,3-Dichlorobenzene | 1,4-Dichlorobenzene |
For the same reason, there is only onephenol (hydroxybenzene), but threebenzenediols; and onetoluene (methylbenzene), but threetoluols, and threexylenes.
On the other hand, the second replacement (by the same substituent) may preserve or even increase the symmetry of the molecule, and thus may preserve or reduce the number of equivalence classes for the next replacement. Thus, the four remaining hydrogens inmeta-dichlorobenzene still fall into three classes, while those ofortho- fall into two, and those ofpara- are all equivalent again. Still, some of these 3 + 2 + 1 = 6 substitutions end up yielding the same structure, so there are only three structurally distincttrichlorobenzenes:1,2,3-,1,2,4-, and1,3,5-.
 |  |  |
| 1,2,3-Trichlorobenzene | 1,2,4-Trichlorobenzene | 1,3,5-Trichlorobenzene |
If the substituents at each step are different, there will usually be more structural isomers.Xylenol, which is benzene with one hydroxyl substituent and two methyl substituents, has a total of 6 isomers:
 |  |  |
| 2,3-Xylenol | 2,4-Xylenol | 2,5-Xylenol |
 |  |  |
| 2,6-Xylenol | 3,4-Xylenol | 3,5-Xylenol |
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.
For example, there are nine structural isomers with molecular formulaC3H6O having different bond connectivities. Seven of them are air-stable at room temperature, and these are given in the table below.
| Name | Molecular structure | Melting point (°C) | Boiling point (°C) | Comment |
|---|---|---|---|---|
| Allyl alcohol |  | –129 | 97 | |
| Cyclopropanol |  | 101–102 | ||
| Propionaldehyde |  | –81 | 48 | Tautomeric with prop-1-en-1-ol, which has bothcis andtrans stereoisomeric forms |
| Acetone |  | –94.9 | 56.53 | Tautomeric with propen-2-ol |
| Oxetane |  | –97 | 48 | |
| Propylene oxide |  | –112 | 34 | Has twoenantiomeric forms |
| Methyl vinyl ether |  | –122 | 6 |
Two structural isomers are theenoltautomers of thecarbonyl isomers (propionaldehyde and acetone), but these are not stable.[13]