Structural isomer

Structural isomerism, also known as constitutional isomerism in the terminology of the International Union of Pure and Applied Chemistry (IUPAC), refers to a fundamental relationship between chemical compounds that share the same molecular formula but differ in the connectivity of their atoms. This variation in bonding arrangements results in compounds that may display markedly different physical, chemical, and spectroscopic properties. The concept applies principally to covalent molecular species but can also be extended to polyatomic ions and ionic compounds, where ions of identical overall composition may adopt alternative bonding frameworks.
Structural isomerism was historically described using the term metamerism, although modern nomenclature prefers the more precise classifications established by IUPAC. Structural isomerism represents the most substantial form of isomeric variation, in contrast to stereoisomerism, where the atomic connectivity remains constant and only the spatial arrangement differs. Among structural isomers, several important subclasses are recognised, including skeletal isomers, positional or regioisomers, functional isomers, tautomers, and structural isotopomers.

Background and conceptual framework

Compounds classified as structural isomers contain identical numbers and types of atoms, yet the framework of chemical bonds between these atoms differs. A familiar example is provided by organic compounds sharing the molecular formula C₄H₁₀O, such as butan-1-ol, methyl propyl ether, and diethyl ether. Despite their identical composition, these species exhibit different functional groups and bonding patterns.
Structural isomerism is especially significant in organic chemistry, where carbon’s ability to form diverse bonding patterns allows a high degree of structural variation. The phenomenon also extends to inorganic species and ions. For example, the cyanate ion (OCN⁻) and the fulminate ion (CNO⁻) have the same elemental composition but distinct bonding orders. Similarly, ionic compounds such as ammonium cyanate and urea, both containing the same number of each atom, differ in connectivity and thus represent a classical case of structural isomerism in ionic solids.
The distinction between structural and stereoisomerism is foundational. Stereoisomers such as enantiomers and geometric (cis–trans) isomers possess identical atomic connectivity but differ in spatial arrangement. Structural isomers, by contrast, vary specifically in how atoms are linked.

Skeletal isomerism

Skeletal isomers are compounds whose carbon framework — the molecular “skeleton” — differs in arrangement. This form of isomerism is particularly important in alkanes and other hydrocarbons, where chains may be straight or branched.
A widely cited example involves the isomers of pentane (C₅H₁₂):

• n-Pentane, with a straight-chain structure.
• Isopentane (2-methylbutane), featuring a single branch.
• Neopentane (2,2-dimethylpropane), with a more compact, heavily branched structure.

These variations in carbon connectivity significantly influence melting points, boiling points, densities, and combustion characteristics. Where cyclic structures are involved, the term chain isomerism may be used to describe analogues differing in ring connectivity or chain–ring relationships.

Positional isomerism (regioisomerism)

Positional isomers, or regioisomers, differ in the location of a substituent, functional group, or multiple bond on a common carbon skeleton. These isomers are especially relevant when substituting atoms on symmetrical parent structures.
For example, monohydroxylated derivatives of pentane exist in three positional forms depending on the carbon atom bearing the hydroxyl group. Likewise, unsaturated fatty acids such as linolenic acid and its structural analogue exhibit differing positions of double bonds, even though both possess the same number of carbon–carbon unsaturations. These positional changes can significantly influence reactivity, stability, and biochemical roles.
Positional isomerism is constrained by structural symmetry. A molecule with many symmetrically equivalent positions, such as ethane, affords fewer positional isomers upon substitution than an asymmetrical molecule such as propane.

Functional isomerism

Functional isomers possess different functional groups despite sharing identical molecular formulae. These differences often result in pronounced changes in reactivity, molecular polarity, and characteristic spectroscopic features.
Typical examples include:

• Propanal (an aldehyde) versus acetone (a ketone): although both are C₃H₆O compounds, the functional group arrangement differs, yielding highly distinct chemical properties.
• Ethanol (an alcohol) versus dimethyl ether (an ether): again, the same atomic composition gives rise to different functional group classifications.

Functional isomers frequently exhibit very different infrared spectra because vibrational modes characteristic of each functional group produce distinct absorption patterns. By contrast, structural isomers that do not differ in functional group, such as propan-1-ol and propan-2-ol, show more similar spectroscopic profiles.

Structural isotopomers

Under ordinary circumstances, isotopes of an element are treated equivalently in structural descriptions. However, in spectroscopic investigations such as Raman, nuclear magnetic resonance (NMR), or microwave spectroscopy, isotopic substitution can be significant. When atoms of different isotopes occupy distinct positions in otherwise similar molecules, the resulting species are termed structural isotopomers.
For example, ethene has no structural isomers when isotopes are ignored, but substitution of two protium (^1H) atoms with deuterium (^2H) creates distinct isotopomers such as 1,1-dideuteroethene and 1,2-dideuteroethene. If carbon isotopes are also varied, additional isotopomers arise. Moreover, some isotopomers may also occur as stereoisomers, including cis and trans configurations.

Structural equivalence and molecular symmetry

Structural equivalence arises when atoms in a molecule may be interchanged through symmetry operations without altering molecular structure. These symmetry operations include permutations corresponding to rotations, reflections, or exchanges that leave bonding patterns invariant.
Methane illustrates complete hydrogen equivalence: all four hydrogen atoms are symmetrically interchangeable because any permutation of them maintains its tetrahedral structure. Ethane similarly displays equivalence among its six hydrogens when considering rotation around the carbon–carbon bond. By contrast, propane demonstrates two inequivalent sets of hydrogen atoms due to the asymmetric environment of its central carbon, producing distinct classes.
Structural equivalence strongly influences the count of positional isomers. Ethane’s six equivalent hydrogens allow only one ethanolic isomer when substitution occurs; propane’s division into two equivalence classes permits two distinct propanolic isomers.

Symmetry breaking and substituted derivatives

Substitution of one atom within a symmetric parent molecule typically reduces molecular symmetry and leads to previously equivalent atoms becoming non-equivalent. This effect is pronounced in aromatic chemistry. Benzene’s six hydrogens are structurally equivalent, meaning that monosubstitution yields only a single chlorobenzene species. Once substituted, however, the remaining hydrogens divide into three distinct equivalence classes: ortho, meta, and para positions.
A second identical substitution therefore produces three dichlorobenzene isomers — 1,2-, 1,3-, and 1,4-dichlorobenzene. The same structural logic explains why phenol exists as a single compound, whereas dihydroxybenzenes occur in ortho, meta, and para forms. Analogous principles govern the substitution patterns of toluene, giving rise to multiple isomeric toluols and xylenes.
In some instances, secondary substitution may restore or enhance symmetry, though this is less common. Overall, the interplay between symmetry and structural isomerism forms a crucial basis for predicting the number and type of possible isomers in organic and inorganic chemistry.