Isomerism is the occurrence of distinct chemical species with the same molecular formula but different arrangements of their atoms. Such species are called isomers: they contain the same numbers of atoms of each element, but differ in how those atoms are connected or positioned in space. Isomerism demonstrates that composition alone does not uniquely determine the identity or properties of a molecule. Its two principal categories are constitutional isomerism and stereoisomerism. (openstax.org)
Constitutional isomerism
Constitutional isomerism, also commonly called structural isomerism, occurs when isomers differ in their constitution—the sequence and nature of the bonds connecting their atoms. These differences can involve the molecular skeleton, the position of a substituent, or the type of functional group. (goldbook.iupac.org)
Common descriptive categories include:
- Skeletal or chain isomerism: the atoms form different skeletons. Butane, CH₃CH₂CH₂CH₃, and 2-methylpropane, (CH₃)₃CH, both have the formula C₄H₁₀, but one has an unbranched carbon skeleton and the other a branched one.
- Position isomerism: a substituent or functional group occupies a different position on an otherwise comparable skeleton. Propylamine and isopropylamine, for example, differ in the position of the amino group.
- Functional-group isomerism: the same molecular formula corresponds to different functional groups. Ethanol, CH₃CH₂OH, and dimethyl ether, CH₃OCH₃, both have the formula C₂H₆O, but are an alcohol and an ether, respectively. (openstax.org)
These labels describe particular kinds of constitutional difference rather than fundamentally separate principles. In every case, the decisive distinction is different atomic connectivity. Merely drawing a chain in a different orientation does not create another constitutional isomer. (openstax.org)
Stereoisomerism
Stereoisomerism occurs when species have the same constitution but differ in the three-dimensional arrangement of their atoms. It is therefore necessary to distinguish a molecule’s connectivity from its spatial structure. Stereoisomers can be classified by their mirror-image relationship and by the molecular motions that interconvert them. (publications.iupac.org)
Enantiomers and diastereomers
Enantiomers are stereoisomers that are nonsuperimposable mirror images of one another. Their existence is associated with chirality, or molecular handedness. A common source of chirality in organic chemistry is a tetrahedral carbon atom attached to four different groups. Lactic acid, for example, exists as a pair of enantiomers because its central carbon is attached to H, OH, CH₃, and CO₂H. (openstax.org)
Diastereomers are stereoisomers that are not related as enantiomers. The distinction is relational: a particular stereoisomer may have an enantiomer and also several diastereomers. Diastereomerism includes many differences in the relative arrangement of substituents around double bonds or within cyclic structures. (goldbook.iupac.org)
Cis–trans and E/Z isomerism
Cis–trans isomerism describes differences in the relative positions of groups, especially around double bonds and in rings. In but-2-ene, the two methyl groups can lie on the same side of the carbon–carbon double bond (cis) or on opposite sides (trans). These arrangements are stereoisomeric rather than constitutionally different. (openstax.org)
For suitable double bonds, the more general E/Z nomenclature assigns priorities to the substituents at each end using the Cahn–Ingold–Prelog rules. If the higher-priority groups are on the same side, the configuration is Z; if they are on opposite sides, it is E. Because priority is determined by a formal ranking system, E/Z descriptors are not universally interchangeable with cis/trans. (goldbook.iupac.org)
Conformation and configuration
A conformation is a spatial arrangement obtainable through rotation about formally single bonds; broader usage also includes certain other molecular rearrangements. Alkanes typically occupy several rapidly interconverting conformations. These arrangements need not have the same energy, even though their atomic connectivity remains unchanged. (goldbook.iupac.org)
Conformational differences are distinguished from differences in configuration, such as those between the two enantiomers of a stable tetrahedral stereocentre or between suitable double-bond stereoisomers. The conformational/configurational distinction concerns interconversion, whereas the enantiomer/diastereomer distinction concerns the relationship between spatial structures. These are complementary ways of describing stereoisomerism. (old.iupac.org)
Tautomerism and resonance
Tautomerism is isomerism between readily interconvertible forms called tautomers. A common example is keto–enol tautomerism, in which hydrogen changes its attachment position and the locations of double bonds change. Tautomers are distinct constitutional forms, often present in chemical equilibrium with one another. Tautomerism is not simply rotation or deformation of an unchanged bonding arrangement. (goldbook.iupac.org)
Tautomers must also be distinguished from resonance contributors. Resonance contributors are alternative representations used together to describe the electronic structure of one molecular entity; they are not separate isomers that interconvert. Isomerism concerns distinct atomic arrangements, whereas resonance describes electronic delocalization within a species. (goldbook.iupac.org)
Differences in properties and biological interactions
Constitutional isomers are different compounds and can have different properties despite sharing a molecular formula. A change in skeleton, functional group, or functional-group position can alter chemical behaviour. Consequently, a molecular formula is insufficient to identify a compound without structural information. (openstax.org)
Enantiomers present a special case. Under equivalent achiral conditions, they have the same ordinary physical properties, but rotate plane-polarized light by equal amounts in opposite directions. Their interactions with chiral environments can differ. This distinction is important in biochemistry, where enzymes and other biological binding sites can distinguish between molecular handedness, producing different biological responses to different enantiomers. (openstax.org)