Structural

Chain isomers have the same molecular formula but differ in the branching of the carbon skeleton. The total number of carbons is identical, but some are arranged in a straight (unbranched) chain while others form branched structures. Example: butane (n-butane) vs 2-methylpropane (isobutane), both C₄H₁₀. n-Butane: CH₃–CH₂–CH₂–CH₃; 2-methylpropane: (CH₃)₃CH. Their boiling points differ (n-butane −1 °C vs isobutane −12 °C) because branching reduces surface area and van der Waals forces. Chain isomerism becomes increasingly important as carbon chain length grows.

Position isomers share the same carbon skeleton and functional group(s) but differ in the position of the functional group along the chain or ring. Example: 1-propanol (CH₃CH₂CH₂OH) vs 2-propanol (CH₃CH(OH)CH₃), both C₃H₈O. Similarly, 1-chloropropane (CH₃CH₂CH₂Cl) vs 2-chloropropane (CH₃CHClCH₃) are position isomers. Position isomers can also arise from different positions of a double bond: but-1-ene (CH₂=CHCH₂CH₃) vs but-2-ene (CH₃CH=CHCH₃). These differences in position affect reactivity — for instance, primary and secondary alcohols follow different oxidation patterns.

Functional group isomers contain the same molecular formula but belong to different homologous series because the atoms are connected into entirely different functional groups. Classic pairs include: (a) alcohols and ethers — ethanol (C₂H₅OH) and dimethyl ether (CH₃OCH₃), both C₂H₆O; (b) aldehydes and ketones — propanal (CH₃CH₂CHO) and propanone (CH₃COCH₃), both C₃H₆O; (c) carboxylic acids and esters — ethanoic acid (CH₃COOH) and methyl methanoate (HCOOCH₃), both C₂H₄O₂; (d) alkynes and dienes — but-1-yne (HC≡CCH₂CH₃) and buta-1,3-diene (CH₂=CH–CH=CH₂), both C₄H₆. Because the functional groups are different, these isomers undergo entirely different chemical reactions.

Metamerism is a special case of functional group isomerism found in compounds containing a divalent atom (such as oxygen, sulfur, or nitrogen) flanked by two alkyl groups. The isomers differ in the length and structure of the alkyl groups on either side of the heteroatom, while the overall formula remains the same. Example: diethyl ether (CH₃CH₂–O–CH₂CH₃) and methyl propyl ether (CH₃–O–CH₂CH₂CH₃), both C₅H₁₂O. Similarly, diethyl sulfide and methyl propyl sulfide are metamers. Metamers must belong to the same functional class — unlike functional group isomers, which cross class boundaries. Because only the alkyl distribution changes, physical properties differ only modestly.

Tautomers are special structural isomers that interconvert rapidly and reversibly, usually by the migration of a proton (H⁺) and a shift of a double bond. The most important example is keto-enol tautomerism. In the keto form, a carbonyl group (C=O) exists adjacent to a carbon bearing a hydrogen. In the enol form, that hydrogen has migrated to the oxygen, giving a hydroxyl group (–OH) on a carbon–carbon double bond (C=C). For acetone (propanone): keto form CH₃–C(=O)–CH₃ ⇌ enol form CH₃–C(OH)=CH₂. The position of the equilibrium depends on stability: for simple ketones, the keto form dominates (>99.9%) because C=O bonds (~745 kJ/mol) are stronger than C=C plus O–H. However, for 1,3-dicarbonyl compounds such as acetylacetone (pentan-2,4-dione), the enol form can make up ~80% at room temperature because the enol is stabilised by intramolecular hydrogen bonding and conjugation of the C=C with the remaining C=O group. Tautomerism is not an equilibrium between two stable isolated compounds; it is a dynamic, solution-phase equilibrium. Acid and base catalysts (and even protic solvents) accelerate tautomer interconversion by facilitating proton transfer.

• \ce{CH3-CO-CH3} — Propanone (keto form) (—): The dominant tautomer (~99.9%) containing a carbonyl group (C=O); more stable due to the high bond enthalpy of the C=O bond (~745 kJ mol⁻¹).
• \ce{CH3-C(OH)=CH2} — Propen-2-ol (enol form) (—): The minor tautomer (~0.0025%) containing a vinyl hydroxyl group; less stable for simple ketones but stabilised in 1,3-dicarbonyl systems by conjugation and hydrogen bonding.
• \ce{<=>} — Equilibrium arrow (—): Indicates a rapid, reversible equilibrium; both forms coexist in solution, with the position of equilibrium governed by relative thermodynamic stability.

Problem. Draw all structural isomers of C₄H₈O that contain a carbonyl group (C=O). Classify each isomer pair you identify by the type of structural isomerism involved.

Answer. Three structural isomers: butanal (CH₃CH₂CH₂CHO), 2-methylpropanal ((CH₃)₂CHCHO), and butanone (CH₃COCH₂CH₃). Butanal and 2-methylpropanal are chain isomers; both aldehydes and butanone are functional group isomers.

• Stereoisomerism: Geometric and Optical — Stereoisomers share both molecular formula and connectivity (unlike structural isomers) but differ in three-dimensional arrangement; understanding structural isomerism is an essential prerequisite before studying stereoisomerism.
• Homologous Series and Functional Groups — Functional group isomerism involves compounds from different homologous series sharing the same formula; a clear grasp of what defines each homologous series is necessary to identify functional group isomers.
• IUPAC Nomenclature of Organic Compounds — Correctly naming chain and position isomers requires fluency in IUPAC rules for numbering carbon chains and locating substituents or functional groups.
• Covalent Bond Enthalpies — The relative stability of keto versus enol tautomers is explained by comparing C=O, C=C, C–H, and O–H bond enthalpies; bond energy data is therefore directly relevant to tautomerism.
• Reactions of Carbonyl Compounds — Aldehydes, ketones, carboxylic acids, and esters are all carbonyl-containing functional group isomers; understanding their distinct reactivities reinforces why functional group identity matters more than molecular formula alone.