$word_{count}$: 240

Structural isomers have the same molecular formula but different structural connectivity. Five types exist: (1) Chain isomerism — different carbon skeletons (C5H12: n-pentane, isopentane, neopentane). (2) Position isomerism — same functional group at different positions (1-propanol vs 2-propanol). (3) Functional group isomerism — different functional groups (C2H6O: ethanol vs dimethyl ether; C3H6O: propanal vs acetone). (4) Metamerism — different alkyl groups around the same functional group (diethyl ether vs methyl propyl ether); limited to ethers, ketones, secondary amines, and esters. (5) Tautomerism — dynamic equilibrium involving proton shift, most commonly keto-enol tautomerism. The keto form is thermodynamically favored (>99%) in simple carbonyl compounds, but the enol form dominates when stabilized by conjugation (1,3-dicarbonyl compounds), intramolecular H-bonding (acetylacetone: 76% enol), or aromaticity (phenol is exclusively the "enol" form). For counting structural isomers, the degree of unsaturation formula DBE = $\frac{2C + 2 + N - H - X}{2}$ is essential — it tells you the total number of rings and pi bonds. Oxygen and sulfur do not appear in the formula. A benzene ring accounts for 4 DBE (3 double bonds + 1 ring).