Organic chemistry studies carbon compounds. Carbon is tetravalent and forms strong covalent bonds through hybridisation: sp3 (tetrahedral, 109.5 degrees) as in methane, sp2 (trigonal planar, 120 degrees) as in ethene, and sp (linear, 180 degrees) as in ethyne. As s-character rises (sp3 to sp2 to sp), electronegativity rises and C-H bond length shortens. A sigma bond is formed by axial (head-on) overlap and a pi bond by sideways overlap.
Structures are written as complete, condensed or bond-line formulas; three dimensions are shown with wedge (towards viewer) and dashed (away) lines.
Organic compounds are classified as acyclic (open chain) or cyclic; cyclic ones are alicyclic, benzenoid aromatic, non-benzenoid aromatic, or heterocyclic. A functional group (e.g. -OH, -CHO, -COOH, >C=O) determines chemical behaviour. A homologous series is a set of compounds with the same general formula and functional group differing by -CH2-.
IUPAC nomenclature: identify the longest chain (parent), number it to give substituents the lowest locants, cite substituents alphabetically, and use suffixes for the principal functional group (-ane, -ene, -yne, -ol, -al, -one, -oic acid). The priority order of principal groups is -COOH > -SO3H > esters > acid chloride > amide > nitrile > aldehyde > ketone > -OH > -NH2 > alkene > alkyne. Disubstituted benzenes use ortho (1,2), meta (1,3), para (1,4).
Isomerism: structural isomers (chain, position, functional group, metamerism) differ in connectivity; stereoisomers (geometrical cis-trans and optical) have the same connectivity but different spatial arrangement.
Reaction mechanism fundamentals: covalent bonds break by homolytic fission (gives free radicals) or heterolytic fission (gives carbocations and carbanions). Reagents are nucleophiles (electron-rich, attack positive centres) or electrophiles (electron-deficient). Electronic effects: inductive (permanent sigma polarisation), resonance/mesomeric (delocalisation of pi or lone-pair electrons; resonance hybrid is more stable than any canonical form), electromeric (temporary, reagent-induced complete pi-pair shift), and hyperconjugation (delocalisation of alpha C-H sigma electrons; no-bond resonance). Carbocation, carbanion and free-radical stability orders and acid/base strengths are explained by these effects (e.g. tertiary carbocation most stable; trichloroacetic acid more acidic than acetic acid). Organic reactions are of four types: substitution, addition, elimination and rearrangement.
Purification techniques: sublimation (sublimable solids), crystallisation (differential solubility), simple/fractional distillation (volatility differences), distillation under reduced pressure, steam distillation (steam-volatile water-immiscible compounds), differential extraction (partition between immiscible solvents), and chromatography (adsorption: column and thin-layer with Rf value; partition: paper). Qualitative analysis detects C and H by combustion, and N, S, halogens, P by Lassaigne's sodium fusion test. Quantitative estimation: C and H by combustion (CO2 and H2O), nitrogen by Dumas (volume of N2) or Kjeldahl (ammonia titration) methods, halogens and sulphur by Carius method (AgX and BaSO4), phosphorus as Mg2P2O7, and oxygen by difference or directly.