Alkanes are saturated hydrocarbons whose acyclic members follow the general formula \(C_nH_{2n+2}\). They contain only sigma (single) bonds between carbons and between carbon and hydrogen, with each carbon being \(sp^3\) hybridized in a tetrahedral geometry of roughly 109.5°. Because they lack polar functional groups, alkanes are nonpolar and largely unreactive except toward combustion (with \(O_2\) to give \(CO_2\) and \(H_2O\)) and free-radical halogenation. Their nomenclature uses the suffix -ane (methane, ethane, propane), and IR spectroscopy shows strong C–H stretches near 2850–2960 cm\(^{-1}\) with no other diagnostic peaks. Halogenation proceeds by free-radical substitution through initiation (homolysis), propagation, and termination steps.
Alkenes are defined by a carbon–carbon double bond (C=C) and have the general formula \(C_nH_{2n}\) for acyclic members. The alkene carbons are \(sp^2\) hybridized with trigonal planar geometry and bond angles near 120°, and they take the suffix -ene (ethene, propene). Restricted rotation around the double bond gives rise to cis-trans (E/Z) geometric isomerism. Their characteristic reactions are electrophilic additions—addition of HX, X₂, H₂O, or H₂ across the C=C. Markovnikov's rule predicts that HX addition to an unsymmetrical alkene places H on the carbon with more hydrogens and X on the more substituted carbon, via the more stable carbocation. Cold dilute \(KMnO_4\) or \(OsO_4\) performs syn dihydroxylation to give a vicinal diol, ozonolysis cleaves the double bond to give aldehydes/ketones or carboxylic acids after workup, and Pd, Pt, or Ni catalyzes hydrogenation to a single bond. Anti-Markovnikov hydration uses hydroboration–oxidation (\(BH_3\) then \(H_2O_2/NaOH\)) to give the less substituted alcohol.
Alkynes contain a carbon–carbon triple bond (C≡C) and follow the general formula \(C_nH_{2n-2}\). The alkyne carbons are sp hybridized, giving a linear geometry with 180° bond angles, and they are named with the suffix -yne (ethyne, propyne). Terminal alkynes have an unusually acidic C–H (pKa ≈ 25) that can be deprotonated by strong bases like \(NaNH_2\) to form an acetylide ion, which then reacts with an alkyl halide via \(S_N2\) alkylation to form a new, longer alkyne. The IR spectrum shows a sharp ≡C–H stretch near 3300 cm\(^{-1}\) and a weak C≡C stretch near 2100–2260 cm\(^{-1}\). Because a triple bond contains two pi bonds, two equivalents of HX or H₂ add successively. Terminal alkynes are readily distinguished from internal alkynes by reaction with \(Ag(NH_3)_2^+\) or \(Cu(NH_3)_2^+\), which precipitate insoluble metal acetylides only for terminal alkynes.