Alkenes (C=C) and alkynes (C≡C) are unsaturated hydrocarbons containing one or more multiple carbon-carbon bonds. The presence of pi bonds makes these compounds significantly more reactive than alkanes and provides the basis for addition reactions that are among the most important transformations in organic synthesis. Alkenes have the general formula C_nH_{2n}, while alkynes follow C_nH_{2n-2}.
The carbon atoms in the double bond of alkenes are sp² hybridized, forming three sigma bonds in a trigonal planar arrangement (120° bond angles). The unhybridized p orbitals overlap sideways to form the pi bond, which is weaker (~265 kJ·mol⁻¹) than the sigma bond (~350 kJ·mol⁻¹) and lies above and below the plane of the molecule. In alkynes, the triple bond carbons are sp hybridized (180°, linear), with two sets of perpendicular p orbitals forming two orthogonal pi bonds. The restricted rotation around the double bond gives rise to cis/trans isomerism (E/Z designation in the IUPAC system) in alkenes, where the higher priority substituents on each carbon determine the configuration.
Nomenclature of alkenes follows IUPAC rules with the suffix -ene. The parent chain must contain the double bond, which is given the lowest possible number. For alkynes, the suffix is -yne. For compounds containing both, the -ene takes precedence in numbering, and the compound is named as an -enyne. When the two substituents on a double bond are different, the Cahn-Ingold-Prelog priority rules are applied to assign E (entgegen, opposite) or Z (zusammen, together) configuration. The stability of alkenes increases with the degree of substitution — tetrasubstituted > trisubstituted > disubstituted > monosubstituted. The Saytzeff rule states that the more substituted alkene is the major product in elimination reactions.
The characteristic reaction of alkenes is electrophilic addition across the double bond. Hydrogenation (H₂/Pd, Pt, or Ni) reduces alkenes to alkanes. Halogenation adds Br₂ or Cl₂ to give vicinal dihalides; the bromine color discharge is a qualitative test for unsaturation. Hydrohalogenation adds HX following Markovnikov’s rule: the hydrogen atom adds to the less substituted carbon, placing the halogen on the more substituted carbon. Hydration (H₂O/H⁺) produces alcohols, also following Markovnikov regiochemistry. The oxymercuration-demercuration route provides Markovnikov alcohols without rearrangement, while hydroboration-oxidation gives anti-Markovnikov alcohols with syn stereochemistry.
Oxidative cleavage of alkenes provides structural information and synthetic intermediates. Dihydroxylation with OsO₄ or KMnO₄ (cold, dilute) adds two hydroxyl groups with syn stereochemistry. Ozonolysis (O₃ followed by reductive or oxidative workup) cleaves the double bond to produce aldehydes, ketones, or carboxylic acids, allowing the determination of double bond position in unknown alkenes. The position of the double bond is identified from the cleavage products, making ozonolysis a powerful tool in structure elucidation.
Terminal alkynes (RC≡CH) have distinctive acidity with pKa ≈ 25, making them significantly more acidic than alkenes (pKa ≈ 44) or alkanes (pKa ≈ 50). The acidity arises from the high s-character of the sp-hybridized carbon (50% s-character), which stabilizes the conjugate base through increased electronegativity. Sodium amide (NaNH₂) or strong organolithium bases can deprotonate terminal alkynes to form alkynide anions, which act as nucleophiles in alkylation reactions with alkyl halides to form longer-chain alkynes. Alkenes also undergo polymerization — both addition polymerization (radical or cationic/anionic mechanisms) to form polyolefins such as polyethylene, polypropylene, and PVC, making alkenes critically important in the plastics industry.
