Alkene

Alkene are unsaturated hydrocarbons characterized by the presence of at least one carbon-carbon double bond (C=C). This double bond is a hybrid structure, consisting of one sigma (σ) bond and one pi (π) bond. The sigma bond is formed by the direct overlap of atomic orbitals, providing a strong, stable connection.

The pi bond, however, arises from the lateral overlap of unhybridized p orbitals above and below the internuclear axis. This pi electron cloud is more diffuse and less tightly held, making it the primary site of reactivity for alkene. This electron-rich region readily undergoes electrophilic addition reactions, where electron-seeking species attack the pi system, leading to the breaking of the pi bond and the formation of new sigma bonds.

This fundamental reactivity underpins their extensive use in synthesis.

The industrial production of alkenes is predominantly linked to the refining of crude oil and the processing of natural gas. Thermal cracking and catalytic cracking are the cornerstone processes for breaking down larger hydrocarbon molecules into smaller, more valuable alkenes. Steam cracking, a high-temperature process (750–950 °C) using steam as a diluent, is particularly important for producing light alkenes like ethene, propene, and butenes from feedstocks such as naphtha, gas oil, and ethane.

These light alkenes are the primary building blocks for the vast petrochemical industry. Their availability and relatively low cost make them ideal starting materials for synthesizing a multitude of downstream products, from plastics and synthetic rubbers to solvents and detergents.

The ability of alkene to undergo polymerization is arguably their most significant contribution to modern society. This process involves the sequential addition of monomer units to form long polymer chains. Different polymerization mechanisms, such as free-radical, cationic, anionic, and coordination polymerization (e.g., Ziegler-Natta catalysis), allow for precise control over polymer structure, molecular weight, and tacticity.

For instance, ethene polymerizes to form polyethylene (PE), one of the most widely produced plastics globally, with various grades like HDPE and LDPE offering distinct properties for applications ranging from packaging films to rigid containers. Propene polymerizes to polypropylene (PP), known for its strength and heat resistance, used in automotive parts, textiles, and durable goods. The ongoing research in polymerization catalysis continues to expand the range of achievable polymer properties and applications.

While polymerization dominates alkene utilization, their reactivity supports a broad spectrum of other chemical transformations. Electrophilic addition reactions are central, enabling the synthesis of alcohols (hydration), alkyl halides (hydrohalogenation), and vicinal dihalides (halogenation). For example, the hydration of ethene yields ethanol, a crucial solvent and biofuel precursor.

Hydroboration-oxidation provides a regioselective route to alcohols, often following anti-Markovnikov addition. Alkenes also participate in oxidation reactions, yielding epoxides, diols, or cleavage products depending on the reagent. Furthermore, they are key intermediates in Diels-Alder cycloadditions, a powerful tool for forming cyclic compounds. Ethylene's role as a plant hormone (phytohormone) highlights its biological significance, influencing fruit ripening, senescence, and flowering, with applications in agriculture for post-harvest management.