alkylenes

Alkylenes, commonly referred to as alkenes, are a class of unsaturated hydrocarbons characterized by the presence of at least one carbon-carbon double bond (C=C). This structural feature distinguishes them from alkanes, which contain only single bonds, and alkynes, which feature triple bonds. The general molecular formula for alkenes is CnH2n, where *n* represents the number of carbon atoms in the chain.

The simplest alkene is ethene (ethylene), with the formula C2H4, consisting of two carbon atoms connected by a double bond. The double bond in alkenes is formed by the sharing of four electrons between the two carbon atoms—two pairs of electrons—resulting in a stronger bond than a single covalent bond but less stable than a triple bond. This bond configuration gives alkenes unique chemical reactivity, particularly in addition reactions where the double bond is broken to form new single bonds with other atoms or groups.

Alkenes are found in various natural and synthetic sources, including crude oil, where they are produced during refining processes. They are also prevalent in plant biology, where they play roles in growth regulation and stress responses. Industrially, alkenes are crucial intermediates in the production of plastics, synthetic rubbers, detergents, and solvents. For example, ethylene is a key feedstock in the manufacture of polyethylene, a widely used plastic.

The naming of alkenes follows IUPAC nomenclature, where the suffix *-ene* is appended to the parent alkane name. The position of the double bond is indicated by the lowest possible number, and substituents are numbered accordingly. For instance, propene (C3H6) has a double bond between the first and second carbon atoms, while but-1-ene (C4H8) has its double bond between the first and second carbons in a four-carbon chain.

Alkenes exhibit geometric isomerism, known as cis-trans or E-Z isomerism, due to restricted rotation around the double bond. This leads to distinct spatial arrangements of atoms, influencing physical properties and reactivity. The presence of the double bond also makes alkenes more reactive than alkanes, particularly toward electrophilic addition reactions, where polar molecules or ions attack the electron-rich double bond.