Olefins, which are unsaturated hydrocarbons containing one or more carbon–carbon double bonds, are broadly divided into alkenes and cycloalkenes. The general chemical formula of an alkene is C_nH_2n.

Doubly bonded carbons are sp² hybridized and have a trigonal planar geometry. The double bond is composed of a σbond formed by the overlap of hybrid orbitals and a πbond produced by the lateral overlap of unhybridized 2p orbitals on both the carbons. Each carbon atom is bonded to two hydrogen atoms through sp²–s orbital overlap. As the unhybridized p electrons have higher energy than the electrons in the hybrid orbitals, the π bond electrons generally have higher energy than the σ bond electrons. Therefore, electrophiles react preferentially with the π bond electrons of alkenes.

Ethylene, propylene, and butylene exist as colorless gases. Alkenes containing 5 to 14 carbon atoms are liquids, and those containing 15 or more carbon atoms are solids. Alkenes, being effectively nonpolar, are insoluble in water but soluble in nonpolar solvents.

The boiling points of alkenes increase with the molecular massas the intermolecular forces become stronger with the increasing size of the molecules.

Table 1: Physical Properties of Alkenes and Cycloalkenes

Alkenes occur abundantly in nature. For example, ethylene, the simplest alkene, is found in nature as a plant hormone that affects the ripening of fruits. Lycopene and carotenes are the polyenes responsible for the red, orange colors of fruits and vegetables, like tomatoes and carrots. Also, alkenes are the structural frames of various plant essential oils and insect pheromones.