Alkenes contain a double bond that is composed of one sigma and one pi bond between two carbon atoms. The sigma bond has similar properties to those found in alkanes, while the pi bond is more reactive. The carbon atoms in the double bond are sp2 hybridized, forming a planar structure as shown in the ethylene molecule below Figure 1. Rotation around the double is disfavored, so alkenes form fairly stable isomers depending on the positioning of substituents on the same (cis) or opposite (trans) sides of the double bond. These isomers are diastereoisomers.
The melting and boiling points of alkenes are determined by the regularity of the packing of these molecules. Alkene isomers that can achieve more regular packing have higher melting and boiling points than molecules with the same molecular formula but weaker dispersion forces. Alkenes are non-polar, and they are both immiscible in water and less dense than water. They are generally soluble in organic solvents. In addition, they do not conduct electricity.
Alkenes can be produced through the dehydration of their respective aliphatic alcohols.
Alkenes are more reactive than their related alkanes due to the relative instability of the double bond. They are more likely to participate in a variety of reactions including combustion, addition, hydrogenation, and halogenation reactions. Alkenes can also be reacted, typically in the presence of a catalyst, to form polymers.