A cycloalkene or cycloolefin is a type of alkene hydrocarbon which contains a closed ring of carbon atoms and either one or more double bonds, but has no aromatic character. Some cycloalkenes, such as cyclobutene and cyclopentene, can be used as monomers to produce polymer chains. Due to geometrical considerations, smaller cycloalkenes are almost always the cis isomers, and the term cis tends to be omitted from the names. trans-Cycloalkenes with 7 or less carbons in the ring will not occur under normal conditions because of the large amount of ring strain needed. In larger rings (from around 8 atoms), cis–trans isomerism of the double bond may occur.
Cycloalkenes follow a similar nomenclature system to alkenes, but the carbons are numbered starting at a carbon on the double bond and then through the double bond and around the ring. This method is used to keep the index numbers small.
Cycloalkenes with a small ring have about 20° more bond angle strain than a cycloalkane of the same size. This is because the bond angle for an alkene, C-C=C, is 122°, while the bond angle for an alkane, C-C-C, is 112°. When these carbons form a small ring, the alkene which has a larger bond angle will have to compress more than the alkane causing more bond angle strain.
Cycloalkenes have a lower melting point than cycloalkanes of the same size. The lowered melting point is due to the double bond preventing the compound from compact packing.