cycloalkene

In organic chemistry, 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. Cycloalkenes require considerable p-orbital overlap in the form of a bridge between the carbon-carbon double bond; however, this is not feasible in smaller molecules due to the increase of strain that could break the molecule apart. In greater carbon number cycloalkenes, the addition of substituents decreases strain. trans-Cycloalkenes with 7 or fewer carbons in the ring will not occur under normal conditions because of the large amount of ring strain needed. In larger rings (8 or more atoms), cis–trans isomerism of the double bond may occur. This stability pattern forms part of the origin of Bredt's rule, the observation that alkenes do not form at the bridgehead of many types of bridged ring systems because the alkene would necessarily be trans in one of the rings.

==Examples==