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
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File:Cyclopropene 2D skeletal.svg|Cyclopropene
File:Cyclobutene.svg|Cyclobutene
File:Cyclopentene.svg|Cyclopentene
File:Cyclohexene.svg|Cyclohexene
File:Cycloheptene.svg|Cycloheptene
File:Cyclopentadien.svg|Cyclopentadiene
File:1,3-cyclohexadiene.svg|1,3-Cyclohexadiene
File:1,4-cyclohexadiene.svg|1,4-Cyclohexadiene
File:Cycloheptatriene.svg|Cycloheptatriene
File:1,5-Cyclooctadiene.svg|1,5-Cyclooctadiene
File:Cis-cyclooctene.png|cis-cyclooctene
File:(S)-(+)-trans-Cyclooctene Structural Formula V.1.svg|trans-cyclooctene
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Nomenclature
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. <gallery>
File:Cycloalkene nomenclature.svg|1-methylcyclohexene
File:3-methylcyclohexene nomenclature.svg|3-methylcyclohexene
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Properties
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 generally reflect physical properties of their cycloalkane. In physical states, only the smaller cycloalkenes are gases while the others are mostly liquid. These molecules are also more reactive than cycloalkanes due to increased electron density shifts of the double bond.
Trans isomers
As previously mentioned, cis-isomers of cycloalkenes exhibit more stability than trans-isomers; however, on an experimental and computational level, this property is only applicable to cycloalkenes with 10 carbons or less. As the number of carbons increase, the possibility of a trans-isomer occurring also increase. This process can be used to form cycloalkenes of either E or Z configurations, depending on the stereochemistry of the second ring strain.
center|frameless|Formation of a cycloalkane via ring closing metathesis
Birch reduction
Birch reduction is a possible method to reduce aromatic compounds into cycloalkenes, specifically cyclohexadiene.
center|frameless|Formation of a cycloalkane via Birch Reduction
Diels-Alder reaction
The Diels-Alder reaction, also known as cycloaddition, combines a conjugated diene and an alkene to form cycloalkene. This is a concerted process, with bonds forming and breaking simultaneously. However, other cycloalkenes, such as Cyclooctatetraene, can be formed as a result of this reaction. Addition of heat or photolysis causes a reversible reaction that causes one pi bond to become a sigma bond, which closes the ring and creates a cycloalkene.