cykloalkene

Cycloalkenes are cyclic hydrocarbons containing at least one carbon-carbon double bond. The name "cycloalkene" is derived from the Greek word "kyklos" meaning circle, the prefix "alk-" indicating four carbon atoms, and the suffix "-ene" signifying the presence of a double bond. The simplest cycloalkene is cyclopropene, a three-membered ring with one double bond. As the ring size increases, the geometry and reactivity of the double bond can change.

The double bond in a cycloalkene restricts the rotation of the carbon atoms involved. This rigidity can lead to the existence of geometric isomers, known as cis and trans isomers, if the substituents on the double bond carbons are different. However, in small rings (e.g., cyclopropene, cyclobutene), the trans isomer is highly strained and often does not exist or is very unstable. Larger cycloalkenes (e.g., cyclooctene) can accommodate trans double bonds more readily.

Cycloalkenes undergo reactions characteristic of alkenes, such as addition reactions across the double bond, including hydrogenation, halogenation, and hydrohalogenation. The cyclic structure can also influence the regiochemistry and stereochemistry of these reactions. Due to ring strain, smaller cycloalkenes like cyclopropene can be more reactive than their open-chain counterparts. Cycloalkenes are important intermediates in organic synthesis and are found in various natural products.