Cycloalkenes

Cycloalkenes are cyclic hydrocarbons that contain at least one carbon–carbon double bond within the ring. They are the unsaturated counterparts of cycloalkanes. For monocyclic cycloalkenes with a single internal double bond, the general formula is CnH2n−2; additional double bonds in the ring decrease the hydrogen count further.

Common examples include cyclopentene (C5H8), cyclohexene (C6H10), cycloheptene (C7H12), and cyclooctene (C8H14). In these compounds, the double bond is part of the ring, and the ring adopts a geometry constrained by the need to accommodate the sp2-hybridized carbons. Small cycloalkenes often exhibit higher ring strain compared with their acyclic counterparts, which influences their reactivity and physical properties.

Reactivity of cycloalkenes generally follows that of alkenes, with typical addition reactions across the C=C bond. Ring strain makes the smaller cycloalkenes more reactive toward hydrogenation and other additions than unstrained alkenes. Some cycloalkenes also exist as stereoisomers; trans-cycloalkenes are rare due to strain, with trans-cyclooctene being notable as the smallest isolable example in nature and synthesis.

Preparation of cycloalkenes commonly involves dehydrohalogenation or dehydration of cycloalkanols, or dehydrogenation of saturated rings, often under catalytic conditions. They also appear as intermediates in organic synthesis and as building blocks in natural products and materials chemistry.

See also: cycloalkane, alkene, and cycloalkadienes.