cyclobuten

Cyclobutene is a small cyclic hydrocarbon in which a four-membered ring contains a single carbon–carbon double bond. Its chemical formula is C4H6. The ring is highly strained because the internal angles in a four-membered ring are well below the ideal tetrahedral angle, and the presence of a double bond within the ring compounds the strain further. As a result, cyclobutene and many of its substituted derivatives are quite reactive relative to larger cycloalkenes.

Cyclobutene can be prepared by several routes, often starting from cyclobutanol or other cyclobutane derivatives through dehydration or dehydrogenation steps. It also appears as an intermediate in the synthesis of more complex bicyclic or polycyclic structures. Substituted cyclobutenes retain the same four-membered ring framework with one endocyclic double bond, and their reactivity is shaped by both ring strain and the nature of the substituents.

Reactivity and applications are dominated by the strain of the ring and the double bond. Cyclobutenes readily participate in electrocyclic reactions, most notably ring-opening to 1,3-butadiene under thermal or photochemical conditions; the two modes follow different stereochemical pathways in line with Woodward–Hoffmann rules. They also serve as reactive dienophiles in Diels–Alder reactions, enabling the construction of more complex bicyclic and polycyclic systems. Substituents on the ring can influence both the rate of ring opening and the outcome of cycloaddition reactions, making cyclobutenes useful building blocks in organic synthesis.

See also: cyclobutane, cyclobutene derivatives, electrocyclic reactions, butadiene.