fourmembered

Four-membered rings are cyclic structures in which four atoms form the ring. This class includes carbocyclic rings such as cyclobutane, as well as heterocyclic rings like oxetane (containing oxygen), azetidine (nitrogen), and thietane (sulfur), and unsaturated variants such as cyclobutene and cyclobutanone. The term covers both saturated and unsaturated rings and those with heteroatoms, across organic chemistry.

A defining feature of four-membered rings is substantial ring strain. The ideal bond angles expected for sp3 or sp2 centers cannot be achieved in a four-membered framework, leading to higher energy than larger rings. Carbon-containing four-membered rings often adopt a puckered conformation to relieve some strain, while heteroatoms can alter the geometry and reactivity of the ring. The strain contributes to distinctive chemical behavior, including a greater propensity for ring-opening reactions and for cycloadditions that form or transform small rings.

Synthesis and occurrence vary by type. Four-membered rings can be formed by cycloaddition reactions, such as [2+2] cycloadditions to give cyclobutanes or by cyclization of suitable precursors to yield oxetanes and azetidines. They can also arise by ring contraction or rearrangement from larger rings in certain synthetic sequences.

Applications and significance of four-membered rings arise from their high reactivity and unique geometry. They serve as reactive intermediates in organic synthesis, and their presence in pharmaceuticals, agrochemicals, and natural products can impart distinctive properties that influence biological activity and binding.