endocyclic

Endocyclic is an adjective used in chemistry to describe features that are contained within a ring structure. In this usage, a bond, atom, or functional group is endocyclic if all of its participating atoms are members of the ring. By contrast, exocyclic refers to a bond or substituent that is attached to the ring but lies outside the ring framework.

In organic chemistry, endocyclic bonds are those formed between two atoms that are both part of the ring. Endocyclic double bonds occur inside the ring, as in cyclohexene, where the C=C bond connects two ring atoms. In aromatic systems such as benzene, the pi bonds involved are considered part of the endocyclic framework of the ring.

An exocyclic bond or substituent, by contrast, is attached to a ring atom but lies outside the ring. A common example is methylenecyclopentane, in which a methylene group (=CH2) is exocyclic to the cyclopentane ring because the external carbon forms the double bond with a ring carbon.

In polycyclic or fused-ring systems, the designation endocyclic can depend on the ring being considered; a bond may be endocyclic to more than one ring or require specification with respect to a particular ring. Endocyclic is also used in synthetic chemistry to describe endocyclization, a process that forms a new bond within the same ring, as opposed to exocyclization, which forms a bond external to the ring.