adjective. External to a chemical ring structure.

Pertaining to the Exocyclica; having an eccentric anus, as a clypeastroid or spatangoid seaurchin.

adjective. (of a cyclic compound) not forming part of, or not attached to the ring

"Formaldehyde also affects nucleic acids by forming adducts with exocyclic nitrogen atoms in nucleotides"

acronyms  baa-rated  break-point  carbon-carbon  electro-negative