Nonpericyclic

Nonpericyclic refers to reactions in organic chemistry that do not proceed through a concerted, cyclic rearrangement of bonding electrons in a single transition state. This stands in contrast to pericyclic reactions, which involve a continuous cycle of bond-making and bond-breaking in one step and are often analyzed with Woodward–Hoffmann orbital symmetry rules. Nonpericyclic processes typically do not exhibit the stereochemical outcomes predicted by those rules because their mechanisms proceed via discrete intermediates or stepwise sequences.

In nonpericyclic pathways, bond changes are not synchronized in a cyclic transition state. Reactions commonly proceed through ionic, radical, or diradical intermediates and may involve separate steps for bond formation and bond cleavage. This can lead to different kinetic and stereochemical patterns compared with pericyclic reactions. Mechanistic evidence for nonpericyclic behavior is usually obtained from kinetic studies, isolation or trapping of intermediates, isotopic labeling, or computational analyses that show a stepwise energy profile rather than a single cyclic transition state.

Typical examples include reactions driven by ionic mechanisms, such as SN1 and SN2 substitutions, and E1 and E2 eliminations, as well as radical additions and cyclizations that proceed via intermediate species. Many rearrangements and rearrangements with charge separation also fall into the nonpericyclic category. The term is used to emphasize that such processes are not governed by the orbital symmetry considerations central to pericyclic chemistry.

Notes: the distinction can be nuanced; some reactions may appear to straddle classifications or involve complex multi-step pathways that resemble pericyclic-like features in parts of their coordinate space.