synthon

A synthon is an idealized fragment that represents a reactive center in the planning of chemical synthesis. The term was introduced by Elias J. Corey in the context of retrosynthetic analysis to describe fragments that would produce the target molecule by making a single bond formation. Synthons are conceptual, not necessarily present in the actual molecule or the reagents used; they function as guides for designing a synthetic route.

Key concepts associated with synthons include the distinction between synthons and synthetic equivalents. A synthon denotes a hypothetical fragment, while a synthetic equivalent is a real-world reagent or combination of reagents that can deliver the same reactive center in practice. This distinction allows chemists to evaluate which commercially available starting materials or reagents can be used to realize a proposed disconnection.

Synthons are commonly categorized by the type of bond formation they imply, such as electrophilic synthons (places in the target where an electrophile would fork the linkage) and nucleophilic synthons (places where a nucleophile would form a bond). In retrosynthesis, the target molecule is dissected into synthons, and subsequent steps identify practical equivalents that can realize those synthons under known reactions.

The synthon concept remains influential in teaching, planning, and computer-aided synthesis planning, where fragment-based thinking supports modular and convergent route design. It provides a framework for linking abstract disconnections to real, available reagents.