Unsynthesizable

Unsynthesizable refers to designs, methods, or compounds that cannot be produced or realized through a synthesis process using current methods, tools, or knowledge. The term is most commonly used in digital hardware design and in chemistry, where it denotes limitations in translating ideas into a tangible implementation or in forming a target from available precursors.

In digital hardware design, synthesis is the automated process of converting a high-level description, such as register-transfer level code, into a gate-level netlist that can be manufactured. Code or constructs that do not map to actual hardware are considered unsynthesizable. Common unsynthesizable elements include delay statements and time-based constructs, initial blocks, file input/output, and real-number data types. While such code may be fully simulatable, synthesis tools typically ignore or fail on these constructs. To maintain portability and reliability, developers separate simulation-only code from synthesizable modules and may guard simulation blocks with directives. Some advanced tools offer limited synthesis support for restricted constructs, but the general best practice is to avoid unsynthesizable features when creating portable hardware designs.

In chemistry and materials science, unsynthesizable describes compounds that cannot be prepared with available reagents, routes, or stability constraints. This may arise from unstable intermediates, highly hazardous conditions, or a lack of a practical synthetic sequence. The term is often used cautiously, since many targets are described as “not yet synthesized” rather than truly impossible. Researchers may pursue alternative synthetic pathways or computational assessments to evaluate feasibility, highlighting the ongoing gap between theoretical possibility and practical realization.