What chemical was once a anesthetic and also by criminals to knock out their victims?

The chemical you are referring to is likely chloroform. Historically used as an anesthetic in surgeries, chloroform gained notoriety for its misuse by criminals to incapacitate victims, as it can induce unconsciousness when inhaled. Its sedative properties made it appealing for such purposes, but its use has since been largely discontinued in medical settings due to safety concerns.