In advanced chemistry labs and classrooms around the world, one rule has quietly endured for nearly a century. Introduced in the early 20th century, it has been repeated often enough to pass almost without question, shaping how organic structures are imagined, drawn and dismissed.
Yet within this long-standing assumption lies a structural limitation that has defined what chemists consider impossible to make. Its boundaries have rarely been tested in practice, in part due to the difficulty of proving an exception without violating fundamental chemical principles.
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