Solution to 120 year-old puzzle reveals new chemical phenomenon

Among the many chemical mysteries emerging in the 19th century, acetaldehyde phenylhydrazone (APH) have proved to be one of the strangest and most confounding. German scientist Emil Fischer, winner of the 1902 chemistry Nobel prize for sugar and purine synthesis, first made APH in 1877, while exploring phenylhydrazine’s reactivity. In 1896, he reported that unusually there were three interchangeable forms, differentiated by their melting points, 63−65°C, 80°C and 98−101°C.