Why does slicing onions make you cry? The culprit is syn-propanethial S-oxide. Exposing allinase enzymes in onions to air generates 1-propenesulfenic acid. Another enzyme rearranges the acid to form this unusual volatile compound. When it hits your eyes, it stimulates the lachrymal glands to produce tears.
W. Niegisch and W. H. Stahl determined syn-propanethial S-oxide’s molecular formula in 1956. After many researchers attempted to establish its structure, E. Block and co-workers pinned it down in 1979 and showed that it has the syn, or Z, configuration. About 5% of the natural substance has the anti (E) configuration.
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