thiols Thiols, also named as mercaptan, are a class of organic compounds containing a sulfhydryl group (SH). They are similar to the alcohols and phenols but with the oxygen atom replaced by a sulfur atom. Actually, thiols are responsible for high volatility and unpleasant odor resembling that of garlic or rotten eggs. Thiols also contribute to the telltale odor skunks sprays in self-defense and the scent of freshly chopped onions. A concentration of 10 ppb can be recognized by human nose. Because of their disagreeable odours, it is easy to say whether thiols exist in petroleum and natural gas. They are also the same compounds responsible for the smell associated with bad breath and flatulence in humans. However, not the odors properties of all thiols are offensive. The thiols in many botanicals lead to their aromatic properties. For example, thioterpineol, a thiol in grapefruit, contributes to the distinctive yet not unpleasant fragrance of grapefruit. Thiols also are involved in heady aroma familiar to those brew beer or wine at home.