Aliphatic thiols, R–SH, were formerly known as mercaptans because of their affinity for mercury compounds (Latin: corpus mercurium aptans). They can be regarded as organic derivatives of hydrogen sulfide or thio analogs of alcohols.
According to IUPAC nomenclature the names of aliphatic thiols are constructed by adding the ending “thiol” to the name of the corresponding alkane (e.g., ethanethiol). If substituents with higher priorities are present, the prefix “mercapto” is used instead (e.g., mercaptoacetic acid).
Thiols are found in a wide range of natural materials. Low molecular mass thiols are responsible for the aromas of various foods, such as cheese, milk, coffee, cabbage, and bread. They are probably liberated during processing through enzymatic cleavage of sulfides, disulfides, and other sulfur‐containing substances 12. Thiols can also be detected in a number of plants 13. Methanethiol occurs in radishes and various alliums, such as onions, leeks, and garlic. The characteristic odor of fresh grapefruit juice is due to the presence of 1‐p‐menthene‐8‐thiol in high dilution 14. Asparagus contains 3‐mercapto‐ and 3,3′‐dimercaptoisobutyric acids 15. Butanethiol and 2‐methylbutanethiol have been found in the defense secretions of skunks 16. Low molecular mass alkanethiols are formed in the degradation of biological material during putrefaction 17, and consequently are found in varying quantities in most crude petroleum, depending on its origin 18.
l‐Cysteine, one of the natural amino acids found in proteins and a substance that participates in an important reversible redox system with the disulfide cystine, is of considerable biochemical interest, as is coenzyme A (CoA−SH), whose acetyl derivative (CoA−S−Ac) is a key participant in the metabolism of carbohydrates, fats, and proteins.
In 1834 W. C. Zeise achieved the first thiol synthesis by preparing ethanethiol from barium hydrogensulfide and calcium ethyl sulfate 19.