Thiols are a class of organic compounds characterized by the presence of a sulfur-hydrogen bond, known as the sulfhydryl or \(\text{-SH}\) group, which is structurally analogous to the hydroxyl (\(\text{-OH}\)) group in alcohols. The systematic naming of these compounds, governed by the International Union of Pure and Applied Chemistry (IUPAC), provides a standardized language for chemists worldwide. This system is important not only for clarity in scientific communication but also for safety, as many simple thiols are known for their distinct, often potent, odors. By following a set of established rules, even complex thiol structures can be named unambiguously.
Identifying the Thiol Functional Group and Parent Chain
The first step in naming any thiol is to identify the longest continuous chain of carbon atoms that specifically includes the carbon atom to which the \(\text{-SH}\) group is attached. This longest chain forms the basis, or parent name, of the compound, and its length determines the alkane name used, such as propane for a three-carbon chain or pentane for a five-carbon chain.
A slightly longer carbon chain may exist elsewhere in the molecule, but if it does not contain the \(\text{-SH}\) group, it cannot be selected as the parent chain. The integrity of the parent chain is prioritized by the presence of the functional group. This rule ensures that the primary chemical identity of the molecule, defined by the thiol group, is reflected in its base name.
Applying the Thiol Suffix and Locant
Once the parent chain is correctly identified, the next step is to modify its alkane name to indicate the presence of the thiol functional group. The IUPAC convention dictates that the final ‘e’ from the corresponding alkane name is dropped, and the suffix “\(\text{-thiol}\)” is added. For instance, a two-carbon parent chain, ethane, becomes ethanethiol.
To specify the exact location of the \(\text{-SH}\) group along the carbon chain, a positional number, or locant, must be used. This is achieved by numbering the parent chain beginning from the end that gives the carbon atom bonded to the \(\text{-SH}\) group the lowest possible number. This priority rule means the thiol group takes precedence over other substituents like alkyl groups or halogen atoms for numbering.
The locant is placed directly before the suffix, differentiating isomers like propan-1-thiol from propan-2-thiol. If the \(\text{-SH}\) group is at the very end of the chain, the “1” locant is sometimes understood and omitted in simpler cases, though including it is always clearer and more systematic.
Naming Complex Thiols with Substituents
Many thiols feature additional groups attached to the parent chain, and these groups must be named as substituents using prefixes. These secondary substituents, which include alkyl groups like methyl and ethyl, or halogens like chloro and bromo, are located by their own positional numbers. The numbering of the parent chain, which was already established to give the \(\text{-SH}\) group the lowest number, is maintained for locating these substituents.
Each substituent is named and numbered, and then all substituents are listed in alphabetical order before the parent name. For example, the chloro- prefix would be listed before the methyl- prefix, regardless of their position numbers. The full name then follows the format: (Substituent Locant)-(Substituent Name)(Parent Chain Locant)-thiol.
A complete example is 2-chloro-3-methylbutan-1-thiol, which indicates a four-carbon chain with the thiol on the first carbon, a chlorine atom on the second, and a methyl group on the third. The \(\text{-SH}\) group maintains its priority throughout this process, ensuring its position dictates the foundational numbering of the compound.
Nomenclature When -SH is a Lower Priority Group
In some molecules, the thiol group is not the highest-priority functional group present, meaning another group, such as a carboxylic acid (\(\text{-COOH}\)) or an aldehyde (\(\text{-CHO}\)), must take the final suffix position in the name. The thiol group is ranked lower than many oxygen-containing counterparts, including alcohols. When this occurs, the \(\text{-SH}\) group is no longer indicated by the “\(\text{-thiol}\)” suffix.
Instead, the \(\text{-SH}\) group is treated as a secondary substituent and is named using a prefix. The older, but still widely recognized, prefix for the \(\text{-SH}\) group is “\(\text{mercapto-}\)“. The more current IUPAC recommendation, however, favors the use of the prefix “\(\text{sulfanyl-}\)“.
An example of this usage is 3-mercaptopropanoic acid, where the carboxylic acid group determines the parent name, and the \(\text{-SH}\) group is a substituent on the third carbon. The choice between using a suffix or a prefix is determined entirely by the functional group’s priority within the molecule. Regardless of the prefix used, the locant indicating the \(\text{-SH}\) group’s position must still be included.