IUPAC Nomenclature of Thiols Study Guide

Fundamentals of Thiol Nomenclature

Thiols, historically known as mercaptans, are organic compounds characterized by the presence of a sulfhydryl group ( $-SH$ ) bonded to a carbon atom. In accordance with the International Union of Pure and Applied Chemistry (IUPAC) standards, the systematic naming of thiols bears a strong similarity to the nomenclature of alcohols, though there are critical differences regarding suffix application and the retention of vowels in the parent name. Effective nomenclature requires the identification of the longest carbon chain containing the sulfhydryl group, proper numbering to prioritize the functional group, and the correct assembly of substituents in alphabetical order.

IUPAC Rules for Simple and Branched Alkanethiols

The naming of thiols begins with identifying the longest continuous carbon chain that contains the $-SH$ group, which defines the parent alkane. The carbon chain must be numbered from the end nearest to the sulfhydryl group so that the carbon atom bearing the $-SH$ group receives the lowest possible locant. Notably, when naming thiols, the terminal "e" of the alkane name is retained. For example, a two-carbon thiol is named ethanethiol ( $CH_3CH_2SH$ ), as opposed to ethanol where the "e" is dropped. If branches are present, they are identified as substituents. Their positions are indicated by numerical locants based on the numbering established for the $-SH$ group, and they are listed alphabetically in the final name (e.g., 2-methylbutane-1-thiol).

Nomenclature of Dithiols and Polythiols

Compounds that contain multiple sulfhydryl groups are designated as polythiols. For molecules containing two $-SH$ groups, the suffix "-dithiol" is used; for three groups, "-trithiol" is utilized. Similar to monothiols, the final "e" of the parent alkane is preserved to prevent the collision of consonants (e.g., ethane-1,2-dithiol). When naming these compounds, it is necessary to provide a locant for each sulfhydryl group present. The numbering of the parent chain is chosen to provide the lowest possible set of locants for the $-SH$ groups.

Detailed Analysis of Problem 1

Problem 1 presents a structure with the fragments $CH_2CH_2CH_3$ , $CH_2CCH_3$ , and an $SH$ group. Based on these components, the molecule is identified as 2-methylpentane-2-thiol. The longest carbon chain containing the functional group consists of five carbons, establishing "pentane" as the parent alkane. The sulfhydryl group and a methyl substituent are both located at the second carbon position ( $C_2$ ) when numbering from the end closest to the functional group. Following IUPAC protocol, the name is constructed as 2-methylpentane-2-thiol.

Detailed Analysis of Problem 2

Problem 2 involves the fragments $HS$ and $SH$ . This indicates a molecule with two sulfhydryl functional groups, classified as a dithiol. In such cases, the systematic name is derived from the parent alkane chain connecting the two groups. If the groups are on adjacent carbons, the molecule is named ethane-1,2-dithiol ( $HSCH_2CH_2SH$ ). The primary rule demonstrated here is the use of the numerical locants for both groups and the addition of the "-dithiol" suffix to the full alkane name.

Detailed Analysis of Problem 3

Problem 3 shows the structure $CH_3CH(SH)CH_2CH(CH_3)CH_2CH_3$ . To name this molecule, the longest chain is identified as a six-carbon hexyl chain. Numbering begins from the left to give the $-SH$ group the lower locant of $2$ . This numbering scheme places a methyl substituent at carbon $4$ . Consequently, the IUPAC name is assembled as 4-methylhexane-2-thiol. This example highlights the priority of the $-SH$ group over alkyl substituents during the numbering process.

Detailed Analysis of Problem 4

Problem 4 provides the group $SH$ , which often refers to cyclic thiols or basic alkanethiols in these exercise sets. If the sulfhydryl group is attached to a cyclohexane ring, the name is cyclohexanethiol. In cyclic systems, the carbon atom attached to the $-SH$ group is automatically designated as carbon $1$ , and the locant is typically omitted from the name unless other substituents are present. If the structure is the simplest possible thiol, it is methanethiol ( $CH_3SH$ ).

Questions & Discussion

The transcript for Page 27 focuses on the exercise: "Name the following thiols using the IUPAC No" (IUPAC Nomenclature). The discussion involves four distinct problems that range from branched monothiols to dithiols. Key pedagogical points include:

Why is the "e" retained in thiols (e.g., pentanethiol) but not in alcohols (e.g., pentanol)? The "e" is retained because "thiol" starts with a consonant, whereas "ol" starts with a vowel.
How are dithiols numbered? They are numbered to give the lowest possible locants to the sulfhydryl groups, regardless of alkyl branch positions.
What is the priority of the $-SH$ group? In the absence of higher-priority groups like alcohols or carboxylic acids, the $-SH$ group determines the numbering of the parent chain.