Structure and Nomenclature of Thiols and Sulfides

Thiols and sulfides are sulfur analogs of alcohols and ethers, respectively, where sulfur replaces oxygen. Thiols contain the –SH (sulfhydryl) functional group bonded to a hydrocarbon, while sulfides have a central sulfur atom bonded to two hydrocarbon groups. Both exhibit bent geometry, but the larger sulfur atom creates longer C–S and S–H bonds and smaller bond angles compared to their oxygen counterparts.

Symmetrical sulfides have two identical hydrocarbon groups bonded to the central sulfur atom, while asymmetrical sulfides have two different groups. Both types follow the same structural pattern with sulfur at the center, but the identity of the flanking groups determines their classification and influences their chemical properties and nomenclature.

Common names of thiols are derived by adding the suffix 'mercaptan' to the name of the alkyl group bonded to the –SH group. For example, methanethiol is commonly called methyl mercaptan. This naming convention reflects the historical terminology used in organic chemistry for thiol compounds.

IUPAC names for thiols use the suffix 'thiol' appended to the parent hydrocarbon chain name while retaining the letter 'e'. The parent chain is numbered from the end closest to the –SH group, with locants indicating the position of the sulfhydryl group and any substituents. When the sulfhydryl group is a substituent, it is prefixed as 'mercapto' to the parent chain name.

Common names of sulfides are assigned by adding the suffix 'sulfide' to follow the names of the two alkyl groups listed in alphabetical order. For example, methyl ethyl sulfide contains a methyl and ethyl group bonded to sulfur. This straightforward naming approach reflects the two hydrocarbon substituents present in the molecule.

To derive IUPAC names of sulfides, the longest carbon chain is identified as the parent alkane. The smaller alkyl group containing the sulfur atom is named as an alkylthio substituent and prefixed to the parent alkane name. This systematic approach ensures consistent naming based on chain length and substituent position.

The C–S bond is longer than the C–O bond because sulfur is a larger atom than oxygen. Similarly, the S–H bond of thiols is longer than the O–H bond of alcohols due to sulfur's greater atomic radius. This size difference also results in smaller bond angles in thiols and sulfides compared to their oxygen analogs, affecting their bent geometry.