sulfide
/ SUL-fide /
A sulfide is the sulfur analog of an ether: take an ether's bridging oxygen, R-O-R', and swap it for a sulfur atom to get R-S-R'. Sulfur sits directly below oxygen in the periodic table, so it forms the same two single bonds, but its larger, softer, more polarisable atom gives sulfur compounds a distinct personality. A sulfide is also called a thioether (the prefix thio- always signals sulfur replacing oxygen).
The closely related thiol is the sulfur version of an alcohol: R-S-H instead of R-O-H, sometimes called a mercaptan. Two practical differences from their oxygen cousins stand out. First, sulfur compounds are smelly — the rotten-egg odour of hydrogen sulfide, the skunk's spray, the warning smell added to natural gas, and the aroma of garlic and onions all come from thiols and sulfides. Second, sulfur is a much better nucleophile than oxygen and its lone pairs are more reactive, so sulfides readily react further (for instance with an alkyl halide to give a positively charged sulfonium salt), where an ether would just sit there inert.
Sulfur's most important role is biological, in proteins. The amino acid cysteine carries a thiol (-SH), and two cysteine thiols can be oxidised together to form a disulfide bond (-S-S-), a covalent cross-link that staples a folded protein into shape. These disulfide bridges hold together insulin, antibodies, hair keratin, and countless enzymes; breaking and re-forming them is literally what a permanent-wave hair treatment does. The thioether in the amino acid methionine, meanwhile, is a key methyl-group carrier in metabolism.
Dimethyl sulfide, CH3-S-CH3, is the ether analog with S in place of O; ethanethiol, CH3-CH2-S-H, is the alcohol analog (the smell added to natural gas). Two cysteine -SH groups oxidise to a disulfide -S-S- that locks a protein's shape.
Swap an ether's O for S and you get a sulfide — bigger, softer, more nucleophilic, and smellier.
Sulfur, being larger and more polarisable, is a stronger nucleophile but a weaker base than oxygen; and unlike ether oxygen it is easily oxidised — to a sulfoxide, then a sulfone — which oxygen ethers do not do.