thiol
/ THY-ol /
Take an alcohol's hydroxyl group and swap the oxygen for sulfur, oxygen's bulkier cousin one row down the periodic table, and you get a thiol: an S-H group on carbon. If you have ever smelled rotten eggs, skunk spray, or the warning odour added to natural gas, you have met thiols. The nose detects them at astonishingly low concentrations.
A thiol is a functional group with the structure C-S-H, sometimes called a mercaptan or, when on a protein, a sulfhydryl group. Sulfur sits just below oxygen, so a thiol echoes an alcohol but with revealing differences. The S-H bond is weaker and longer than O-H, and sulfur is larger and more polarizable, so thiols are noticeably more acidic than alcohols and the sulfur is a much softer, stronger nucleophile. Hydrogen bonding through S-H is feeble, so thiols boil lower and smell volatile. A reaction worth knowing: two thiols can be oxidized together to form a disulfide bond, R-S-S-R, and this can be reversed by reduction.
Thiols are quietly essential in biology and chemistry. The amino acid cysteine carries a thiol, and disulfide bridges between cysteines staple proteins into their working shapes — the curl of hair and the folding of insulin both depend on them. In the lab, the strong, soft nucleophilicity of sulfur makes thiols valuable reagents. The infamous smell is just the most memorable trait, not the most important one.
Ethanethiol, CH3CH2SH, is added in tiny amounts to odourless natural gas so leaks can be smelled. In proteins, two cysteine S-H groups oxidize to a disulfide (-S-S-) that locks the folded shape in place.
An S-H group: pungent, more acidic than an O-H, and able to pair up into disulfide bonds.
A thiol is the sulfur analogue of an alcohol, but it is more acidic and a softer, stronger nucleophile — sulfur's larger, more polarizable atom, not its smell, drives its chemistry.