Thiol

A thiol (/ˈθaɪɒl/) is any organosulfur compound of the form R−SH, where R represents an alkyl or other organic substituent. Thiols are the sulfur analogue of alcohols (that is, sulfur takes the place of oxygen in the hydroxyl group of an alcohol), and the word is a portmanteau of 'thio-' + 'alcohol', with the first word deriving from Greek θεῖον (theion) meaning 'sulfur'. The –SH functional group itself is referred to as either a thiol group or a sulfhydryl group. A thiol (/ˈθaɪɒl/) is any organosulfur compound of the form R−SH, where R represents an alkyl or other organic substituent. Thiols are the sulfur analogue of alcohols (that is, sulfur takes the place of oxygen in the hydroxyl group of an alcohol), and the word is a portmanteau of 'thio-' + 'alcohol', with the first word deriving from Greek θεῖον (theion) meaning 'sulfur'. The –SH functional group itself is referred to as either a thiol group or a sulfhydryl group. Many thiols have strong odors resembling that of garlic or rotten eggs. Thiols are used as odorants to assist in the detection of natural gas (which in pure form is odorless), and the 'smell of natural gas' is due to the smell of the thiol used as the odorant. Thiols are sometimes referred to as mercaptans. The term 'mercaptan' /mərˈkæptæn/ was introduced in 1832 by William Christopher Zeise and is derived from the Latin mercurium captāns (capturing mercury) because the thiolate group (RS−) bonds very strongly with mercury compounds. Thiols and alcohols have similar connectivity. Because sulfur is a larger element than oxygen, the C−S bond lengths – typically around 180 picometres in length – is about 40 picometers longer than a typical C−O bond. The C−S−H angles approach 90° whereas the angle for the C−O−H group is more obtuse. In the solid or liquids, the hydrogen-bonding between individual thiol groups is weak, the main cohesive force being Van der Waals interactions between the highly polarizable divalent sulfur centers. The S−H bond is much weaker than the O−H bond as reflected in their respective bond dissociation energy (BDE). For CH3S−H, the BDE is 366 kJ/mol (87 kcal/mol), while for CH3O−H, the BDE is 440 kJ/mol (110 kcal/mol). Due to the small difference in the electronegativity of sulfur and hydrogen, an S−H bond is moderately polar. In contrast, O−H bonds in hydroxyl groups are more polar. Thiols have a lower dipole moment relative to the corresponding alcohol.