Thiol

Thioalcohols are organic chemical compounds that have one or more aliphatic or aromatic -bound thiol groups (-SH ) functional groups. Thioalcohols corresponding alcohols, whose oxygen atom is replaced by a sulfur atom. How alcohols and phenols formally from water ( H2O) derived which thioalcohols and thiophenol derivatives of hydrogen sulfide are (H2S). Alkanethiols are directly derived from the alkanes with thiol group. Thioalcohols thiophenols, together with the group of thiols.

Nomenclature

Thioalcohols called because of their ability to bind mercury (II ) ions, mercaptans (from Latin Mercurium captans: Mercury catching ). The names of individual alkane thiols are formed from the name of the corresponding alkane and the suffix -thiol, such as methanethiol, ethanethiol, etc. For molecules with a functional group of higher priority, the prefix mercapto (or thiol, outdated and sulfhydryl ) will be used. In addition, many alkanethiols trivial names, such as ethyl mercaptan for ethanethiol.

Production

By nucleophilic substitution of alkyl halides, alkyl sulfonates or sulfates with hydrogen sulfides:

Byproducts thioether here:

Formation of thioethers can be suppressed by an excess of hydrogen sulfide.

Free of thioethers obtained thioalcohols by alkylation of thiourea with alkyl halides and subsequent heating with sodium hydroxide. As an intermediate here forms a Isothioronium salt:

The synthesis can also be effected by reaction of alcohols with hydrogen sulfide over aluminum oxide as a catalyst:

Properties

Chemical Properties

As homologues of alcohols - sulfur is in the same major group as oxygen - go thioalcohols a similar reactions. Its salts are called thiolates; the anions are good nucleophiles. Furthermore, they are reducing agents, for example, often used in protein chemistry.

Since the SH bond is weaker than the OH bond, thioalcohols are more acidic than the analogous alcohols and form already in aqueous NaOH solution thiolates. Furthermore, thiols can be oxidatively dimerize to disulfides in contrast to alcohols; This is important in the case of proteins in which two amino acid chains can be linked via the cysteine ​​units, cystine derivatives are formed.

Physical Properties

Thioalcohols lower boiling point than the analogous alcohols, because sulfur does not form hydrogen bonds due to the lower electronegativity of the hydrogen.

Toxicity

Volatile thiols, especially the short-chain homologues, particularly obnoxious smell. They have a toxic effect on the central nervous system.

Occurrence

As flavorings can be found thioalcohols inter alia, in milk, cheese, onions and garlic. They are free with degradation and putrefaction of organic material, such as biogas plants. Several thioalcohols occur in the glandular secretion of the skunk. Other examples of the presence of mercaptans and the condensate from the crude natural gas. The natural gas condensate from the developed by Qatar and Iran South Pars gas field can contain up to 3000 ppm of mercaptans.

Examples of thioalcohols

• Acetylcysteine
• Dimercaptosuccinic
• Dimercaptopropanesulfonic acid
• Ethanethiol ( ethyl mercaptan )
• Dithiothreitol (DTT)
• Dithioerythritol (DTE)
• Captopril
• Coenzyme A
• Cysteine
• Penicillamine
• 1- propanethiol
• 2- propanethiol
• Glutathione
• Homocysteine
• Mesna
• Methanethiol ( methyl mercaptan )
• Thiophenol

Application

Especially linear thiols such hexadecanethiol find in recent times increasingly being used in the construction of self-assembling monolayers ( Self Assembling Monolayers SAM). The high affinity of the thiol group to coinage metals, especially gold, leads to the fact that the thiols assemble spontaneously in a highly ordered layer when a gold substrate of a thiol solution is exposed. Thiols are suitable functionalized, such as ssDNA or proteins, this system forms the basis for biosensors (Biacore ). The metal-chelating property is used in various antidotes against heavy metal poisoning (eg lead, mercury or cadmium). Another antidotischer effect lies in the reducing property of the sulfhydryl group.

In addition, combustible gases such as natural gas or camping gas mixtures of methane thiol, ethane thiol, propane-1 -thiol and 2-methyl -propane -2-thiol (tert -butyl mercaptan, TBM ) is added as odorants. You create the " gas smell " because natural gas or methane are odorless itself, and contribute to the safe use of these fuels in. One example is the accidental release of the odorant on 16 January 2001 in the city of Bern laboratory, after which the old town was closed due to gas smell to traffic.