# Thiols

General structure of thiols with the blue marked thiol group (mercapto group)

Thioalcohols are organic-chemical compounds that have one or more aliphatically or aromatically bound thiol groups (also mercapto groups ) (−SH) as functional groups . Thioalcohols correspond to alcohols whose oxygen atom has been replaced by a sulfur atom. Just as alcohols and phenols are formally derived from water (H 2 O), thioalcohols and thiophenols are derivatives of hydrogen sulfide (H 2 S). Alkanethiols are derived directly from alkanes with a thiol group. Together with the thiophenols, thioalcohols form the group of thiols.

## nomenclature

Due to their ability to bind mercury (II) ions, thioalcohols are also called mercaptans (from the Latin Mercurium captans: catching mercury). The names of individual alkanethiols are formed from the corresponding name of the alkane and the suffix -thiol , e.g. methanethiol, ethanethiol, etc. For molecules with a functional group of higher priority, the prefix mercapto- (or sulfanyl- , also sulfhydryl- ) is used. In addition, many alkanethiols have common names , e.g. B. ethyl mercaptan for ethanethiol.

## Manufacturing

Through nucleophilic substitution on alkyl halides , alkyl sulfonates or alkyl sulfates with hydrogen sulfides :

${\ displaystyle {\ ce {RX + SH- -> R-SH + X-}}}$
${\ displaystyle {\ ce {X = Cl, Br, I, R-SO3 ^ {-}, RO-SO3 ^ {-}, SO4 ^ {2-}}}}$

Thioethers are produced as by-products :

${\ displaystyle {\ ce {R-SH + SH- <=> RS- + H2S}}}$
${\ displaystyle {\ ce {RS- + RX -> RSR + X-}}}$

The formation of thioethers can be suppressed by using an excess of hydrogen sulfide.

Free of thioethers obtained thioalcohols by alkylation of thiourea with alkyl halides, followed by heating with aqueous sodium hydroxide solution . An isothioronium salt is formed here as an intermediate:

The synthesis can also take place by reacting alcohols with hydrogen sulfide on aluminum oxide as a catalyst :

${\ displaystyle {\ ce {R-OH + H2S -> R-SH + H2O}}}$

Thiols are also formed by sulfhydrogenation of terminal olefins with hydrogen sulfide:

${\ displaystyle {\ ce {R-CH = CH2 + H2S -> R-CH2CH2SH}}}$

## properties

### Chemical properties

As homologues of alcohols - sulfur is in the same main group as oxygen - thioalcohols enter into similar reactions. Their salts are called thiolates ; the anions are good nucleophiles . Furthermore, they are reducing agents that are widely used, for example, in protein chemistry .

Since the SH bond is weaker than the OH bond, thioalcohols are more acidic than the analogous alcohols and form thiolates even in aqueous NaOH solution. Furthermore, unlike alcohols, thiols can be oxidatively dimerized to disulfides ; This is important for proteins , in which two amino acid chains on cysteine may be linked units, where cystine - derivatives arise.

### Physical Properties

Thioalcohols boil lower than the analogous alcohols, because sulfur forms weaker hydrogen bonds due to its lower electronegativity to hydrogen.

### toxicity

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

## Occurrence

Western blotchy skunk ( Spilogale gracilis ), the anal secretion of which contains thioalcohols.
Thio alcohols in the glandular secretions of the skunk.

Thioalcohols u. a. in milk , cheese , onions and garlic . They are released from organic material during degradation and putrefaction processes , e.g. B. in biogas plants . Several thioalcohols are found in the skunk's glandular secretions . Further examples of the occurrence of thiols are crude oil and the condensate from natural gas production. The natural gas condensate from the South Pars gas field developed by Qatar and Iran can contain up to 3000 ppm of mercaptans.

## application

Linear thiols in particular, such as hexadecanethiol , have recently been used increasingly in the construction of self-assembling monolayers ( self-assembling monolayers , SAM). The high affinity of the thiol group for coin metals , especially gold , means that the thiols spontaneously assemble in a highly ordered layer when a gold substrate is exposed to a thiol solution. If the thiols are suitably functionalized, for example with ss DNA or proteins, this system forms the basis for biosensors (Biacore). The metal-complexing property is used in various antidotes against heavy metal poisoning (e.g. lead , mercury or cadmium ). Another antidotic effect is 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-2-propanethiol ( tert TBM -Butylmercaptan,) as odorant added. They create the “gas smell” because natural gas or methane themselves are odorless and contribute to the safe use of these fuels . One example of this is the unintentional release of the odorant on January 16, 2001 in Bern's city ​​laboratory , after which the old town was closed to traffic due to the smell of gas .