Are thiols poisonous

There are compounds of sulfur that are very similar to those of alcohols. There are different terms for this class of compounds: One calls it Thiols, sulfhydryls or Mercaptans.

What distinguishes thiols structurally from alcohols is that they have a sulfur atom instead of an oxygen atom. Your general formula is therefore instead of R-OH

R-SH

The thiol group (-SH) is also known as the sulfhydryl or mercapto group. If we compare alcohol with water in terms of structure and properties, we have to think of hydrogen sulfide when we think of thiols. The name mercaptan also reminds us of this. That comes from the Latin mercurium captans = Catching / filling mercury. Because many thiols (like H2S) compounds that are difficult to dissolve with heavy metal ions (Pb, Zn, Hg), in this case a mercury salt, which is deposited as a precipitate.
Another characteristic of thiols is their extremely unpleasant odor, which suggests the presence of sulfur. By the way, you can read here what organic sulfur compounds have to do with skunk, hangover, roast coffee and lily plants.


Thiols have low boiling points
In contrast to the alcohols, the mercaptans have much lower boiling points. For example, methyl mercaptan (CH3SH) at 6 C. In comparison, methanol has a boiling point of 65 C. The reason for this is very simple: Compared to oxygen, sulfur has a less electronegative character, which is why the mercaptan molecules associate less. The S ∙∙∙∙ H-S hydrogen bonds are much weaker than those of oxygen and their bond is easy to break. That is also the reason why they are so easily volatile.
In addition, they are much less water-soluble than alcohols, as they can hardly form stable hydrogen bonds to oxygen.


Thiols form disulfide bridges
In proteins, an interplay between the formation and dissolution of disulfide bridges plays an important role. They contribute to the chain conformation of the α-helix structure and the β-sheet structure and can, among other things, form compact protein molecules through interactions between the side chains of the amino acids. The resulting tertiary structure gives the proteins their final three-dimensional shape.
To form disulfide bridges from thiols, only a little oxygen is required:

2 R-S-H + O2 ———> R-S-S-R + H2O

This reaction can be reversed by reduction. Incidentally, the splitting and formation of the -S-S linkage is also responsible for the skin formation on the milk.


Representation of mercaptans
To synthesize mercaptans, alkyl halides are reacted with potassium or sodium hydrogen sulfide. The type of reaction is a nucleophilic substitution.

R-Cl + NaSH --——> R-SH + NaCl

It should be noted that both raw materials must first be produced. Also, you will only get a good yield if the alkyl group (-R) is primary.


use
Mercaptans serve as chemical intermediates and are added in small quantities to gases that are odorless. Due to the terrible smell of these compounds, toxic gases that otherwise cannot be smelled can be perceived even in very small added concentrations of the thiol.


The descendants of the thiols
Thioester
Analogous to the derivatives of the alcohols, there are also derivatives of the thiols. One of them is the thioester. These are ester-like compounds in which the oxygen atom of the ester bond has been replaced by a sulfur atom. It looks like this:

An important thioester is e.g. B. the acetyl-CoA.

Thioether
Another derivative is the thioether. These are ether-like compounds in which the oxygen atom has also been replaced by a sulfur atom. The general formula is R.1-S-R2. The amino acid methionine is a thioether.