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Thiols

Some thiols are used to impart their odor to combustible gases, in order to detect a leak of these gases, which given their characteristics can be very dangerous. Thiols have similarity to alcohols, since they have an OH group, while thiols have a SH group.

Thiol with sulfhydryl group in blue

However, they also have several differences: the OH group is more polar than the SH group, and sulfur is an atom smaller than oxygen and less electronegative.

This determines that the molecules of the thiols do not form hydrogen bonds neither with the water nor with each other. Therefore, the solubility of thiols in water is low and their boiling point is lower than that of alcohols with a similar carbon chain.

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Properties of thiols

The smelly scent of skunks is mainly made up of thiols

Generalities

The OH group of alcohols and the SH group of thiols share a set of physical and chemical properties because oxygen and sulfur belong to the same chemical group in the Periodic Table (group VIA).

Thiols form thioethers, thioacetals, and thioesters, which are analogous to chemical compounds formed by alcohols, such as ethers, acetals, and esters. However, the SH and OH bonds present some differences that will be reflected in the properties of thiols and alcohols.

Odor

A characteristic of low molecular weight thiols is their unpleasant odor, similar to garlic and onion. This odor is of such magnitude that the human smell has a threshold of one part of thiol per ten billion parts of air.

Ethanethiol is added to natural gas as a safety measure, thus detecting any flammable but odorless leaks in and of itself. The odor of thiols decreases as the number of carbons in their chain increases, due to the decrease in volatility and the proportion of sulfur in the thiol.

The SH bond of thiols has a lower dipole moment than the OH bond of alcohols. Furthermore, the sulfur atom is smaller in size and less electronegativity than the oxygen atom.

This determines that the molecules of the thiols cannot form hydrogen bonds neither with the water nor with each other, causing a difference between the physical properties of the thiols and the alcohols. For example: methanethiol (CH 3 SH) has a boiling point of 6ºC , while methanol (CH 3 OH) boils at 65ºC.

Therefore, at room temperature (25 ° C), methanol is a liquid and methanethiol is a gas.

In summary: thiols have a lower boiling point than the corresponding alcohols. For the same reasons, the solubility of thiols in water and other polar solvents is very low. Meanwhile, low molecular weight alcohols are very soluble in water.

Reactivity

The SH bond is weaker than the OH bond, so the SH bond dissociates more easily, making thiols stronger acids than alcohols. Most alcohols have a pKa between 16 and 18, while thiols have a pKa around 11.

This allows thiols to be easily dissolved by sodium hydroxide to be converted to its conjugated base thiolate (RS  ): a very strong nucleophilic agent.

The SH group can be oxidized to form disulfide bonds (-SS-) that are present in the oxidation of glutathione: a thiol present in cells and that has an antioxidant function.

Bromine or other halogens, as well as oxygen, can act on a thiol to form disulfide bridges:

2 RSH + Br → RSS-R ‘+ 2 HBr

The RSSR ‘compounds are known as organic sulfides and alkylthiols, R 2 S.

On the other hand, strong oxidizing agents, such as sodium hypochlorite and hydrogen peroxide, can act on thiols to produce sulfonic acids:

RSH + H 2 O 2    → RSO 3 H + 3 H 2 O

Nomenclature

The IUPAC nomenclature for thiols is very similar to that of alcohols: the name of the alkane from which it derives is added the suffix ‘thiol’.

Thus, for example, the thiol derived from butane is called 1-butanethiol, CH 3 CH 2 CH 2 CH 2 SH, indicating with the number the position of the SH group on the carbons of the chain.

Similarly, there is another nomenclature, again, similar to that of alcohols: the word ‘alcohol’ is replaced by ‘mercaptan’ in the names of alcohols. For example, CH 3 SH, analogous to methyl alcohol, CH 3 OH, is called methyl mercaptan or methyl mercaptan.

Another example would be CH 3 CH 2 CH (SH) CH 3 , called 2-butanethiol, sec-butyl mercaptan, or sec-butyl mercaptan. That is, the identity of R is emphasized in the formula RSH. Hence, the other nomenclature standards for alcohols are also met for thiols.

And finally, when the SH group is present in a larger structure or where more important groups predominate, the prefix sulfhydryl or mercapto is used.

For example, the compound CH 3 CH 2 CH (SH) COCH 3 is called 3-mercapto-2-pentanone. Note that SH has a lower priority than oxygenated groups (with the exception of ether), so it usually corresponds to lower numerals.

Synthesis

One method of synthesis of thiols consists of the reaction of thiourea with an alkyl halide, which produces an intermediate salt of isothiouronium. This is hydrolyzed by sodium hydroxide to obtain thiol and urea.

CH 3 SH is prepared industrially by the reaction of hydrogen sulfide with methanol, in the presence of acid catalysts:

CH 3 OH + H 2 S → CH 3 SH + H 2 O

In another production method, an alkyl halide is reacted with sodium hydrosulfide:

RX + NaSH → RSH + NaX

Applications

Fixation of enzymes bound to nanoparticles

Thiols are used as bridges to attach nanoparticle-enzyme complexes to gold electrodes. For example, thiols are used to fix the nanoparticle-uricase complex, an enzyme that intervenes in the conversion of uric acid to allantoin.

Thiol-ene reaction

It is an organic reaction between a thiol and an alkene, called a “click” reaction; Among other characteristics, it is a high-performance, wide-ranging reaction that creates by-products that can be removed by non-chromatographic methods, and is also easy to perform.

The thiol-ene reaction has applications in the synthesis of thiosugar: sugar present in salicinol, an inhibitor of α-glucosidases. Salicinol has been isolated from plants used in Sri Lanka and India in the treatment of diabetes.

Additionally, the thiol-ene reaction is used in carbohydrate chemistry, polymerizations, surface chemistry, synthetic chemistry, and peptide chemistry.

Odorants

Cooking gas is odorless, but thanks to its composition of low molecular weight thiols it has a distinctive odor

Low molecular weight aliphatic thiols are incorporated into natural gases so that their strong odor serves as a warning against a gas leak or leak.

Medicine

Penicillamine (C 5 H 11 NO 2 S) is a drug used in the treatment of Wilson’s disease and rheumatoid arthritis.

And captopril (C 9 H 15 NO 3 S), on the other hand, is used in the treatment of high blood pressure and congestive heart disease.

Both are examples of thiols with medicinal applications.

Biological

Thiol groups are present, among other compounds with biological function, in cysteine, coenzyme A and glutathione.

Cysteine ​​is an amino acid capable of forming disulfide bridges in its protein chain, being therefore responsible for the tertiary structure of a protein. Likewise, it can form disulfide bridges between different protein chains, originating the quaternary structure of proteins.

Cysteine ​​is found in the active centers of numerous enzymes and also has an antioxidant action.

Glutathione is a thiol that has a very important intracellular antioxidant action, conferring protection to the cells and the macromolecules that compose it.

Coenzyme A, on the other hand, is a thiol that is involved in the synthesis and oxidation of fatty acids. In addition, coenzyme A in the form of acetylcoenzyme A, intervenes in the initiation of the citric acid cycle or Krebs cycle.

Examples

Finally, some names of thiols will be listed together with their molecular formulas:

-Tioacetic acid (C 2 H 4 OS)

-Coenzyme A (C 21 H 36 N 7 O 16 P 3 S)

-Reduced glutathione (C 10 H 17 N 3 O 6 S)

-Cysteine ​​(C 3 H 7 NO 2 S)

-Mercaptoethanol (C 2 H 6 OS)

-Methanethiol (CH 3 SH)

-1-Propanethiol (C 3 H 7 SH)

-Butanethiol (C 4 H 9 SH)

-Tiophenol or thiomerosal (C 6 H 6 S)

-Grapefruit mercaptan (C 10 H 18 S)

-Hexadecanethiol (C 6 H 34 S)

-Penicillamine (C 6 H 11 NO 2 S)

-Captopril (C 9 H 15 NO 3 S)

-Ethanethiol (C 2 H 6 S)

-2-propenethiol (C 3 H 6 S)

-Α-lipoic acid or lipoamide (C 8 H 14 O 2 S 2 ). It is a cyclic disulfide.

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