Thiol

Source: Wikipedia, the free encyclopedia.
Thiol with a   blue highlighted sulfhydryl group.

In

hydroxyl
(−OH) group of an alcohol), and the word is a blend of "thio-" with "alcohol".

Many thiols have strong odors resembling that of

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 (/mərˈkæptæn/)[3] or mercapto compounds,[4][5][6] a term introduced in 1832 by William Christopher Zeise and is derived from the Latin mercurio captāns ('capturing mercury')[7] because the thiolate group (RS−) bonds very strongly with mercury compounds.[8]

Structure and bonding

Thiols having the structure R−SH, in which an

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 energies (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).[10]

An S−H bond is moderately

dipole moment
relative to their corresponding alcohols.

Nomenclature

There are several ways to name the alkylthiols:

Physical properties

Odor

Many thiols have strong

Lawrence C. Katz and co-workers showed that MTMT functioned as a semiochemical, activating certain mouse olfactory sensory neurons, attracting female mice.[17] Copper has been shown to be required by a specific mouse olfactory receptor, MOR244-3, which is highly responsive to MTMT as well as to various other thiols and related compounds.[18] A human olfactory receptor, OR2T11, has been identified which, in the presence of copper, is highly responsive to the gas odorants (see below) ethanethiol and t-butyl mercaptan as well as other low molecular weight thiols, including allyl mercaptan found in human garlic breath, and the strong-smelling cyclic sulfide thietane.[19]

Thiols are also responsible for a class of

yeast
and the "skunky" odor of beer that has been exposed to ultraviolet light.

Not all thiols have unpleasant odors. For example, furan-2-ylmethanethiol contributes to the aroma of roasted coffee, whereas grapefruit mercaptan, a monoterpenoid thiol, is responsible for the characteristic scent of grapefruit. The effect of the latter compound is present only at low concentrations. The pure mercaptan has an unpleasant odor.

In the United States, natural gas distributors were required to add thiols, originally ethanethiol, to natural gas (which is naturally odorless) after the deadly New London School explosion in New London, Texas, in 1937. Many gas distributors were odorizing gas prior to this event. Most currently-used gas odorants contain mixtures of mercaptans and sulfides, with t-butyl mercaptan as the main odor constituent in natural gas and ethanethiol in liquefied petroleum gas (LPG, propane).[20] In situations where thiols are used in commercial industry, such as liquid petroleum gas tankers and bulk handling systems, an oxidizing catalyst is used to destroy the odor. A copper-based oxidation catalyst neutralizes the volatile thiols and transforms them into inert products.

Boiling points and solubility

Thiols show little association by

soluble in water and other polar solvents than alcohols of similar molecular weight. For this reason also, thiols and their corresponding sulfide functional group isomers
have similar solubility characteristics and boiling points, whereas the same is not true of alcohols and their corresponding isomeric ethers.

Bonding

The S−H bond in thiols is weak compared to the O−H bond in alcohols. For CH3X−H, the bond enthalpies are 365.07±2.1 kcal/mol for X = S and 440.2±3.0 kcal/mol for X = O.[21] Hydrogen-atom abstraction from a thiol gives a thiyl radical with the formula RS•, where R = alkyl or aryl.

Characterization

Volatile thiols are easily and almost unerringly detected by their distinctive odor. Sulfur-specific analyzers for

ammonium hydroxide
to give a red colour.

Preparation

In industry, methanethiol is prepared by the reaction of hydrogen sulfide with methanol. This method is employed for the industrial synthesis of methanethiol:

CH3OH + H2S → CH3SH + H2O

Such reactions are conducted in the presence of acidic catalysts. The other principal route to thiols involves the addition of hydrogen sulfide to alkenes. Such reactions are usually conducted in the presence of an acid catalyst or UV light. Halide displacement, using the suitable organic halide and sodium hydrogen sulfide has also been used.[23]

Another method entails the alkylation of sodium hydrosulfide.

RX + NaSH → RSH + NaX (X = Cl, Br, I)

This method is used for the production of thioglycolic acid from chloroacetic acid.

Laboratory methods

In general, on the typical laboratory scale, the direct reaction of a

isothiouronium salt, which is hydrolyzed in a separate step:[24][25]

CH3CH2Br + SC(NH2)2 → [CH3CH2SC(NH2)2]Br
[CH3CH2SC(NH2)2]Br + NaOH → CH3CH2SH + OC(NH2)2 + NaBr

The thiourea route works well with primary halides, especially activated ones. Secondary and tertiary thiols are less easily prepared. Secondary thiols can be prepared from the ketone via the corresponding dithioketals.[26] A related two-step process involves alkylation of thiosulfate to give the thiosulfonate ("Bunte salt"), followed by hydrolysis. The method is illustrated by one synthesis of thioglycolic acid:

ClCH2CO2H + Na2S2O3 → Na[O3S2CH2CO2H] + NaCl
Na[O3S2CH2CO2H] + H2O → HSCH2CO2H + NaHSO4

Organolithium compounds and Grignard reagents react with sulfur to give the thiolates, which are readily hydrolyzed:[27]

RLi + S → RSLi
RSLi + HCl → RSH + LiCl

Phenols can be converted to the thiophenols via rearrangement of their O-aryl dialkylthiocarbamates.[28]

Thiols are prepared by reductive dealkylation of sulfides, especially benzyl derivatives and thioacetals.[29]

Thiophenols are produced by S-arylation or the replacement of diazonium leaving group with sulfhydryl anion (SH−):[30][31]

ArN+
2 + SH− → ArSH + N2

Reactions

Akin to the chemistry of alcohols, thiols form

esters respectively. Thiols and alcohols are also very different in their reactivity, thiols being more easily oxidized than alcohols. Thiolates are more potent nucleophiles than the corresponding alkoxides
.

S-Alkylation

Thiols, or more specific their conjugate bases, are readily alkylated to give sulfides:

RSH + R′Br + B → RSR′ + [HB]Br  (B = base)

Acidity

Thiols are easily deprotonated.

pKa of 6, versus 10 for phenol. A highly acidic thiol is pentafluorothiophenol
(C6F5SH) with a pKa of 2.68. Thus, thiolates can be obtained from thiols by treatment with alkali metal hydroxides.

Synthesis of thiophenolate from thiophenol

Redox

Thiols, especially in the presence of base, are readily oxidized by reagents such as bromine and iodine to give an organic disulfide (R−S−S−R).

2 R−SH + Br2 → R−S−S−R + 2 HBr

Oxidation by more powerful reagents such as sodium hypochlorite or hydrogen peroxide can also yield sulfonic acids (RSO3H).

R−SH + 3 H2O2 → RSO3H + 3 H2O

Oxidation can also be effected by oxygen in the presence of catalysts:[33]

2 R–SH + 1⁄2 O2 → RS−SR + H2O

Thiols participate in thiol-disulfide exchange:

RS−SR + 2 R′SH → 2 RSH + R′S−SR′

This reaction is important in nature.

Metal ion complexation

With metal ions, thiolates behave as ligands to form transition metal thiolate complexes. The term mercaptan is derived from the Latin mercurium captans (capturing mercury)[7] because the thiolate group bonds so strongly with mercury compounds. According to hard/soft acid/base (HSAB) theory, sulfur is a relatively soft (polarizable) atom. This explains the tendency of thiols to bind to soft elements and ions such as mercury, lead, or cadmium. The stability of metal thiolates parallels that of the corresponding sulfide minerals.

Thioxanthates

Thiolates react with carbon disulfide to give thioxanthate (RSCS−
2).

Thiyl radicals

Main article:
aryl.[6] They arise from or can be generated by a number of routes, but the principal method is H-atom abstraction from thiols. Another method involves homolysis of organic disulfides.[34] In biology thiyl radicals are responsible for the formation of the deoxyribonucleic acids, building blocks for DNA. This conversion is catalysed by ribonucleotide reductase (see figure).[35] Thiyl intermediates also are produced by the oxidation of glutathione, an antioxidant in biology. Thiyl radicals (sulfur-centred) can transform to carbon-centred radicals via hydrogen atom exchange equilibria. The formation of carbon-centred radicals could lead to protein damage via the formation of C−C bonds or backbone fragmentation.[36]

Because of the weakness of the S−H bond, thiols can function as

free radicals.[37]

Biological importance

The catalytic cycle for ribonucleotide reductase, demonstrating the role of thiyl radicals in producing the genetic machinery of life.

Cysteine and cystine

As the functional group of the

quaternary structure of multi-unit proteins by forming fairly strong covalent bonds between different peptide chains. A physical manifestation of cysteine-cystine equilibrium is provided by hair straightening technologies.[38]

Sulfhydryl groups in the

heavy metal poisoning
.

Drugs containing thiol group

6-Mercaptopurine
(anticancer) Captopril (antihypertensive)
D-penicillamine
(antiarthritic) Sodium aurothiolate (antiarthritic)[39]

Cofactors

Many cofactors (non-protein-based helper molecules) feature thiols. The biosynthesis and degradation of fatty acids and related long-chain hydrocarbons is conducted on a scaffold that anchors the growing chain through a thioester derived from the thiol Coenzyme A. The biosynthesis of methane, the principal hydrocarbon on Earth, arises from the reaction mediated by coenzyme M, 2-mercaptoethyl sulfonic acid. Thiolates, the conjugate bases derived from thiols, form strong complexes with many metal ions, especially those classified as soft. The stability of metal thiolates parallels that of the corresponding sulfide minerals.

In skunks

The defensive spray of

skunks consists mainly of low-molecular-weight thiols and derivatives with a foul odor, which protects the skunk from predators. Owls are able to prey on skunks, as they lack a sense of smell.[40]

Examples of thiols

See also

References

  1. ^ Dictionary Reference: thiol Archived 2013-04-11 at the Wayback Machine
  2. ^ θεῖον Archived 2017-05-10 at the Wayback Machine, Henry George Liddell, Robert Scott, A Greek–English Lexicon
  3. ^ Dictionary Reference: mercaptan Archived 2012-11-13 at the Wayback Machine
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  8. ^ See:
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  19. ^ "Copper key to our sensitivity to rotten eggs' foul smell". chemistryworld.com. Archived from the original on 10 May 2017. Retrieved 3 May 2018.
  20. ^ Roberts, J. S., ed. (1997). Kirk-Othmer Encyclopedia of Chemical Technology. Weinheim: Wiley-VCH.[page needed]
  21. ^ Luo, Y.-R.; Cheng, J.-P. (2017). "Bond Dissociation Energies". In J. R. Rumble (ed.). Handbook of Chemistry and Physics. CRC Press.
  22. ^ Man, Pascal P. "Sulfur-33 NMR references". www.pascal-man.com. Archived from the original on 23 August 2017. Retrieved 3 May 2018.
  23. doi:10.1002/0471238961.2008091518150205.a01
  24. ^ Speziale, A. J. (1963). "Ethanedithiol". Organic Syntheses; Collected Volumes, vol. 4, p. 401..
  25. doi:10.15227/orgsyn.021.0036
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  26. ^ S. R. Wilson, G. M. Georgiadis (1990). "Mecaptans from Thioketals: Cyclododecyl Mercaptan". Organic Syntheses; Collected Volumes, vol. 7, p. 124..
  27. ^ E. Jones and I. M. Moodie (1990). "2-Thiophenethiol". Organic Syntheses; Collected Volumes, vol. 6, p. 979..
  28. ^ Melvin S. Newman and Frederick W. Hetzel (1990). "Thiophenols from Phenols: 2-Naphthalenethiol". Organic Syntheses; Collected Volumes, vol. 6, p. 824..
  29. ^ Eliel, Ernest L.; Lynch, Joseph E.; Kume, Fumitaka; Frye, Stephen V. (1993). "Chiral 1,3-oxathiane from (+)-Pulegone: Hexahydro-4,4,7-trimethyl-4H-1,3-benzoxathiin". Organic Syntheses; Collected Volumes, vol. 8, p. 302.
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  35. PMID 12797828
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  36. PMID 20882988
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  37. ISBN 978-1-4160-5897-7
    . Sulfhydryls are scavengers of free radicals, protecting chemical damage induced by either ionizing radiation or alkylating agents.
  38. ^ Reece, Urry; et al. (2011). Campbell Biology (Ninth ed.). New York: Pearson Benjamin Cummings. pp. 65, 83.
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  40. ^ "Understanding Owls – The Owls Trust". theowlstrust.org. Archived from the original on 5 February 2018. Retrieved 3 May 2018.

External links

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