Mercaptobenzothiazole

File:Mercaptobenzothiazole equilibrium.svg

The molecule is planar with a C=S double bond, so the name mercaptobenzothiazole is a misnomer, a more appropriate name could be benzothiazoline-2-thione. Solution measurements by NMR spectroscopy could not measure the presence of the thiol tautomer that the name implies, instead it exists as a thione/dithiocarbamate and the hydrogen appears on the nitrogen in the solid state, gas-phase, and in solution.{{cite journal |last1=Chesick |first1=J. P. |last2=Donohue |first2=J. |year=1971 |title=The Molecular and Crystal Structure of 2-Mercaptobenzothiazole |journal=Acta Crystallographica Section B: Structural Crystallography and Crystal Chemistry |volume=27 |issue=7 |pages=1441–1444 |doi=10.1107/S0567740871004102}} Theory indicates that the thione tautomer is about 39 kJ/mol lower in energy than the thiol, and a hydrogen-bonded dimer of the thione has even lower energy.{{cite journal |last1=Wu |first1=Feng-Ling |last2=m. Hussein |first2=Waleed |last3=p. Ross |first3=Benjamin |last4=p. Mcgeary |first4=Ross |year=2012 |title=2-Mercaptobenzothiazole and its Derivatives: Syntheses, Reactions and Applications |journal=Current Organic Chemistry |volume=16 |issue=13 |pages=1555–1580 |doi=10.2174/138527212800840964}} At alkaline pH greater than 7 the deprotonated thiolate form is most abundant. A protonated form could not be observed in the pH range 2-11.{{Cite journal |last1=Galvão |first1=Tiago L. P. |last2=Kuznetsova |first2=Alena |last3=Gomes |first3=José R. B. |last4=Zheludkevich |first4=Mikhail L. |last5=Tedim |first5=João |last6=Ferreira |first6=Mário G. S. |date=March 2016 |title=A computational UV–Vis spectroscopic study of the chemical speciation of 2-mercaptobenzothiazole corrosion inhibitor in aqueous solution |url=http://link.springer.com/10.1007/s00214-016-1839-3 |journal=Theoretical Chemistry Accounts |language=en |volume=135 |issue=3 |doi=10.1007/s00214-016-1839-3 |s2cid=102219996 |issn=1432-881X}}

The compound has been produced by many methods. The industrial route entails the high temperature reaction of aniline and carbon disulfide in the presence of sulfur, which proceeds by this idealized equation:{{cite book |doi=10.1002/047084289X.rn00575 |chapter=Benzothiazole-2-Thiol |title=Encyclopedia of Reagents for Organic Synthesis |date=2005 |last1=Bashiardes |first1=George |isbn=0-471-93623-5 }}

:{{chem2|C6H5NH2 + CS2 + S → C6H4(NH)SC\dS + H2S}}

The traditional route is the reaction of 2-aminothiophenol and carbon disulfide:

:{{chem2|C6H4(NH2)SH + CS2 → C6H4(NH)SC\dS + H2S}}

This method was developed by the discoverer of the compound, A. W. Hoffmann. Other routes developed by Hoffmann include the reactions of carbon disulfide with 2-aminophenol and of sodium hydrosulfide with chlorobenzothiazole.{{cite journal|title=Zur Kenntniss des o-Amidophenylmercaptans |pages=1788–1797|journal=Chem. Ber.|year=1887|volume=20|author=A. W. Hofmann|doi=10.1002/cber.188702001402|url=https://zenodo.org/record/1425477}} Further synthetic advances were reported in the 1920s that included demonstration that phenyldithiocarbamates pyrolyze to benzothiazole derivative.{{cite journal|author=Sebrell, L. B. |author2=Boord, C. E.|title=Preparation and properties of 1-mercaptobenzothiazole, its homologs and derivatives|journal=J. Am. Chem. Soc.|year=1923|volume=45|issue=10|pages=2390–2399|doi=10.1021/ja01663a023|bibcode=1923JAChS..45.2390S }}

The compound is insoluble in water but dissolves upon the addition of base, reflecting deprotonation.{{cite book |doi=10.1002/047084289X.rn00575|chapter=Benzothiazole-2-Thiol |title=Encyclopedia of Reagents for Organic Synthesis |year=2005 |last1=Bashiardes |first1=George |isbn=0471936235 }}

Treatment with Raney nickel results in monodesulfurization, giving benzothiazole:

:{{chem2|C6H4(NH)SC\dS + Ni → C6H4(N)SCH + NiS}}

The benzo ring undergoes electrophilic aromatic substitution at the position para to nitrogen.

S-alkylation of mercaptobenzothiazole gives thioether, which can be oxidized to the sulfone. This sequence sets the stage for the Modified Julia olefination.

Oxidation gives mercaptobenzothiazole disulfide. This disulfide reacts with amines to give sulfenamide derivatives such 2-morpholinodithiobenzothiazole. These compounds are used in sulphur vulcanization, where they act as accelerators.

File:Di(benzothiazool-2-yl)disulfide.svg|Mercaptobenzothiazole disulfide (MBTS)

File:N,N-dicyclohexyl-2-benzothiazolesulfenamide.svg|Dicyclohexyl-2-benzothiazolesulfenamide (DCBS)

File:Sodium mercaptobenzothiazole.svg|Sodium mercaptobenzothiazole

Using 2-mercaptobenzothiazole, rubber vulcanizes with less sulfur and at milder temperatures, both factors give a stronger product. This effect was reported by workers at Pirelli and at Goodyear Tire & Rubber. Lorin B. Sebrell won the 1942 Charles Goodyear Medal for his work on mercaptobenzothiazole.

In polymerization, it finds use as a radical polymerization inhibitor, chain transfer agent, reforming agent, and additive for photoinitiators.{{Cite web|url=https://www.parchem.com/news-articles/2-Mercaptobenzothiazole-the-Hemi-Ultra-Vulcanization-Accelerator-N000170.aspx|title = 2-Mercaptobenzothiazole: The Hemi-Ultra Vulcanization Accelerator}}

The compound has also been used in the past in the gold-mining industry for the froth flotation of gold from ore residue as part of the extraction process.{{cite journal |author= CABASSI, PAJ |display-authors=etal |title=The improved flotation of gold from the residues of Orange Free State ores|journal=Journal of the South African Institute of Mining and Metallurgy |volume=83 |issue=11|pages=270–276|date=November–December 1983|issn=0038-223X|url=http://reference.sabinet.co.za/webx/access/journal_archive/0038223X/1417.pdf}}

Sodium salt is used as a biocide and preservative in adhesives (especially based on latex, starch, casein, and animal glues), paper, textiles. Often found together with sodium dimethyldithiocarbamate as e.g. Vancide 51. Zinc salt is used as a secondary accelerator in latex foam vulcanization.{{Cite book | url=https://books.google.com/books?id=pKrBNbkE2c0C&q=mercaptobenzothiazole+use&pg=PA877 | title=Handbook of Green Chemicals| isbn=9781890595791| last1=Ash| first1=Michael| year=2004| publisher=Synapse Info Resources}}

It can be added to oil-based hydraulic fluids, heat-transfer fluids (oils, antifreezes), cutting fluids and other mixtures as a corrosion inhibitor, effective for copper and copper alloys.{{Cite web|url=https://www.irowater.com/2-mercaptobenzothiazole-mbt-uses/|title = 2 Mercaptobenzothiazole (MBT) Uses|date = 25 June 2019}}

It is also used in veterinary dermatology.{{Cite web | url = https://www.drugs.com/international/mercaptobenzothiazole.html | title = Mercaptobenzothiazole | publisher = drugs.com }}

In electroplating it is used as a brightener for copper sulfate baths, at about 50-100 milligrams/liter. Also can be added to silver cyanide baths.

Mercaptobenzothiazole has a low toxicity in mice, with LD50 of >960 mg/kg.

In 2016, it was identified by the World Health Organization as probably carcinogenic to humans.{{cite web |url=https://www.telegraph.co.uk/news/health/news/12176416/Chemical-found-in-babies-dummies-and-condoms-probably-causes-cancer.html |title=Chemical found in babies' dummies and condoms 'probably causes cancer'|author=Chris Graham |date=February 28, 2016 |website=The Telegraph |access-date=February 29, 2016 }}

It causes allergic contact dermatitis.{{citation |url=http://www.worksafebc.com/contact_us/research/funding_decisions/assets/pdf/2009/RS2009-OG10.pdf |title=British Columbia Fails to Meet the North American Screening Standards: What are the Implications for Workers with Allergic Contact Dermatitis? |author1=Gillian de Gannes |author2=Sayali Tadwalkar |author3=Aaron Wong |author4=Nino Mebuke |name-list-style=amp |year=2013 |publisher=WorkSafeBC |url-status=dead |archive-url=https://web.archive.org/web/20160109114827/http://www.worksafebc.com/contact_us/research/funding_decisions/assets/pdf/2009/RS2009-OG10.pdf |archive-date=2016-01-09 }} The derivative morpholinylmercaptobenzothiazole is a reported allergen in protective gloves, including latex, nitrile, and neoprene gloves.{{citation |author1=Rose, R.F. |author2=Lyons, P. |author3=Horne, H. |author4=Wilkinson, S.M. |year=2009 |title=A review of the materials and allergens in protective gloves |journal=Contact Dermatitis |volume=61 |issue=3 |pages=129–137 |doi=10.1111/j.1600-0536.2009.01580.x|pmid=19780770 |s2cid=25877257 }}

It becomes air-borne as a result of wear on car tires, and is able to be inhaled.{{citation |author1=Avagyan, R. |author2=Sadiktsis, I. |author3=Bergvall, C. |author4=Westerholm, R. |year=2014 |title=Tire tread wear particles in ambient air—a previously unknown source of human exposure to the biocide 2-mercaptobenzothiazole |journal=Environmental Science and Pollution Research |volume=21 |issue=19 |pages=11580–11586 |doi=10.1007/s11356-014-3131-1|pmid=25028318 |s2cid=9147927 }} Evidence suggests that benzothiazoles biodegrade readily.{{cite journal |doi=10.1016/j.envint.2010.06.004 |title=Review of 'emerging' organic contaminants in biosolids and assessment of international research priorities for the agricultural use of biosolids |date=2011 |last1=Clarke |first1=Bradley O. |last2=Smith |first2=Stephen R. |journal=Environment International |volume=37 |issue=1 |pages=226–247 |pmid=20797791 |bibcode=2011EnInt..37..226C }}

Benzothiazoles were long ago found to strongly influence the course of the vulcanization of rubber.{{cite journal |last1=Geer |first1=W. C. |last2=Bedford |first2=C. W. |date= January 24, 1925 |title= The History of Organic Accelerators in the Rubber Industry|journal= Industrial and Engineering Chemistry|volume= 17|issue= 4|pages= 393–396 |doi=10.1021/ie50184a021 |author1-link=William C. Geer }}

{{Reflist}}

{{Authority control}}

Category:Benzothiazoles

Category:Rubber

Category:Dithiocarbamates

Category:Veterinary drugs

Category:IARC Group 2A carcinogens