Editing Thiol (section)

==Physical properties==
===Odor===
Many thiols have strong [[odor]]s resembling that of [[garlic]]. The odors of thiols, particularly those of low molecular weight, are often strong and repulsive. The spray of [[skunk]]s consists mainly of low-molecular-weight thiols and derivatives.<ref>{{cite journal|journal=Journal of Chemical Ecology|volume=1|issue=4|year=1978|title=Some Chemical Constituents of the Scent of the Striped Skunk (''Mephitis mephitis'')|author1=Andersen K. K.|author2=Bernstein D. T.|doi=10.1007/BF00988589|pages=493–499|s2cid=9451251}}</ref><ref>{{cite journal|journal=Journal of Chemical Education|volume=55|issue=3|year=1978|title=1-Butanethiol and the Striped Skunk|author= Andersen K. K., Bernstein D. T. |doi=10.1021/ed055p159|pages=159–160|bibcode=1978JChEd..55..159A |last2= Bernstein }}</ref><ref>{{cite journal|journal=Tetrahedron|volume=38|issue=13|year=1982 |title=Chemical Constituents of the Defensive Secretion of the Striped Skunk (''Mephitis mephitis'')|author1=Andersen K. K.|author2=Bernstein D. T.|author3=Caret R. L.|author4=Romanczyk L. J., Jr. |doi=10.1016/0040-4020(82)80046-X|pages=1965–1970}}</ref><ref>{{cite journal|journal=Journal of Chemical Ecology|volume=28|issue=9|year=2002|title=Volatile Components in Defensive Spray of the Hooded Skunk, ''Mephitis macroura''|author1=Wood W. F.|author2=Sollers B. G.|author3=Dragoo G. A.|author4=Dragoo J. W.|doi=10.1023/A:1020573404341|pages=1865–70|pmid=12449512|bibcode=2002JCEco..28.1865W |s2cid=19217201}}</ref><ref>{{cite web|url=http://users.humboldt.edu/wfwood/chemofskunkspray.html|title=Chemistry of Skunk Spray|access-date=January 2, 2008|author=William F. Wood|publisher=Dept. of Chemistry, [[Humboldt State University]]|url-status=live|archive-url=https://web.archive.org/web/20101008140758/http://users.humboldt.edu/wfwood/chemofskunkspray.html|archive-date=October 8, 2010}}</ref> These compounds are detectable by the human nose at concentrations of only 10 parts per billion.<ref>{{Cite journal | doi= 10.1084/jem.1.2.323|last=Aldrich|first= T.B.|title=A Chemical Study of the Secretion of the Anal Glands of ''Mephitis mephitiga'' (Common Skunk), with Remarks on the Physiological Properties of This Secretion|journal=J. Exp. Med.|volume=1|issue=2|pages=323–340|year=1896|pmid=19866801|pmc=2117909}}</ref> Human [[sweat]] contains (''R'')/(''S'')-3-methyl-3-sulfanylhexan-1-ol (3M3SH), detectable at 2 parts per billion and having an onion-like (S enantiomer) and fruity, grapefruit-like odor (R enantiomer).<ref>{{Cite journal |last1=Troccaz |first1=Myriam |last2=Starkenmann |first2=Christian |last3=Niclass |first3=Yvan |last4=van de Waal |first4=Matthijs |last5=Clark |first5=Anthony J. |date=July 2004 |title=3-Methyl-3-sulfanylhexan-1-ol as a Major Descriptor for the Human Axilla-Sweat Odour Profile |url=https://onlinelibrary.wiley.com/doi/10.1002/cbdv.200490077 |journal=Chemistry & Biodiversity |language=en |volume=1 |issue=7 |pages=1022–1035 |doi=10.1002/cbdv.200490077 |pmid=17191896 |issn=1612-1872}}</ref> (Methylthio)methanethiol (MeSCH<sub>2</sub>SH; MTMT) is a strong-smelling volatile thiol, also detectable at parts per billion levels, found in male [[mouse]] urine. [[Lawrence C. Katz]] and co-workers showed that MTMT functioned as a [[semiochemical]], activating certain mouse olfactory sensory neurons, and attracting female [[Mouse|mice]].<ref>{{cite journal|last1=Lin|first1=Dayu|last2=Zhang|first2=Shaozhong|last3=Block|first3=Eric|last4=Katz|first4=Lawrence C.|year=2005|title=Encoding social signals in the mouse main olfactory bulb| journal=Nature|volume=434|issue=7032|pages=470–477|doi=10.1038/nature03414|bibcode=2005Natur.434..470L|pmid=15724148|s2cid=162036}}</ref> [[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.<ref>{{cite journal|last1=Duan|first1=Xufang|last2=Block|first2=Eric|last3=Li|first3=Zhen|last4=Connelly|first4=Timothy|last5=Zhang|first5=Jian|last6=Huang|first6=Zhimin|last7=Su|first7=Xubo|last8=Pan|first8=Yi|last9=Wu|first9=Lifang|last10=Chi|first10=Q.|last11=Thomas|first11=S.|last12=Zhang|first12=S.|last13=Ma|first13=M.|last14=Matsunami|first14 = H.|last15=Chen|first15=G.-Q.|last16=Zhuang|first16=H.|year=2012|title=Crucial role of copper in detection of metal-coordinating odorants|journal=Proc. Natl. Acad. Sci. U.S.A. |volume=109|issue=9|pages=3492–3497|doi=10.1073/pnas.1111297109|bibcode=2012PNAS..109.3492D|pmid=22328155|pmc=3295281|display-authors=8|doi-access=free}}</ref> 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 [[Tert-Butylthiol|''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]].<ref>{{cite web|url=https://www.chemistryworld.com/news/copper-key-to-our-sensitivity-to-rotten-eggs-foul-smell/1017492.article|title=Copper key to our sensitivity to rotten eggs' foul smell|website=chemistryworld.com|access-date=3 May 2018|url-status=live|archive-url=https://web.archive.org/web/20170510113946/https://www.chemistryworld.com/news/copper-key-to-our-sensitivity-to-rotten-eggs-foul-smell/1017492.article|archive-date=10 May 2017}}</ref>

Thiols are also responsible for a class of [[wine fault]]s caused by an unintended reaction between sulfur and [[yeast (wine)|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 [[terpene|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 [[Tert-Butylthiol|''t''-butyl mercaptan]] as the main odor constituent in natural gas and [[ethanethiol]] in [[liquefied petroleum gas]] (LPG, propane).<ref name=Roberts>{{Cite book|editor=Roberts, J. S.|title=Kirk-Othmer Encyclopedia of Chemical Technology|publisher=Wiley-VCH|location=Weinheim|date=1997}}{{page needed|date=May 2019}}</ref> In situations where thiols are used in commercial industry, such as liquid petroleum gas tankers and bulk handling systems, an oxidizing [[Catalysis|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 [[hydrogen bond]]ing, both with water molecules and among themselves. Hence, they have lower [[boiling point]]s and are less [[soluble]] in water and other [[solvent#Polarity, solubility, and miscibility|polar solvents]] than alcohols of similar molecular weight. For this reason also, thiols and their corresponding sulfide functional group [[structural isomer|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 CH<sub>3</sub>X−H, the bond enthalpies are {{val|365.07|2.1|u=kcal/mol}} for X = S and {{val|440.2|3.0|u=kcal/mol}} for X = O.<ref>{{cite book|chapter=Bond Dissociation Energies|author1=Luo, Y.-R. |author2=Cheng, J.-P. |title=Handbook of Chemistry and Physics|editor=J. R. Rumble|year=2017|publisher=CRC Press}}</ref>  Hydrogen-atom abstraction from a thiol gives a [[thiyl radical]] with the formula RS<sup>•</sup>, where R = alkyl or aryl.