View source for Thiol


==Overview==
[[Image:Thiol-group.png|thumb|Sulfhydryl]]
In [[organic chemistry]], a '''thiol''' is a compound that contains the functional group composed of a [[sulfur]] atom and a [[hydrogen]] atom (-SH). Being the sulfur analogue of an [[alcohol]] group (-OH), this functional group is referred to either as a ''thiol group'' or a ''sulfhydryl group''. More traditionally, thiols are often referred to as '''mercaptans'''.

==Nomenclature==
When a thiol group is a [[substituent]] on an [[alkane]], there are several ways of naming the resulting thiol:
* The preferred method (used by the [[IUPAC]]) is to add the suffix ''-thiol'' to the name of the alkane. The method is nearly identical to [[Alcohol#Nomenclature|naming an alcohol]]. Example: CH<sub>3</sub>SH would be ''methanethiol''.
* An older method, the word ''mercaptan'' replaces ''alcohol'' in the name of the equivalent alcohol compound. Example: CH<sub>3</sub>SH would be ''methyl mercaptan''. (CH<sub>3</sub>OH would be [[methanol|methyl alcohol]])
* As a prefix, the term ''mercapto''- is used. Example: [[mercaptopurine]].

==Etymology==
The term ''mercaptan'' comes from the [[Latin]] ''mercurius captans'', meaning 'laying hold of mercury,' because the –SH group binds tightly to the element [[mercury (element)|mercury]].

==Physical Properties==
===Odor===
Many thiols are colorless [[liquid]]s having an [[odor]] resembling that of [[garlic]]. The odor of thiols is often strong and repulsive, particularly for those of low molecular weight. Thiols bind strongly to [[skin]] [[protein]]s, and are responsible for the intolerable, persistent odor produced by the spraying of [[skunk]]s. [[Natural gas]] distributors began adding various forms of pungent thiols, usually [[ethanethiol]], to [[natural gas]], which is naturally odorless, after the deadly [[1937]] [[New London School explosion]] in [[New London, Texas]]. Thiols are also responsible for a class of [[wine fault]]s caused by an unintended reaction between sulfur and [[yeast]]. However, not all thiols have unpleasant odors. For example, [[grapefruit mercaptan]], a [[terpene|monoterpenoid]] thiol, is responsible for the characteristic scent of [[grapefruit]].

===Boiling points and solubility===
Due to the small [[electronegativity]] difference between sulfur and hydrogen, an S-H bond is practically [[nonpolar]] [[covalent]]. Therefore, the S-H bond in the thiols have a lower [[dipole moment]] as compared to the alcohol's O-H bond. Thiols show little association by [[hydrogen bond]]ing, with both 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. Thiols are as soluble and have similar boiling points to isomeric [[sulfides]].

==Chemical Properties==
===Synthesis===
The methods used in making thiols are analogous to those used to make alcohols and ethers. The reactions are quicker and higher yielding because sulfur anions are better nucleophiles than oxygen atoms.

Thiols are formed when a [[halogenoalkane]] is heated with a solution of [[sodium hydrosulfide]]

: CH<sub>3</sub>CH<sub>2</sub>Br + NaSH heated in ethanol(aq) → CH<sub>3</sub>CH<sub>2</sub>SH + NaBr

In addition, [[disulfides]] can be readily reduced by reducing agents such as lithium aluminium hydride in dry ether to form two thiols.

: R-S-S-R' → R-SH + R'-SH

===Reactions===
The thiol group is the sulfur analog of the [[hydroxyl group]] (-OH) found in [[alcohol]]s. Since [[sulfur]] and [[oxygen]] belong to the same [[periodic table group]], they share some similar chemical [[chemical bond|bonding]] properties. Like alcohol, in general, the deprotonated form RS<sup>−</sup> (called a '''thiolate''') is more chemically reactive than the protonated thiol form RSH

The chemistry of thiols is thus related to the chemistry of alcohols: thiols form [[thioether]]s, [[thioacetal]]s and [[thioester]]s, which are analogous to [[ethers]], [[acetal]]s, and [[esters]]. Furthermore, a thiol group can react with an [[alkene]] to create a thioether. (In fact, biochemically, thiol groups may react with [[vinyl]] groups to form a [[thioether linkage]].) 

===Acidity===
The sulfur atom of a thiol is quite [[Nucleophile|nucleophilic]], rather more so than the oxygen atom of an alcohol. The thiol group is fairly acidic with a usual [[pKa|pK<sub>a</sub>]] around 10 to 11. In the presence of a [[base (chemistry)|base]], a thiolate [[anion]] is formed which is a very powerful nucleophile. The group and its corresponding anion are readily [[Redox|oxidized]] by reagents such as [[bromine]] to give an organic [[disulfide]] (R-S-S-R).

: 2R-SH + Br<sub>2</sub> → R-S-S-R + 2HBr

Oxidation by more powerful reagents such as [[sodium hypochlorite]] or [[hydrogen peroxide]]  yield [[sulfonic acid]]s (RSO<sub>3</sub>H).

: 2R-SH + 2H<sub>2</sub>O<sub>2</sub> → RSO<sub>3</sub>H + 2H<sub>2</sub>O

==Biological importance==
As the functional group of the [[amino acid]] [[cysteine]], the thiol group plays an important role in biological systems. When the thiol groups of two cysteine residues (as in monomers or constituent units) are brought near each other in the course of [[protein]] folding, an [[oxidation|oxidation reaction]] can create a [[cystine]] unit with a [[disulfide bond]] (-S-S-). Disulfide bonds can contribute to a protein's [[tertiary structure]] if the cysteines are part of the same [[peptide]] chain, or contribute to the [[quaternary structure]] of multi-unit proteins by forming fairly strong covalent bonds between different peptide chains. The [[heavy chains|heavy]] and [[light chain]]s of [[antibodies]] are held together by disulfide bridges. Also, the kinks in curly [[hair]] are a product of cystine formation. Permanents take advantage of the oxidizability of cysteine residues. The chemicals used in hair straightening are [[reductant]]s that reduce cystine disulfide bridges to free cysteine sulfhydryl groups, while chemicals used in hair curling are [[oxidant]]s that oxidize cysteine sulfhydryl groups to form cystine disulfide bridges. Sulfhydryl groups in the [[active site]] of an [[enzyme]] can form [[noncovalent]] bonds with the enzyme's [[substrate (biochemistry)|substrate]] as well, contributing to [[catalysis|catalytic activity]]. Active site cysteine residues are the functional unit in [[cysteine protease]]s.

==Examples of thiols==
* [[Methanethiol]] - CH<sub>3</sub>SH
* [[Ethanethiol]] - C<sub>2</sub>H<sub>5</sub>SH
* [[Coenzyme A]]
* [[Lipoamide]]
* [[Glutathione]]
* [[Cysteine]]
* [[Dithiothreitol]]/[[dithioerythritol]] (an [[epimer]]ic pair)
*[[2-Mercaptoindole]]

== See also ==
* [[Thial]]
* [[Disulfide bond]]
* [[Thiol-disulfide exchange]]

== External links ==
*[http://dysa.northwestern.edu/CurrOrgChem.pdf Applications, Properties, and Synthesis of w-Functionalized n-Alkanethiols and Disulfides — the Building Blocks of Self-Assembled Monolayers] by D. Witt, R. Klajn, P. Barski, B.A. Grzybowski at Northwestern University.
*[http://www.factmonster.com/ce6/sci/A0832739.html Mercaptan], by The Columbia Electronic Encyclopedia
*[http://www.columbiagaspamd.com/community_outreach/mercaptan.htm What is Mercaptan?], by Columbia Gas of Pennsylvania and Maryland
* [http://chemistry.about.com/od/chemistryfaqs/f/blsmell.htm What Is the Worst Smelling Chemical?], by About Chemistry

{{Functional Groups}}

[[Category:Thiols| ]]
[[Category:Functional groups]]
[[Category:Organosulfur compounds]]

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