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Tue, October 21, 2014,18:29

Mercaptans

Thiol / mercaptans
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.
What is Mercaptan( in natural gas industries)? Natural gas in its native state is colorless and odorless. Mercaptan is the additive that is added to natural gas to make it easier to detect in case of a leak. The most important thing to know about mercaptan is that it stinks. Some people compare it to the smell of rotten eggs.
In a concentrated form, its smell is almost unbearable. And it takes only a few parts per million of mercaptan to give natural gas a smell. That is precisely why we add it to natural gas. If we did not add mercaptan, it would be hard for you to know that unlit natural gas was coming from your stove after you left the valve turned on. And leaks from furnaces and hot water heaters would be nearly impossible to detect without expensive equipment. So mercaptan's smell is a very valuable safety feature.
Mercaptans contain sulfur. That's what makes them smell. The kind we use blends well with natural gas and, in a gaseous state, has much the same properties as natural gas, so it will also rise and dissipate with natural gas.
There are other uses for mercaptans in industry, including jet fuel, pharmaceuticals and livestock feed additives. They are used in many chemical plants. Mercaptans are less corrosive and less toxic than similar sulfur compounds found naturally in rotten eggs, onions, garlic, skunks, and, of course, bad breath. In other word, forms of mercaptan can be found in things that smell.
 
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 naming an alcohol. Example: CH3SH would be methanethiol.
  • An older method, the word mercaptan replaces alcohol in the name of the equivalent alcohol compound. Example: CH3SH would be methyl mercaptan. (CH3OH would be methyl alcohol)
  • As a prefix, the terms sulfanyl or mercapto are 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.
Physical properties
Odor
Many thiols are colorless liquids having an odor resembling that of garlic. The odor of thiols is often strong and repulsive, particularly for those of low molecular weight. (A close selenium analog, butyl seleno-mercaptan, is responsible for the intolerable, persistent odor produced by the spraying of skunks.) Thiols bind strongly to skin proteins. 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 faults caused by an unintended reaction between sulfur and yeast. However, not all thiols have unpleasant odors. For example, grapefruit mercaptan, a 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 bonding, with both water molecules and among themselves. Hence, they have lower boiling points and are less soluble in water and other 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
CH3CH2Br + NaSH heated in ethanol(aq) → CH3CH2SH + 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 alcohols. Since sulfur and oxygen belong to the same periodic table group, they share some similar chemical bonding properties. Like alcohol, in general, the deprotonated form RS (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 thioethers, thioacetals and thioesters, which are analogous to ethers, acetals, 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 nucleophilic, rather more so than the oxygen atom of an alcohol. The thiol group is fairly acidic with a usual pKa around 10 to 11. In the presence of a base, a thiolate anion is formed which is a very powerful nucleophile. The group and its corresponding anion are readily oxidized by reagents such as bromine to give an organic disulfide (R-S-S-R).
2R-SH + Br2 → R-S-S-R + 2HBr
Oxidation by more powerful reagents such as sodium hypochlorite or hydrogen peroxide yield sulfonic acids (RSO3H).
R-SH + 3H2O2 → RSO3H + 3H2O
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 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 and light chains 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 reductants that reduce cystine disulfide bridges to free cysteine sulfhydryl groups, while chemicals used in hair curling are oxidants 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 as well, contributing to catalytic activity. Active site cysteine residues are the functional unit in cysteine proteases.
Examples of thiols
  • Methanethiol
  • Ethanethiol
  • propanethiol
  • Tertiary butylthiol
  • Coenzyme A
  • Lipoamide
  • Glutathione
  • Cysteine
  • 2-Mercaptoethanol
  • Dithiothreitol/dithioerythritol (an epimeric pair)
  • 2-Mercaptoindole
 
Methyl Mercaptan (CH3)SH

The simplest organic sulfur.   It has a single carbon, the sulfur and enough hydrogens to fill all the bonds.  It is also called methanethiol.   It is produced naturally by decomposition of organic matter and smells like rotting cabbage.
 
Ethyl Mercaptan, (CH3) (CH2) SH

Thiol version of ethanol, the active ingredient in some beverages.
 
Propyl Mercaptan, (CH3)2 (CH2) SH

There are two versions, both of which exist in natural gas supplies.  Normal, or n-Propyl Mercaptan has the sulfur hanging off one end of a three carbon chain.  iso-Propyl Mercaptan has the sulfur attached to the middle carbon of the three carbon chain.
 
Tertiary Butyl Mercaptan, (CH3)3CSH

Conceptually, there are several versions of butyl mercaptans, but only one seems to have a lifetime that permits it to exist in natural gas supplies. The form that is stable enough to by useful as an odorant in gas is the tertiary butyl arrangement.
  References
  • http://www.factmonster.com/ce6/sci/A0832739.html
  • http://yokedlife.net/mercaptans.aspx
  • www.wikipedia.org
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