Thiol
In organic chemistry, a thiol, or thiol derivative, is any organosulfur compound of the form, where R represents an alkyl or other organic substituent. The functional group itself is referred to as either a thiol group or a sulfhydryl group, or a sulfanyl group. Thiols are the sulfur analogue of alcohols, and the word is a blend of "thio-" with "alcohol".
Many thiols have strong odors resembling that of garlic, cabbage or rotten eggs. Thiols are used as odorants to assist in the detection of natural gas. The smell of natural gas is due to the addition of thiol.
Nomenclature
Thiols are sometimes referred to as mercaptans or mercapto compounds, a term introduced in 1832 by William Christopher Zeise and is derived from the Latin mercurio captāns because the thiolate group bonds very strongly with mercury compounds.There are several ways to name the alkylthiols:
- The suffix -thiol is added to the name of the alkane. This method is nearly identical to naming an alcohol and is used by the IUPAC, e.g. CH3SH would be methanethiol.
- The word mercaptan replaces alcohol in the name of the equivalent alcohol compound. Example: CH3SH would be methyl mercaptan, just as CH3OH is called methyl alcohol.
- The term sulfhydryl- or mercapto- is used as a prefix, e.g. mercaptopurine.
Physical properties
Odor
Many thiols have strong odors resembling that of garlic. The odors of thiols, particularly those of low molecular weight, are often strong and repulsive. The spray of skunks consists mainly of low-molecular-weight thiols and derivatives. These compounds are detectable by the human nose at concentrations of only 10 parts per billion. Human sweat contains /-3-methyl-3-sulfanylhexan-1-ol, detectable at 2 parts per billion and having an onion-like and fruity, grapefruit-like odor. methanethiol 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 mice. 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. A human olfactory receptor, OR2T11, has been identified which, in the presence of copper, is highly responsive to the gas odorants 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.Thiols are also responsible for a class of wine faults caused by an unintended reaction between sulfur and yeast, as well as 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. Concentrated samples have an unpleasant odor.
In the United States, distributors are required to add thiols, originally ethanethiol, to natural gas after the deadly New London School explosion in New London, Texas, in 1937, although many 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. 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 hydrogen bonding, both with 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. 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.Structure and bonding
Thiols having the structure R−S−H, in which an alkyl group is attached to a sulfhydryl group, are referred to as alkanethiols or alkyl thiols. Thiols and alcohols have similar connectivity. Because sulfur atoms are larger than oxygen atoms, C−S bond lengths—typically around 180 picometres—are about 40 picometers longer than typical C−O bonds. C−S−H angles approach 90°, whereas the angle for the C−O−H group is more obtuse. In solids and liquids, the hydrogen-bonding between individual thiol groups is weak, and thus thiols are more volatile than the corresponding alcohols. The main cohesive forces for thiols involves 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. For CH3S−H, the BDE is, while for CH3O−H, the BDE is. 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 gas chromatographs are useful. Spectroscopic indicators are the D2O-exchangeable SH signal in the 1H NMR spectrum. The νSH band appears near 2400 cm−1 in the IR spectrum. In the nitroprusside reaction, free thiol groups react with sodium nitroprusside and 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: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.
Another method entails the alkylation of sodium hydrosulfide.
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 haloalkane with sodium hydrosulfide is inefficient owing to the competing formation of sulfides. Instead, alkyl halides are converted to thiols via an S-alkylation of thiourea. This multistep, one-pot process proceeds via the intermediacy of the isothiouronium salt, which is hydrolyzed in a separate step: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. A related two-step process involves alkylation of thiosulfate to give the thiosulfonate, followed by hydrolysis. The method is illustrated by one synthesis of thioglycolic acid:
Organolithium compounds and Grignard reagents react with sulfur to give the thiolates, which are readily hydrolyzed:
Phenols can be converted to the thiophenols via rearrangement of their O-aryl dialkylthiocarbamates.
Thiols are prepared by reductive dealkylation of sulfides, especially benzyl derivatives and thioacetals.
Thiophenols are produced by S-arylation or the replacement of diazonium leaving group with sulfhydryl anion :