Odorants, thiols

A thiol contains an —SH group covalently bonded to carbon. Sulfur is just below oxygen in the periodic table, so a thiol is somewhat similar to an alcohol. Still, the chemical and physical properties of thiols differ significantly from those of alcohols. For example, whereas alcohols have inoffensive odors, thiols smell bad. The stench of skunk scent is due to thiols, including 3-methylbutanethiol. Thiols are important in proteins because of their abilities to form S—S linkages, which we describe in Section 13-1. 

Comparative aroma dilution analyses of the headspace of aqueous solutions, containing either the total volatiles isolated from a fresh coffee brew or these volatiles mixed with the melanoidins isolated from coffee brew, revealed drastic losses of odorous thiols, 2-furfurylthiol, 3-methyl-2-butenethiol, 3-mercapto-3-methylbutyl formate, 2-methyl-3-furanthiol, and methanethiol, in the presence of melanoidins.509 The first compound was affected most, the reduction being 16-fold, and was accompanied by an overall reduction in roasty-sulfury aroma. The rapid loss of thiols was confirmed by stable-isotope dilution analysis. [2H]-NMR and LC-MS gave strong evidence that the thiols become covalently bound via Maillard-derived pyrazinium compounds. 

Boudrilloux, R, Darriet, R, Henry, R., Lavigne-Cruege, V., Dubourdieu, D. (1998). Identification of volatile and powerful odorous thiols in Bordeaux red wine varieties. J. Agric. Eood Chem., 46, 3095-3099. 

Thiol-Ene Systems. Systems that combine thiols [such as a trithiol (16)] with ene comonomers, such as allyl ethers [like trimethylol propane diallyl ether (17)] or acrylates, were first considered in the 1970s however, because of their unpleasant odor, thiols were abandoned, and acrylates became the monomer of choice for industrial implementation. 

As with other thiols, which contain the -SH group, this compound is noted for its foul odor. Thiols are added to natural gas so that their odor can warn of gas leaks. Dimethylsulfide, also shown here, is a volatile compound released by ocean-dwelling microorganisms to the atmosphere... 

At one time thiols were named mercaptans Thus CH3CH2SH was called ethyl mercaptan according to this system This nomenclature was abandoned beginning with the 1965 revision of the lUPAC rules but is still sometimes encountered When one encounters a thiol for the first time especially a low molecular weight thiol its most obvious property is its foul odor Ethanethiol is added to natural gas so that leaks can be detected without special equipment—your nose is so sensitive that it can detect less than one part of ethanethiol m 10 000 000 000 parts of arr The odor of thiols weakens... 

The main by-products of this synthesis type are sulfides and the isomer resulting from the Markownikoff addition to the alkene. For example, in the synthesis of 1-butanethiol (eq. 4), 5-thianonane, C H SC H, and 2-butanethiol are produced as by-products. The 2-butanethiol has uses as a herbicide intermediate and a gas odorant blend component and is further processed. The 5-thianonane is incinerated or reprocessed for fuel value. Sulfides account for up to 10% of the thiols produced. Another 2—5% is the Markownikoff addition product. 

In general, rubber manufacturers balance thiol reactivity and odor. The stmcture of the thiol plays a significant role in its abiUty to be transported within the polymer matrix, particularly in emulsion polymerizations, ie, mixed water—monomer emulsion. The odor of light thiols is generally too strong for most mbber manufacturers, as it is generally hard to remove residual odors from polymers. 

Another area in which sulfur compounds have long found use is in the area of agricultural chemicals. Many of these materials had been produced by the manufacturer of the agricultural chemicals, but difficulties in containing odor and the use of hydrogen sulfide in heavily populated areas again pushed toward specialization by several companies. A Hst of agricultural chemicals, and the thiol that is used or has been used in production, follows ... 

Organosulfur chemicals serve a diverse group of industries. In these appHcations ways have been found to control the odor of the thiols, thus utilizing these compounds effectively in a wide range of products. 

The toxicology and procedures for handling thiols used in gas odorants have been reviewed (56), as have thiols as a whole (57). A partial listing of toxicological results is shown in Table 4. 

A listing of known TLV values and odor thresholds for a variety of thiols is given ia Table 5. ... 

Thiol spills are handled ia the same manner that all chemical spills are handled, with the added requirement that the odor be eliminated as rapidly as possible. In general, the leak should be stopped, the spill should be contained, and then the odor should be reduced. The odor can be reduced by sprayiag the spill area with sodium hypochlorite (3% solution), calcium hypochlorite solution (3%), or hydrogen peroxide (3—10% solution). The use of higher concentrations of oxidant gives strongly exothermic reactions, which iacrease the amount of thiol ia the vapor, as well as pose a safety ha2ard. The apphcation of an adsorbent prior to addition of the oxidant can be quite helpful and add to the ease of cleanup. 

Thiols are shipped ia every conceivable container size. Dmms and cans can be of carbon steel for most thiols, provided color is not a determining factor. Tmck, rail, and isocontainer shipments should be set up to utilize a vapor return line from the tank to the shipping container. This substantially minimizes the amount of odor that escapes. Phillips Petroleum Company and Atochem North America can supply further information regarding the handling and properties of many thiols. 

The most striking characteristic of thiols is their appalling odor. Skunk scent, for instance, is caused primarily by the simple thiols 3-methyl-1-butanethiol and 2-butene-l-thiol. Volatile thiols such as ethanethiol are also added to natural gas and liquefied propane to serve as an easily detectable warning in case of leaks. 

Thiol (s), 652, 667-668 disulfides from, 668 electrostatic potential map of, 75 from alkyl halides, 667 hybridization of, 20 naming, 667 odor of, 667 oxidation of, 668 pKa of, 604... 

Because of their inherent penetrating smell, certain organic sulfur compounds are used for odorization. Repellents from the skunk contain compounds such as trans-2-butene-l-thiol and 3-methyl-1-butanethiol. Ethylmercaptan, because of its extremely low odor threshold, is the favorite compound used as an odorant in natural gas and liquid propane for leak detection. Tetrahydro-thiophene is also often used. Common odorization reagents are summarized in Table 15-1 and Figures 15-1 to 15-3. 

Skunks deter predators by release of a liquid spray containing seven major volatile components classified as thiols (compounds containing the -SH functional group) and acetate derivatives of thiols (characterized by the SC(0)CH3 functionality). In particular, two of the more odiferous components responsible for the strongly repellent odor of the skunk s secretion are 2-butene-1-thiol (Fig. 13.2.1) and 3-... 

Dithiothreitol (DTT) and dithioerythritol (DTE) are the trans and cis isomers of the compound 2,3-dihydroxy-1,4-dithiolbutane. The reducing potential of these versatile reagents was first described by Cleland in 1964. Due to their low redox potential (—0.33 V) they are able to reduce virtually all accessible biological disulfides and maintain free thiols in solution despite the presence of oxygen. The compounds are fully water-soluble with very little of the offensive odor of the 2-mercaptoethanol they were meant to replace. Since Cleland s original report, literally thousands of references have cited the use of mainly DTT for the reduction of cystine and other forms of disulfides. 

For most odour nuisance problems, chemical plants, refineries, livestock production, food processing, rendering, water purification plants etc., the compounds responsible for the odour are known. So chemical analysis of the odour can be limited to these odorants, and selective concentrating techniques can be used. Selective concentrating methods are based on specific absorption techniques, using particular chemical reactions of odorant classes. Sometimes several absorption methods have to be used in order to describe the odour problem, thus increasing the labor cost of the analysis. On the other hand absorption methods allow better quantitative results. Selective absorption of odorants from air produces a far less complex mixture. We developed or are developing several of these methods for aldehydes, amines, acids, thiols etc. 

The open-framework materials catalyze peroxide-based oxidations such as sulfoxidation, but significantly more noteworthy, 6-Tb catalyzes 02-based oxidations and does so at ambient or nearly ambient temperature (96). A case in point is the oxidation of thiols, a common odorant in human environments and a mildly toxic class of compounds, to disulfides that have almost no odor and are less toxic. The stoichiometry for oxidation of a representative substrate, 71-propane thiol, was established to be that in Eq. (1). After 30 days at 45 °C, 6-Tb produces ca. 19 turnovers of disulfide product based on the bis(trisester)Vg groups in the open-framework material, using only ambient air as the oxidant. 

Striped skunk defensive secretion contains a number of compounds but two are largely responsible for the odor tra 5 -2-buten-l-thiol and 3-methylbutane-l-thiol ... 

These are volatile compounds and do have rather dreadful smells. They also occur in skunk spray in the form of their acetate esters. These are much less volatile and so contribute less to the immediate odor. However, on contact with water, they are slowly converted to the free thiols, a reaction that accounts for the persistence of skunk odor for days or weeks. The chemical components of skunk defensive secretion vary somewhat from species to species but thiols play a prominent role in all cases. 

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