Aliphatic Mercaptans Thiols

Again analogous to alcohols, primary mercaptans split out H2S to give a strong M - 34 peak, the resulting ion then eliminating ethylene thus the homologous series M - H2S - (CH2=CH2) arises. 

Secondary and tertiary mercaptans cleave at the a-carbon atom with loss of the largest group to give a prominent peak M - CH3, M - C2H5, M — C3H7,. . . . However, a peak at m/z 47 may also appear as a rearrangement peak of secondary and tertiary 

FIGURE 2.15. Di-M-pentyl sulfide. The table (upper right) has reset the molecular ion at an intensity of 100% for discussion of the molecular ion cluster. 

In long-chain mercaptans, the hydrocarbon pattern is superimposed on the mercaptan pattern. As for alcohols, the alkenyl peaks (i.e., m/z 41, 55, 69,. . . ) are as large or larger than the alkyl peaks (m/z 43, 57, 

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PROP Compounds containing the -SH group bound to carbon. Also called thiols. SAFETY PROFILE Generally they have a very offensive odor that may cause nausea and headache. High concentrations of vapor can produce unconsciousness with cyanosis, cold extremities, and rapid pulse. A common air contaminant. Dangerous when heated to decomposition they almost always emit highly toxic fumes of SO. They may react with water, steam, or acids to produce toxic and flammable vapors. Aliphatic mercaptans are flammable. They can react violently with powerful oxidizers such as Ca(OCl)2. 

From the reaction of the sodium salt of an aliphatic or aromatic mercaptan (thiol) with hexachlorocyclotriphospha-zatriene (1,3,5,2,4,6-triazatriphosphorine 2,2,4,4,6,6-hexa-... 

Thiols, also called aliphatic mercaptans, with the formula RSH, are the sulfur analogs of alcohols. Thiols generally show stronger molecular ion peaks than the equivalent alcohols. Looking back at Table 10.4, sulfur has a signihcant isotope. This gives rise to an enhanced M + 2 peak in the mass spectrum of sulfur-containing compounds. 

Maskow et al. found that aromatic sulfides are generally much more effective than aliphatic sulfides, but, in most cases, neither works very well at HCI concentrations above 10%. However, dipropargyl sulfide is one of the best, if not the best, single inhibitor ever discovered for steel in HCI. It is very effective over wide ranges of temperature and acid concentration. Mercaptans (thiols) are also excellent steel corrosion inhibitors indeed, they received considerable attention in the 1950s and 1960s. Since then, interest in them has waned, perhaps as a result of their odoriferous and toxic nature and their tendency to form H2S in situ. 

Of the general formula, R - S — H, where R represents an aliphatic or cyclic radical, the thiols —also known as mercaptans— are acidic in behavior owing to their S—H functional group they are corrosive and malodorous. Their concentration in crude oils is very low if not zero, but they are created from other sulfur compounds during refining operations and show up in the light cuts, as illustrated in Table 8.6. 

Fairchild EJ, Stokinger HE Toxicologic studies on organic sulfur compounds. 1. Acute toxicity of some aliphatic and aromatic thiols (mercaptans). Am Ind Elyg Assoc J 19 171-189, 1958... 

The direction of ring opening of nnsymmetnea) episulfides by thiols in the presence of either acid or base catalyst appears to be rather non-selective.1 Mixtures of primary and tertiary mercaptan have been obtained by reaction of isobutylene Bulfido with some of the simple aliphatic thiols such as -butanethiol in the presence of catalytic quantities of either boron trifluoride or sodium ethoxide (Eq. 43). 

Whereas aliphatic and aromatic thiols cleaved the C—Sn bond in tetraalkylstannanes to give trialkylorganylthiostannanes768,855, the analogous reaction in organosilicon chemistry is absolutely unusual. According to Seyferth789,818 (1957), the vinyl group was especially easy to cleave from tin atom by mercaptans. 

Alkyl sulfides and thiols. Some alkyl thiols and sulfides, notably those from commonly ingested Allium sativum (garlic) and Allium cepa (onion) (Alliaceae), are variously bioactive as odorants and antimicrobials. Propanethial S-oxide (CH3-CH2-CH=S=0) is a lachrymatory irritant principle of onion. Allicin (S-oxodiallydisulfide CH2=CH—CH2-SO-S-CH2— CH=CH2), diallyldisulfide (CH2=CH-CH2-S-S-CH2-CH=CH2) and diallylsulfide (CH2=CH—CH2—S—CH2-CH=CH2) are major odorants of garlic that are reactive and irritant because of the allyl groups. Dimethyl disulfide (CH3—S—S-CH3), dipropyl disulfide (CH3-CH2-CH2-S-S-CH2-CH2-CH3), methyl allyl disulfide (CH3-S-S-CH2-CH=CH2) and propane-1-thiol (CH3-CH2—CH2—SH) are further Allium odorants. Methane thiol (methyl mercaptan CH3—SH) is a widespread plant volatile and notably derives from anaerobic bacterial degradation of cysteine as in human flatus and bad mouth odour. The aliphatic disulfides allicin and ajoene inhibit proinflammatory expression of iNOS. 

Hydrogen atom donors such as non-nucleophilic tertiary thiols or tri-n-butyltin hydride are extremely efficient traps for the capture of the alkyl radical R derived from O-acyl thiohydroxamates, thus providing a very efficient method for reductive decarboxylation (Scheme 3). In practical terms, the use of the mercaptan is preferred since the tertiary alkyl pyridyl disulfide can be easily removed during work up by a simple acid extraction. The reaction has been successfully applied to a very wide range of complex substrates [8] possessing primary, secondary, or tertiary aliphatic carboxylic acids, and reactions at room temperature or below require only photolysis from a simple tungsten lamp and often involve in situ O-acyl thiohy-droxamate derivatization. 

It is interesting to point out that the thiol nature has a major influence on the hydrothiolation rates. Hence, changing the aliphatic thiol from primary to secondary resulted in signiflcant rate depression, consistent with a steric impediment at the turnover-limiting alkyne insertion (step ii. Scheme 14). The use of benzyl mercaptan allows an increase of the rate by nearly three times. The effect on the alkyne was found to be minor and by increasing the alkyne steric encumbrance minor changes in rates were observed. 

Aliphatic homoallyl mercaptans and aldehydes yield trisubstituted tetrahydrothiopyrans with good diastereoselectivity through an InCb-mediated Prins reaction. Similar selectivity is observed with l-phenyl-3-butene-l-thiol and benzaldehyde scrambling is noted with substituted benzaldehydes (Scheme 41) <01JOC739>. 

During study of the chiral thiourea 12-catalyzed sulfa-Michael addition of aliphatic thiols to nitroolefins, benzylic mercaptans were subjected to the reaction with nitrodienes (Scheme 8) [17]. Exclusive formation of 1,4-adducts was observed in moderate to good chemical yield with moderate to fair enantioselectivity. 

Compounds containing a carbon-bonded mercapto group (sulfhydryl group, -SH), called mercaptans or thiols, are the sulfur analogues of hydroxy compounds. Altogether, lower aliphatic thiols are compounds exhibiting an intense, often penetrating. 

Particularly important substances for the basic flavour of baked and cooked meat are aliphatic thiols (such as methanethiol), sulfides (such as dimethyldisulfide, dimethytrisulfide and dimethyltetra-sulfide), aldehydes (such as acetaldehyde, 2-methylpropanal and 3-methylbutanal), furans, pyridines and thiophenes with a mer-capto group in position C-3 and their corresponding disulfides and some other aliphatic and heterocyclic sulfur compounds. Examples of important aliphatic thiols are 3-mercaptobutan-2-one (8-179) and 3-mercaptopentan-2-one found in cooked beef. A mixture of 3-mercapto-2-methylpentane-l-ol diastereoisomers (Figure 8.86) has a broth-like, sweaty and leek-hke flavour. Very low odour threshold concentrations and an odour reminiscent of roasted meat are found in 2-methylfuran-3-thiol (8-180), which also occurs in beef broth, roasted coffee and other foods, 2,5-dimethylfuran-3-thiol, their corresponding disulfides and 2-furanmethanethiol (furfuryl mercaptan). The typical aroma of roast beef is found in... 

The characteristic aroma of blackcurrants (varieties oiRibes nigrum, Grossulariaceae) is mainly a consequence of aliphatic and aUcycUc thiols. The bearer of the characteristic cat odour is 4-methoxy-2-methyl-2-thiol, called blackcurrant mercaptan (8-124), which occurs also in olive oil and green tea. Other important components are 4-mercapto-4-methylpentan-2-one called cat ketone (8-125) and (lS,4R)-p-menthan-8-thiol-2-one (see Section 8.2.13.1.1). Cat ketone is also found in grapefruits, some hop cultivars, basil and some aromatic wines. The leaves contain about 0.7% essential oil, whose main component isp-cymol, the sulfur compounds of which contribute to the strong smell, mainlyp-menthan-8-thiol-2-one. 

Other Physical Properties.—Conformational information for thiols and sulphides can be obtained from enthalpy of formation data. Proton-transfer of benzyl-mercaptan (protonation at sulphur with CF3SO3H, and de-protonation by imidazole) has been studied. Thermodynamic parameters for acidity constants of substituted benzenethiols, and for hydrogen-bonding interactions between alkanols and di-n-octyl sulphide and the analogous ether and N-methylamine, reveal a dominant electronic influence of the orM<>-substituent, and substantially higher hydrogen-bond acceptor ability for the aliphatic sulphide than is generally assumed. ... 

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