Thiophene, detection

This reaction enables the detection of sulfur monoxide and other reduced sulfur compounds that can react with ozone to initially form sulfur monoxide. The chemiluminescence intensity depends on the type of compound, with thiols giving the largest response followed by alkyl sulfides, hydrogen sulfide, and thiophenes. Detection limits in the parts per billion range can be achieved for thiols. Alkenes, which also react with ozone to give emission centered at 354 nm, potentially interfere but selectivity can be achieved with a suitable optical filter. 

The presence of thiophene and its derivatives in crude oils was detected in 1899, but until 1953, the date at which the methyl-thiophenes were identified in kerosene from Agha Jari, Iran crude oil, it was believed that they came from the degradation of sulfides during refining operations. Finally, their presence was no longer doubted after the identification of benzothiophenes and their derivatives (Table 8.9), and lately of naphthenobenzothiophenes in heavy cuts. 

Commercial benzene may contain thiophene C H S, b.p. 84°, which cannot be separated by distillation or by fractional crystallisation. The presence of thiophene may be detected by shaking 3 ml. of benzene with a solution of 10 mg. of isatin in 10 ml. of concentrated sulphuric acid and allowing the mixture to stand for a short time a bluish-green colouration is produced if thiophene is present. The thiophene may be removed from benzene by any of the following methods —... 

Properties.—Mobile, colourless liquid m. p. 5 4 b. p. So 4 sp. gr. o 874 at 20°. Coal-tar benzene usually contains a little thiophene, C4H4S, which may be detected by dissolving a few crystals of isatin (see p. 229) in concentrated sulphuiicacid and shaking up with the benzene. If thiophene is present, a blue colour is produced (indophenin reaction). 

Systematic studies on disubstituted thiophenes are scarce, but some differences between the various series of disubstituted thiophenes have been detected. According to Imoto et al., the main effect of substituents in 2,5-disubstitution in thiophenes appears in the wavelength shift, whereas the main effect of substituents in the 2,4-disub-stituted thiophenes appears in the intensity changes of the bands. The intensities of the first absorption band of some 2,6-disubstituted thiophenes have been calculated from the spectroscopic momenta of the substituents. The UV absorption of thiophene was displaced... 

It is thus apparent that the selectivity of a reagent toward thiophene and benzene can differ appreciably, and this difference in selectivity is also strongly noticeable in the proportions of 2- and 3-isomers formed. Although in certain reactions no 3-isomer has been detected, appreciable amounts have been found in other reactions. Thus 0.3% of the 3-isomer has been found in the chlorination of thiophene.- Earlier results indicated that 5-10% 3-nitrothiophene is formed in the nitration of thiophene and a recent gas-chromatographic analysis by Ostman shows that the mononitrothiophene fraction contains as much as 16% of the 3-isomer. It appears that gas-chromatographic analysis should be very useful for the detection of small amounts of 3-isomers in other substitution reactions. However, from routine analyses of IR spectra, it appears to the present author that the amount of 3-isomers formed in acylation, formylation, and bromina-tion of thiophene are certainly less than a few per cent. 

This activation of the ortho position is most strikingly illustrated in the reactivity of 2,5-dimethylthiophene, which competitive experiments have shown to undergo the SnCb-catalyzed Friedel-Crafts reaction more rapidly than thiophene and even 2-methylthiophene. The influence of the reagent on the isomer distribution is evident from the fact that 2-methoxythiophene is formylated and bromi-nated (with A -bromosuccinimide) only in the 5-position. Similarly, although 3-bromo-2-methylthiophene has been detected in the bromi-nation of 2-methylthiophene with bromine, only the 5-isomer (besides some side-chain bromination) is obtained in the bromination of alkylthiophenes with A -bromosuccinimide. ° However, the mechanism of the latter type of bromination is not established. No lines attributable to 2-methyl-3-thiocyanothiophene or 2-methyl-3-chIoro-thiophene could be detected in the NMR spectra of the substitution products (5-isomers) obtained upon thiocyanation with thiocyanogen or chlorination with sulfuryl chloride. 2-Methyl- and 2-ethyl-thiophene give, somewhat unexpectedly, upon alkylation with t-butyl chloride in the presence of Feds, only 5-t-butyl monosubstituted and... 

NMR spectra of thiophene 77 indicate a small amount (8.4% in CS2) of the second keto tautomer 78. The enol tautomer 76 eould not be detected [78JCS(P1)292]. 

The System described in the previous section has been extended with a sulfur chemiluminescence detector (SCO) for the detection of Sulfur compounds (32). The separated fractions were thiols + sulfides + thiophenes (as one group), benzothio-phenes, dibenzothiophenes and benzonaphtho-thiophenes. These four groups have been subsequently injected on-line into and separated by the GC unit. Again, no overlap between these groups has been detected, as can be seen from Figure 14.20, in which the total sulfur compounds are shown and from Figure 14.21 in which the separated dibenzothiophenes fraction is presented. The lower limit of detection of this method proved to be 1 ppm (mg kg ) sulfur per compound. 

ESR experiments employing in situ photolytic decomposition of the peroxydisulfate anion (S20g ) have been carried out to study the reaction of S04 with aliphatic sulfoxides. In the case of dimethyl sulfoxide three radicals are detected ( CHj, CH3 S02, CH2 S(0)CH3), the proportion being pH-dependent. The reaction is assumed to proceed via an initially formed radical cation (not detected) which would be rapidly hydrated to give an intermediate identical with that generated by OH addition on the sulfoxide. Such a process parallels the rapid hydration of radical cations formed from thiophene in their reactions with SO/ and... 

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. 

The main dilference between thiophene and selenophene is that on flash photolysis in the former SH and not S2, while in the later Se2 and not SeH were detected in the photodecomposition products. This suggests that in selenophene the dissociation takes place at the Se site and that Se2 is formed as a result of the recombination of the Se atoms.24... 

Poly(thiophene) films have also been formed on a Pt foil potentiostatically from 1 M thiophene in 1M LiC104/CH3CN at 1.74 V vs. Ag/AgCl(lM). The films were then removed from the thiophene solution and placed in 1M LiC104/CH3CN and cycled between 0 V to 2.0 V vs. Ag/AgCl to test for the presence of polymer films. At deposition potentials below 1.74 V no film deposition was detected, at higher... 

Along with some disubstituted furan 59, mono-arylation product 58 was isolated when 2-chloro-3,6-diethylpyrazine (57) and furan were refluxed in the presence of Pd(Ph3P)4 and KOAc. In the case of 2-chloro-3,6-dimethylpyrazine (23) and thiophene, monothienylpyrazine 60 was the sole product. When 2-chloro-3,6-diisobutylpyrazine was used as substrate, 9% of the disubstituted thiophene was detected. Analogous to the couplings with furan and thiophene, the heteroaryl... 

A novel thiophene-based molecular scaffold that could be used for optical in vivo imaging of protein aggregates was recently reported by Aslund et al. [36]. This chemically defined anionic pentameric thiophene derivative, p-FTAA (Fig. 21a), could be utilized for direct in vivo detection of A [S deposit in the brain of a transgenic mouse model with AD pathology. The deposits were easily visualized by two-photon... 

Studies by Heinze etal. on donor-substituted thiophenes or pyrroles [33] such as methylthio (= methylsulfonyl) or methoxy-substituted derivatives provide further clear evidence for this reaction pathway. They found, for instance, that 3-methylthiothiophene or 3-methoxythio-phene (2) undergo a fast coupling reaction. However, deposition processes or insoluble film formation could not be detected in usual experiments with these compounds, even at high concentrations. Similarly, the corresponding 3,3 -disubstituted bithiophenes (2a) do not polymerize, but the anodic oxidation of 4,4 -disubstituted bithiophenes (2c) produces excellent yields of conducting polymers. 

Fig. 6.57. El mass spectmm of thiophene. The isotopic pattern of sulfur directly reveals its presence in the ions detected at m/z 45, 58, (69), and 84. Spectrum used by permission of NIST. NIST 2002.

The addition of silyl radicals to double bonds in benzene or substituted benzenes (Reaction 5.2) is the key step in the mechanism of homolytic aromatic substitution with silanes [8,9]. The intermediate cyclohexadienyl radical 2 has been detected by both EPR and optical techniques [21,22]. Similar cyclohex-adienyl-type intermediates have also been detected with heteroaromatics like furan and thiophene [23]. 

---