Hydrogen sulphide sources

Hydrogen sulphide is a key intermediate in the formation of many heterocyclic sulphur compounds. It is produced from cysteine by hydrolysis or by Strecker degradation ammonia, acetaldehyde and mercaptoacetaldehyde are also formed (Scheme 12.4). All of these are reactive compounds, providing an important source of reactants for a wide range of flavour compounds. Scheme 12.6 summarises the reactions between hydrogen sulphide and other simple intermediates formed in other parts of the Maillard reaction. 

The sulphides of the alkali metals and of the alkaline earth metals are readily decomposed by weak acids, even by carbonic acid.3 Ily the use of these sulphides and dilute sulphuric acid it is possible to obtain hydrogen sulphide in a high degree of purity. The hydrosulphides of the alkaline earth metals arc convenient sources of hydrogen sulphide, yielding the gas on treatment with carbon dioxide or even by merely heating.4 The carbon dioxide method is applied on a technical scale in the extraction of sulphur from alkali waste (see p. 50). 

Zielinski, E. Use of argon ionisation detector with <3Ni source for chromatographic determination of hydrogen sulphide, sulfur dioxide and carbon oxy-sulfide. Chem. Anal. 14, 521 (1969). - Z. Anal. Chem. 254, 378 (1971). 

When hydrogen sulphide is the heteroatom source the mechanism is similar to the pyrrole case. 

By 2000, by-product hydrogen sulphide in North America will represeijitj a potential source of a minimum of 15 x 10 m3/year (5 x 10 SCF/yr) of hydrogen, and potential.could be as great as 25 - 35 x 10 m /yr (8.5 - 11.5 x 10 SCF/yr). World-wide the potential is of course even more significant (Table II). 

If an individual generator is desired, a Pyrex test-tube (150 x 20 mm) charged with Aitch-tu-ess or an equivalent mixture (essentially an intimate mixture of a solid paraffin hydrocarbon, sulphur, and asbestos) is utilized this yields hydrogen sulphide when heated by a micro burner and the evolution of gas ceases (but not usually abruptly) when the source of heat is removed. The test-tube type of generator is depicted in Fig. 11.28. The small hole (vent) is covered... 

The SH radical is the only diatomic hydride of the first two periods whose vibrational constants are completely unknown and one of the few common hydrides for which not even an approximate value of the dissociation energy is available. Its spectrum does not appear readily and only one band, the o—o band of the i/ — i7 transition, has ever been observed.It is well known that hydrogen sulphide can be decomposed photochemically into its elements, and although other mechanisms have been proposed it is probable that the primary decomposition is to H and SH. For this reason the flash photolysis of H,S was studied as a probable source of the SH radical,... 

The first autotrophes to use the Sun s energy (i.e. photosynthesizers) would also have been prokaryotic anaerobes hving in aquatic environments, and they used hydrogen sulphide as a source of the hydrogen needed for carbohydrate synthesis, yielding sulphur as a byproduct, not oxygen ... 

There is another, very important and large repository of methane methane hydrates (also known as gas hydrates or clathrates Kvenvolden 1988).They comprise ice in which the interstices of the lattice house small molecules, such as methane, ethane, carbon dioxide and hydrogen sulphide. In fact, enough gas needs to be present to fill 90% of the interstices in order for the hydrate to form, and it has a different crystal structure from normal ice (Sloan 1990). If fully saturated, the most common crystalline structure can hold one molecule of methane for every 5.75 molecules of water, so lm3 of hydrate can contain 164 m3 of methane at STP (see Box 4.8).The solubility of methane in water is insufficient to account for hydrate formation, and a major nearby source is required, typically methanogenesis, based on the dominance of methane (99%) and its very light isotopic composition (813C generally <—60%o see Section 5.8.2). 

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