Mercaptane

Corrosion protection is indispensable, especially concerning certain vulnerable parts of the aircraft such as the combustion chamber and turbine. The potential hazards are linked to the presence of sulfur in various forms mercaptans, hydrogen sulfide, free sulfur, and sulfides. 

It is noteworthy, however, that traces of sulfur can have beneficial effects on the anti-wear resistance of fuel injection pumps. It is thus undesirable to reduce the sulfur content to extremely low values unless additives having lubricating qualities are added. Independently from total sulfur content, the presence of mercaptans that are particularly aggressive towards certain metal or synthetic parts is strictly controlled. The mercaptan content is thereby limited to 0.002% (20 ppm) maximum. The analysis is performed chemically in accordance to the NF M 07-022 or ASTM D 3227 procedures. 

The Doctor Test, NF M 07-029 or ASTM D 484, is conducted in qualitative tests for free sulfur, free H2S, and mercaptans. The specifications require a negative Doctor Test for these three types of contaminants. 

Sometimes analyses are required for particular compounds such as sulfur, chlorine and lead, or for specific components such as mercaptans, hydrogen sulfide, ethers and alcohols. 

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. 

Table 8.7 gives some of the mercaptans identified in crude oils. 

Nature of cut (temperature interval, "Q Mercaptan sulfur, % Total sulfur, % % mercaptan sulfur total sulfur... 

Distribution of mercaptan sulfur among the different cuts of Arabian Light crude oil. 

The sulfides are chemically neutral they can have a linear or ring structure. For molecules of equal carbon number, their boiling points are higher than those of mercaptans they constitute the majority of sulfur containing hydrocarbons in the middie distillates (kerosene and gas oil). 

In the crude, water is found partly in solution and partly in the form of a more-or-less stable emulsion this stability is due to the presence of asphaltenes or certain surfactant agents such as mercaptans or naphthenic acids. 

Mercaptans are naturally present in crude oil (Chapters 1 and 8), or they result from the decomposition of other sulfur compounds during thermai or catalytic cracking operations. 

The sweetening operation consists of converting the mercaptans to disulfides by air oxidation in the presence of a catalyst in a caustic environment. 

Mercaptan sulfur NF M 07-022 ISO 3012 ASTM D 3227 Silver nitrate analysis... 

Sulfur (detection of H2S and mercaptans) ( Doctor Test ) NF M 07-029 ASTM D 325 Sodium plumbite test (coloration of interface)... 

It is important not to allow the reaction mixture to become appreciably alkaline, since the free base then decomposes rapidly yielding benzyl mercaptan, which has an unpleasant odour. 

Dissolve (or suspend) 0-25 g. of the acid in 5 ml. of warm water, add a drop or two of phenolphthalein indicator and neutralise carefully with ca. N sodium hydroxide solution. Then add 2-3 drops of ca. O lN hydrochloric acid to ensure that the solution is almost neutral (pale pink colour). (Under alkaline conditions the reagent tends to decompose to produce the evil-smelling benzyl mercaptan.) If the sodium salt is available, dissolve 0-25 g. in 5 ml. of water, and add 2 drops of ca. 0 -hydrochloric acid. Introduce a solution of 1 g. of S-benzyl-iso-thiuro-nium chloride in 5 ml. of water, and cool in ice until precipitation is Dibasic and tribasic acids will require 0-01 and 0-015 mol respectively. 

Mercaptans (or thio-alcohols or thiols), the sulphur analogues of the alcohols, were formerly prepared by the interaction of an alkyl halide and sodium hydrosulphide in alcoholic solution ... 

The lower members have remarkably disagreeable odours, but the offensive odour diminishes with increasing carbon content until it almost disappears at about CioH2i SH, n-dodecyl (lauryl) mercaptan. 

The dialkyl disulphides may be prepared by the oxidation of an alkaline solution of a mercaptan with iodine, for example ... 

The mixture should be colourless, otherwise difficulty will be experienced in the subsequent purification of the product. If the reaction mixture is coloured by iodine (due to volatilisation of some of the mercaptan), add just sufficient ethyl mercaptan to decolourise it. 

Mercaptans RSH (also thioethers or sulphides R SR and disulphides R SSR") are generally hquids and possess unpleasant odours. 

Alkyl mercaptans are partly soluble in solutions of caustic alkalis, but their salts are hydrolysed in dilute aqueous solution back to the free mercaptans. Thiophenols are soluble in alkah hydroxide solutions. Upon treatment with sodium, hydrogen is evolved. 

Alkyl (or Aryl) 2 4-dinitrophenyl-sulphides (or thioethers) and the corresponding sulphones. Mercaptans react with 2 4-dinitrochlorobenzene in alkaline solution to yield crystalhne thioethers (2 4-dinitrophenyl-sulphides) (I) ... 

The sulphides (I) can be readily oxidised in glacial acetic acid solution by potassium permanganate to the corresponding sulphones (II) the latter exhibit a wide range of melting points and are therefore particularly valuable for the characterisation of mercaptans ... 

Preparation of 2 4-dinitrophenyl-sulphides. Dissolve about 0-5 g. (or 0 005 mol) of the mercaptan in 10-15 ml, of rectified spirit (or in the minimum volume necessary for solution warming is permissible) and add 2 ml. of 10 per cent, sodium hydroxide solution. Mix the resulting sodium mercaptide solution with a solution of 1 g. of 2 4-dinitrochlorobenzene in 5 ml. of rectified spirit. Reaction may occur immediately with precipitation of the thioether. In any case reflux the mixture for 10 minutes on a water bath in order to ensure the completeness of the reaction. Filter the hot solution rapidly allow the solution to cool when the sulphide will crystaUise out. RecrystaUise from alcohol. 

Dinitrothiobenzoates. Mercaptans react with 3 5-dinitro-benzoyl chloride in the presence of pyridine as a catalyst to yield 3 5-di-nitrothiobenzoates ... 

Mix 0-2 g. of 3 5-dinitrobenzoyl chloride, 6 drops of the mercaptan and 1-3 drops of pyridine in a test-tube, and heat the mixture in a beaker of boiling water until fumes of hydrogen chloride cease to appear (15-30 minutes). Add a few drops of water, followed by a drop or two of pyridine to eliminate the excess of the reagent. The product sohdifies upon stirring with a glass rod. Add water, filter, and recrystalUse from dilute alcohol or dilute acetic acid. 

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