Sulfur dioxide, liquid

Liquid SO2 is an important non-aqueous solvent. It is polar and aprotic, and readily solvates anions. Selected properties of SO2 are listed in Table 9.3, and its liquid range is compared with other solvents in Fig. 9.2. 

Melting point/K Boiling point / K Density of liquid/gem Dipole moment/D Relative permittivity 

Liquid dinitrogen tetraoxide, N2O4, undergoes the selfionization shown in eq. 9.13. In this medium, nitrosyl salts such as [N0][C104] behave as acids, and metal nitrates (e.g. NaN03) behave as bases. 

In some ways, this acid-base terminology is unfortunate, since there are other, more common descriptors (e.g. Brpnsted, Lewis, hard and soft). However, the terminology has been helpful in suggesting lines of research for the study of non-aqueous systems, and its use will probably continue. 

Why does NH4CI behave as an acid in liquid ammonia  

Sulfur dioxide is one of the most extensively produced sulfur compound. Irrespective of the source of the sulfur, the first oxidation is in form of sulfur dioxide. Hence it can be called the key chanical for the heavy chemical industry. 

Liquid sulfur dioxide expands by about 10% when warmed from 20 °C to 60 °C under pressure. Pure Uquid sulfur dioxide is a poor conductor of electricity, but high conductivity solutions of some salts in sulfur dioxide can be made. Liquid sulfur dioxide is only sUghtly miscible with water. 

Sulfur dioxide is extranely stable to heat, even up to 2,000 C. It is not explosive or flanunable in admixture with air. The oxidation of sulfur dioxide by air or pure oxygen is a reaction of great commercial importance and is commonly conducted at 400-700 °C in the presence of a catalyst, e.g., vanadium oxide. 

The conventional method of producing sulfur dioxide is by combustion of sulfur followed by absorption-desorption, drying, and cryogenic condensation. This process is capital intensive, hazardous, saddled with corrosion, and involves high utility consumption  

Subsequently pure sulfur dioxide is produced by chemical reduction of sulfur trioxide with sulfur. This route is more direct and attractive due to the fact that sulfur trioxide is produced in a sulfuric acid plant. However, hitherto the process used by IG Farben Industrie of Germany under US patents 3,432,263 and 3,455,652 (1966) adds solid or molten sulfur to 25% oleum at 110 °C giving 98.5% SO2 and 1.5% SO3. The excess sulfur trioxide is further reacted with solid sulfur in a tower and then traces of sulfur trioxide are removed by absorbing in 98% sulfuric acid prior to compression and condensation. 

Sulfur dioxide can function as a very weak Lewis acid or a Lewis base and thereby form a variety of solvates  

With some very strong Lewis acids, hydrolysis occurs to produce an oxyhalide  

The S02 molecule has unshared pairs of electrons on both the sulfur and oxygen atoms. As a result, it forms numerous complexes with transitions metals in which it is known to attach in several ways. These include bonding through the sulfur atom, through an oxygen atom, by both oxygen atoms, and various bridging schemes. In most cases, the complexes involve soft metals in low oxidation states. Another important reaction of sulfur dioxide is known as the insertion reaction, in which it is placed 

This type of reaction will be discussed in more detail in Chapter 22. 

Even though liquid S02 does not autoionize, the acidic species would be SO2 + and the basic species would be S032-. Therefore, the neutralization reaction 

Although no attempt will be made to describe the chemistry of all of the nonaqueous solvents listed in Table 5.4, the survey to this point has included ammonia as a basic solvent and liquid hydrogen fluoride as an acidic solvent. Another solvent that has been extensively utilized in both inorganic and organic chemistry is sulfur dioxide. Accordingly, we will give a brief survey of the chemistry of liquid sulfur dioxide for which the physical properties are presented in Table 5.8. 

Although the dipole moment of S02 is fairly high, the dielectric constant is much lower than that of water and slightly lower than that of ammonia. Consequently, liquid S02 is a 

Density Heat of fusion Heat of vaporization Dipole moment Dielectric constant Specific conductance 

The low conductivity of liquid S02 has been interpreted as arising from ions produced by autoionization  

However, generating doubly charged ions would be highly unfavorable compared to autoionization of water, ammonia, or hydrogen fluoride in which proton transfer occurs. 

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Liquid sulfur dioxide expands by ca 10% when warmed from 20 to 60°C under pressure. Pure liquid sulfur dioxide is a poor conductor of electricity, but high conductivity solutions of some salts in sulfur dioxide can be made (216). Liquid sulfur dioxide is only slightly miscible with water. The gas is soluble to the extent of 36 volumes pet volume of water at 20°C, but it is very soluble (several hundred volumes per volume of solvent) in a number of organic solvents, eg, acetone, other ketones, and formic acid. Sulfur dioxide is less soluble in nonpolar solvents (215,217,218). The use of sulfur dioxide as a solvent and reaction medium has been reviewed (216,219). 

Liquid sulfur dioxide discolors iron, copper, and brass at ca 300 ppm moisture and produces light scale at ca 0.1 wt % moisture and serious corrosion at ca 0.2 wt % or higher moisture content. Copper and brass can be used to handle wet sulfur dioxide where some corrosion can be tolerated, or where the moisture level is low. Wooden tanks are widely used for sulfurous acid preparation, handling, and storage. Sulfite pulp digestors are made of steel lined with acid-resistant brick. 

Shipment and Storage. Liquid sulfur dioxide is commonly shipped in North America using 55- and 90-t tank cars, 20-ton tank tmcks, 1-ton cylinders, and 150-lb cylinders. Cylinders made of specified steel are affixed with the green label for nonflammable gases. The DOT classification is Poison Gas, Inhalation Ha2ard. Purchasers of tank-car quantities are required to have adequate storage faciUties for prompt transfer. 

The Inco flash smelting process produces a very high strength sulfur dioxide gas by using pure oxygen for smelting. Liquid sulfur dioxide is obtained upon compression. 

Fluoride ion attacks the sulfur atom in 2,3-diphenylthiirene 1,1-dioxide to give ck-1,2-diphenylethylenesulfonyl fluoride (23%) and diphenylacetylene (35%). Bromide or iodide ion does not react (80JOC2604). Treatment of S-alkylthiirenium salts with chloride ion gives products of carbon attack, but the possibility of sulfur attack followed by addition of the sulfenyl chloride so produced to the alkyne has not been excluded (79MI50600). In fact the methanesulfenyl chloride formed from l-methyl-2,3-di- -butylthiirenium tetrafluoroborate has been trapped by reaction with 2-butyne. A sulfurane intermediate may be indicated by NMR experiments in liquid sulfur dioxide. 

The formation of 2,3-di-f-butyl-l-methylthiirenium chloride from fran5-3-chloro-4-methylthio-2,2,5,5-tetramethyl-3-hexene is quantitative (by NMR) in liquid sulfur dioxide (Scheme 128) (82JOC590). Similar thiirenium ions are intermediates in the reactions of -thiovinyl derivatives (79MI50600). 

Recovery as liquid sulfur dioxide (absorption of clean dry off-gas in water or chemical absorbtion by ammonium bisulfite or dimethyl aniline),... 

Another improvement (less by-products) has been made over the fuming sulfuric acid process operated commercially for the hydrolysis of 2-perfluoroal-kylethyl iodides. It entails the use of sulfur trioxide in liquid sulfur dioxide [52, 5J (equation 52). 

As reagents concentrated sulfuric acid, hydrochloric acid, liquid sulfur dioxide, thionyl chloride, phosphorus pentachloride, zinc oxide" and even silica gel can be used. Reagents like phosphorus pentachloride (as well as thionyl chloride and others) first convert the hydroxy group of the oxime 1 into a good leaving group ... 

Liquid solvents are used to extract either desirable or undesirable compounds from a liquid mixture. Solvent extraction processes use a liquid solvent that has a high solvolytic power for certain compounds in the feed mixture. For example, ethylene glycol has a greater affinity for aromatic hydrocarbons and extracts them preferentially from a reformate mixture (a liquid paraffinic and aromatic product from catalytic reforming). The raffinate, which is mainly paraffins, is freed from traces of ethylene glycol by distillation. Other solvents that could be used for this purpose are liquid sulfur dioxide and sulfolane (tetramethylene sulfone). 

Nitroaniline (13.8 g., 0.10 mole) (Note 2) is dissolved in a hot solution of 75 ml. of 96% sulfuric acid, 100 ml. of phosphoric acid (density 1.7), and 50 ml. of water in a 1-1. beaker. A stirrer and a thermometer are introduced into the mixture, and the beaker is immersed in an ice bath. A solution of 8.3 g. (0.12 mole) of sodium nitrite in 25 ml. of water is added dropwise to the well-stirred solution at such a rate that the temperature is maintained at 10-15°. Excess nitrite is destroyed by adding sulfamic acid in small portions (Note 3). The mixture is cooled to —10° in an ice-salt bath, and about 50 ml. of liquid sulfur dioxide... 

Inert solvents such as dimethyl ether [22], liquid sulfur dioxide or petroleum ether [23] were used to improve the quality of the sulfated alcohol or the reaction conditions. Solvents immiscible in water, such as petroleum ether [24], carbon tetrachloride [25], or butyl alcohol [26], as well as water-soluble sol-... 

Concentration of sulfur dioxide in the combustion gases As high as 80% - very well suited for processing to sulfuric acid or sulfur or liquid sulfur dioxide Not as high as Inco process... 

In view of the concern about air quality, the recovery and disposal of sulfur dioxide has been the subject of many investigations, though none has yet been really successful (see Table 7.8). It should be mentioned that the options giving rise to highly concentrated sulfur dioxide suitable for liquefaction, or a gas stream sufficiently rich in sulfur dioxide to manufacture sulfuric acid, presuppose the existence of available markets for either liquid sulfur dioxide or sulfuric acid. 

Direct oxidation of diphenylmethanes is of little practical value as color formers. In liquid sulfur dioxide, leuco diphenylmethane 12 (Scheme 2) undergoes hydride abstraction by triphenylcarbenium perchlorate at the benzylic amine position to form immonium ion7 13, whereas in acetonitrile... 

Sodium hydrosulfite is produced through the Formate process where sodium formate solution, sodium hydroxide, and liquid sulfur dioxide reacted in the presence of a recycled stream of methanol solvent. Other products are sodium sulfite, sodium bicarbonate, and carbon monoxide. In the reactor, sodium hydrosulfite is precipitated to form a slurry of sodium hydrosulfite in the solution of methanol, methyl formate, and other coproducts. The mixture is sent to a pressurized filter system to recover sodium hydrosulfite crystals that are dried in a steam-heated rotary drier before being packaged. Heat supply in this process is highly monitored in order not to decompose sodium hydrosulfite to sulfite. Purging is periodically carried out on the recycle stream, particularly those involving methanol, to avoid excessive buildup of impurities. Also, vaporized methanol from the drying process and liquors from the filtration process are recycled to the solvent recovery system to improve the efficiency of the plant. 

Mixtures of the slightly soluble azide with liquid sulfur dioxide became explosive at elevated temperatures. 

During preparation of zinc ethylsulfinate, addition of diethylzinc to liquid sulfur dioxide at — 15°C leads to an explosively violent reaction. Condensation of sulfur dioxide into cold diethylzinc leads to a controllable reaction on warming. 

A previous method of making 4-oximino-4,5,6,7-tetrahydrobenzofurazan by cyclising the oxime with sulfinyl chloride in liquid sulfur dioxide sometimes led to explosive reactions [1]. A new procedure involving aqueous calcium carbonate is quite safe [2],... 

Potassium azide explodes at 120°C when heated in liquid sulfur dioxide. 

This reaction represents a neutralization reaction in liquid sulfur dioxide. It makes no difference that the solvent does not ionize or that SOCl2 is a covalent molecule. The utility of the solvent concept is not that it correctly predicts that solvents undergo some autoionization. The value of the solvent concept is that it allows us to correctly predict how reactions would take place if the solvent ionized. Note that in this case SOCl2 does not ionize, but if it did it would produce S02+ (the acidic species characteristic of the solvent) and Cl-. 

Several explosive species in which three or more azido groups are bonded to tellurium have been well characterized.59 The salt [Te(N3)3][SbF6] is formed from the reaction of Te4[SbF6]2 with potassium azide in liquid sulfur dioxide. The neutral binary tellurium azide Te(N3)4 is prepared by the reaction of TeF4 (in CFC13)60 or TeF6 (in CH3CN)61 with trimethylsilyl azide. 

Deprotonation of S7NF1 produces the thermally unstable, yellow [S7N] anion, which decomposes to the deep blue [S4N] anion.69 The oxidation of S7NH with trifluoroperoxyacetic acid yields S7NH(0), while oxidation with SbCl5 in liquid sulfur dioxide generates the [NS2]+ cation. 

Solutions of hexaphenylethane in liquid sulfur dioxide conduct electricity, suggesting an ionization into triphenylmethyl positive and negative ions. Since the spectrum of triphenylmethide ion was missing from the spectrum of the solution the following equilibrium was postulated ... 

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