Carbonyl, sulfide

Sulfur might be present in inorganic forms elemental S, hydrogen sulfide H2S, carbonyl sulfide COS, or positioned within organic molecules as in the following ... 

Hydroxy-THISs react with electron-deficient alkynes to give nonisol-able adducts that extrude carbonyl sulfide, affording pyrroles (23). Compound 16 (X = 0) seems particularly reactive (Scheme 16) (25). The cycloaddition to benzyne yields isoindoles in low- yield. Further cyclo-addition between isoindole and benzyne leads to an iminoanthracene as the main product (Scheme 17). The cycloadducts derived from electron-deficient alkenes are stable (23, 25) unless highly strained. Thus the two adducts, 18a (R = H, R = COOMe) and 18b (R = COOMe, R = H), formed from 7, both extrude furan and COS under the reaction conditions producing the pyrroles (19. R = H or COOMe) (Scheme 18). Similarly, the cycloadduct formed between 16 (X = 0) and dimethylfumarate... 

In contrast to the 4-hydroxy isomers, the thermally stable 5-hydroxy-THISs add to the C=C bond of cyclopropenylidenes (4. 18, 27. 28). The adducts eliminate carbonyl sulfide, and the strained bond breaks resulting in ring-expansion with formation of pyridin-4-ones. -thiones, or -imines. or 4-alkylidenedihydropvridines (20, X = 0. S.NR. or CRR ) (Scheme 19). 

HydrOxy-THISs react with diethoxycarbonylazine producing a 1,2,4-triazole via addition, elimination of carbonyl sulfide (29). and subsequent loss of the ester groups (Scheme 20) (30). 

Hydroxy-THISs add regioselectively to the C=N bonds of isocyanates or isothiocyanates. The initially formed cycloadducts eliminate carbonyl sulfide with formation of 4-hydroxy- or 4-mercaptoimidazolium hydroxide inner salts (21) (Scheme 21). 4-Hydroxyimidazolium hydroxide... 

The reaction is carried out at low temperature in aqueous medium and then allowed to stand overnight (221). Ammonium thiocarbamate is prepared from a cold saturated solution of ammonium thiocyanate, which is gradually added to dilute sulfuric acid at 25°C. The liberated carbonyl sulfide is passed into a saturated solution of alcoholic ammonia at about 10°C (221). The fairly low yield indicates that the reaction has not been greatly developed. 

Carbon disulfide [75-15-0] is a clear colorless liquid that boils at 46°C, and should ideally be free of hydrogen sulfide and carbonyl sulfide. The reaction with alkaU cellulose is carried out either in a few large cylindrical vessels known as wet chums, or in many smaller hexagonal vessels known as dry chums. In the fully continuous viscose process, a Continuous Belt Xanthator, first developed by Du Pont, is used (15). 

Conversion of carbon in the coal to gas is very high. With low rank coal, such as lignite and subbituminous coal, conversion may border on 100%, and for highly volatile A coals, it is on the order of 90—95%. Unconverted carbon appears mainly in the overhead material. Sulfur removal is faciUtated in the process because typically 90% of it appears in the gas as hydrogen sulfide, H2S, and 10% as carbonyl sulfide, COS carbon disulfide, CS2, and/or methyl thiol, CH SH, are not usually formed. 

Numerous diamines and aromatic dianhydrides have been investigated. WhoUy aromatic Pis have been stmctiirally modified by incorporating various functional groups, such as ether, carbonyl, sulfide, sulfone, methylene, isopropjlidene, perfluoroisopropyUdene, bipyridyls, sdoxane, methyl phosphine oxide, or various combinations of these, into the polymer backbone to achieve improved properties. The chemistry and apphcations of Pis have been described in several review articles (4). 

In the reducing atmosphere of the reactor, sulfur compounds form hydrogen sulfide and small amounts of carbonyl sulfide [463-58-1J, COS, in a molar ratio of approximately 24 1. 

Molecular Sieve Treatment. Molecular sieve treaters can be designed to remove H2S, organic sulfur compounds (including carbonyl sulfide), and water in one step. SoHd-bed units are utilized and regeneration occurs in the same manner as simple, soHd-bed dehydrators. 

Solid-Bed Caustic Treatment. SoHd-bed caustic units utilizing methanol [67-56-1] injection into the LPG feed stream can be used for carbonyl sulfide removal. The methanol—caustic solution must be drained periodically from the beds and discarded. When the soHd bed is exhausted, the spent caustic must be discarded and replaced. The LPG from the treater has a low enough water content to meet the propane specification. 

Petroleum Gases and Naphtha. Methane is the main hydrocarbon component of petroleum gases. Lesser amounts of ethane, propane, butane, isobutane, and some 0 + light hydrocarbons also exist. Other gases such as hydrogen, carbon dioxide, hydrogen sulfide, and carbonyl sulfide are also present. 

PPS dust should be treated as a nuisance particulate. The OSHA permissible exposure limit for respirable dust is 5 mg/m for dust containing no asbestos and less than 1% siUca. The principal decomposition products released during mol ding of PPS and their permissible exposure limits are given in Table 10. Sulfur dioxide and carbonyl sulfide are the most significant off-gases for production of mucous membrane irritation. 

The flash point of PPS, as measured by ASTM D1929, is greater than 500°C. Combustion products of PPS include carbon, sulfur oxides, and carbonyl sulfide. Specific hazards are defined by the OSHA Hazard Communication Standard (158). Based on information in 1995, PPS does not meet any of the hazard definitions of this standard. 

Physica.1 Properties. Carbonyl sulfide [463-58-1] (carbon oxysulfide), COS, is a colorless gas that is odorless when pure however, it has been described as having a foul odor. Physical constants and thermodynamic properties are Hsted ia Table 1 (17,18). The vapor pressure has been fitted to an equation, and a detailed study has been made of the phase equiUbria of the carbonyl sulfide—propane system, which is important ia the purification of propane fuel (19,20). Carbonyl sulfide can be adsorbed on molecular sieves (qv) as a means for removal from propane (21). This approach has been compared to the use of various solvents and reagents (22). 

Table 1. Physical and Thermodynamic Properties of Carbonyl Sulfide...

Chemica.1 Properties. Reviews of carbonyl sulfide chemistry are available (18,23,24). Carbonyl sulfide is a stable compound and can be stored under pressure ia steel cylinders as compressed gas ia equiUbrium with Hquid. At ca 600°C carbonyl sulfide disproportionates to carbon dioxide and carbon disulfide at ca 900°C it dissociates to carbon monoxide and sulfur. It bums with a blue flame to carbon dioxide and sulfur dioxide. Carbonyl sulfide reacts... 

Alternative means for removal of carbonyl sulfide for gas streams iavolve hydrogenation. For example, the Beavon process for removal of sulfur compounds remaining ia Claus unit tail gases iavolves hydrolysis and hydrogenation over cobalt molybdate catalyst resulting ia the conversion of carbonyl sulfide, carbon disulfide, and other sulfur compounds to hydrogen sulfide (25). 

Carbonyl sulfide reacts slowly with aqueous alkah metal hydroxides, which can therefore be used to free carbonyl sulfide from acidic gases. The half-hfe at pH 12 (0.01 N KOH) is 3 min at 22°C. The product initially formed from carbonyl sulfide and aLkaU is the thiocarbonate [534-18-9] which then breaks down to carbonate and sulfide ... 

To effectively remove carbonyl sulfide from a gas stream, special alkaline scmbbiag Hquors are used. These contain sodium aluminate or sodium plumbite, or they are made of alkaUes with a hydrolysis catalyst based on Zn, Fe, Ni, or Cu. Diethanolamine, diglycolamine, or other alkanolamines (qv) mixed with water remove carbonyl sulfide from sour, ie, acid-gas-containing, gas streams (25,26) (see Carbon dioxide). 

Carbonyl sulfide reacts with chlorine forming phosgene (qv) and sulfur dichloride [10545-99-0] and with ammonia forming urea and ammonium sulfide [12135-76-1]. Carbonyl sulfide attacks metals, eg, copper, ia the presence of moisture and is thought to be iavolved ia atmospheric sulfur corrosion (27,28). Its presence ia propane gas at levels above a few ppm may cause the gas to fail the copper-corrosion test. 

Carbonyl sulfide reacts with amines to form thiocarbamates ... 

The photolysis of carbonyl sulfide is a laboratory method for the production of monoatomic sulfur, a short-Hved species (29). 

Occurrence ndPreparation. Carbonyl sulfide is formed by many high temperature reactions of carbon compounds with donors of oxygen and sulfur. A principal route is the foUowiag reaction (30) ... 

This equihbrium favors COS up to ca 500°C. At higher temperatures, COS dissociates increasingly, eg, to 64% at 900°C. The reaction may be mn at 65—200°C to produce carbonyl sulfide if an alkaline catalyst is used (31). A Rhc ne-Poulenc patent describes the manufacture of carbonyl sulfide by the reaction of methanol with sulfur at 500—800°C (32). 

Other important reactions yielding carbonyl sulfide foUow (33,34) ... 

Carbonyl sulfide occurs as a by-product ia the manufacture of carbon disulfide and is an impurity ia some natural gases, ia many manufactured fuel gases and refinery gases, and ia combustion products of sulfur-containing fuels (25). It tends to be concentrated ia the propane fraction ia gas fractionation an amine sweetening process is needed to remove it. 

Carbonyl sulfide is overall the most abundant sulfur-beating compound ia the earth s atmosphere 430—570 parts per trillion (10 ), although it is exceeded by H2S and SO2 ia some iadustrial urban atmospheres (27). Carbonyl sulfide is beheved to origiaate from microbes, volcanoes, and the burning of vegetation, as well as from iadustrial processes. It may be the main cause of atmospheric sulfur corrosion (28). 

Production, Shipment, and Specifications. Carbonyl sulfide is available ia 97% min purity ia cylinders up to 31.8 kg contained weight. It is shipped as a flammable gas. There appears to be no full-scale commercial production of carbonyl sulfide ia the United States. 

Analytical Methods. Detection of carbonyl sulfide ia air can be done by gas chromatography or by combustion to sulfur dioxide and determination of the latter. Where hydrogen sulfide and carbonyl sulfide occur together, the carbonyl sulfide can be determined by combustion after hydrogen sulfide is absorbed by lead acetate, which does not absorb carbonyl sulfide (35). 

Health and Safety Factors. Carbonyl sulfide is dangerously poisonous, more so because it is practically odorless when pure. It is lethal to rats at 2900 ppm. Studies show an LD q (rat, ip) of 22.5 mg/kg. The mechanism of toxic action appears to iavolve breakdowa to hydrogea sulfide (36). It acts principally on the central nervous system with death resulting mainly from respiratory paralysis. Little is known regarding the health effects of subacute or chronic exposure to carbonyl sulfide a 400-p.g/m max level has been suggested until more data are available (37). Carbon oxysulfide has a reported inhalation toxicity in mice LD q (mouse) = 2900 ppm (37). 

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