Sulfur Reducing Agents

Common reducing agents are hydrogen in the presence of metallic or complex catalysts (e.g. Ni, Pd, Pt, Ru, Rh), hydrides (e.g. alanes, boranes, LIAIH, NaBHJ, reducing metals (e.g. Li, Na, Mg, Ca, Zn), and low-valent compounds of nitrogen (e.g. NjHj, NjHJ, phosphorus (e.g. triethyl phosphite, triphenyiphosphine), and sulfur (e.g. HO-CHj-SOjNa = SFS, sodium dithionite = Na S O. ... 

Potassium perchlorate Aluminum plus magnesium, carbon, nickel plus titanium, reducing agents, sulfur, sulfuric acid... 

Starting from Benzene. In the direct oxidation of benzene [71-43-2] to phenol, formation of hydroquinone and catechol is observed (64). Ways to favor the formation of dihydroxybenzenes have been explored, hence CuCl in aqueous sulfuric acid medium catalyzes the hydroxylation of benzene to phenol (24%) and hydroquinone (8%) (65). The same effect can also be observed with Cu(II)—Cu(0) as a catalytic system (66). Efforts are now directed toward the use of Pd° on a support and Cu in aqueous acid and in the presence of a reducing agent such as CO, H2, or ethylene (67). Aromatic... 

The principal material cost for the Chilean iodine producers is sulfur used for the manufacture of sulfur dioxide, the reducing agent for the iodates contained in the leach solutions. Also, the use of sodium carbonate for the neutralization of the depleted solutions is an important cost factor. 

Phosphoric acid, aside from its acidic behavior, is relatively unreactive at room temperature. It is sometimes substituted for sulfuric acid because of its lack of oxidising properties (see SuLFURic ACID AND SULFURTRIOXIDe). The reduction of phosphoric acid by strong reducing agents, eg, H2 or C, does not occur to any measurable degree below 350—400°C. At higher temperatures, the acid reacts with most metals and their oxides. Phosphoric acid is stronger than acetic, oxaUc, siUcic, and boric acids, but weaker than sulfuric, nitric, hydrochloric, and chromic acids. 

Phosphonic acid and hydrogen phosphonates are used as strong but slow-acting reducing agents. They cause precipitation of heavy metals from solutions of their salts and reduce sulfur dioxide to sulfur, and iodine to iodide in neutral or alkaline solution. 

Ultramarine blues are prepared by a high temperature reaction of intimate mixtures of china clay, sodium carbonate, sulfur, siHca, sodium sulfate, and a carbonaceous reducing agent, eg, charcoal, pitch, or rosin. 

Zinin Reduction. The method of reducing aromatic nitro compounds with divalent sulfur is known as the Zinin reduction (57). This reaction can be carried out in a basic media using sulfides, polysulfides, or hydrosulfides as the reducing agent. These reactions can be represented as follows when the counter ion is sodium ... 

In past years, metals in dilute sulfuric acid were used to produce the nascent hydrogen reductant (42). Today, the reducing agent is hydrogen in the presence of a catalyst. Nickel, preferably Raney nickel (34), chromium or molybdenum promoted nickel (43), or supported precious metals such as platinum or palladium (35,44) on activated carbon, or the oxides of these metals (36,45), are used as catalysts. Other catalysts have been suggested such as molybdenum and platinum sulfide (46,47), or a platinum—nithenium mixture (48). 

Preparation. Thiophosgene forms from the reaction of carbon tetrachloride with hydrogen sulfide, sulfur, or various sulfides at elevated temperatures. Of more preparative value is the reduction of trichi oromethanesulfenyl chloride [594-42-3] by various reducing agents, eg, tin and hydrochloric acid, staimous chloride, iron and acetic acid, phosphoms, copper, sulfur dioxide with iodine catalyst, or hydrogen sulfide over charcoal or sihca gel catalyst (42,43). 

Tiichloiomethanesulfenyl chloiide can be reduced to thiophosgene by metals in the presence of acid and by various other reducing agents. The sulfur-bonded chlorine of trichloromethanesulfenyl chloride is most easily displaced by nucleophilic reagents, but under some conditions, the carbon-bound chlorines are also reactive (54). 

Various reducing agents, eg, hydrogen iodide, can abstract chlorine from sulfur monochloride leaving elemental sulfur ... 

Zinc hydrosulfite (zinc dithionite) is a powerhil reducing agent used in bleaching paper and textiles it is prepared from zinc dust and sulfur dioxide ... 

The reducing agents generally used in bleaching include sulfur dioxide, sulfurous acid, bisulfites, sulfites, hydrosulfites (dithionites), sodium sulfoxylate formaldehyde, and sodium borohydride. These materials are used mainly in pulp and textile bleaching (see Sulfur compounds Boron compounds). 

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