Dehydrating agent, sulfuric acid

Phosphoric acid (H3P04) often is used in place of sulfuric acid to dehydrate alcohols. This is because phosphoric acid is less destructive it is both a weaker acid and a less powerful oxidizing agent than sulfuric acid. Dehydration probably proceeds by mechanisms similar to those described for sulfuric acid (Section 15-5C) and very likely through intermediate formation of a phosphate ester ... 

Another dehydration procedure employs 72% aqueous magnesium nitrate (Mg(N03)2 4 H2O) as a dehydrating agent, which gets around some of these difficulties [45]. As in sulfuric acid dehydration, hot nitric acid vapor is fed... 

Perchloric acid Acetic acid, acetic anhydride, alcohols, antimony compounds, azo pigments, bismuth and its alloys, methanol, carbonaceous materials, carbon tetrachloride, cellulose, dehydrating agents, diethyl ether, glycols and glycolethers, HCl, HI, hypophosphites, ketones, nitric acid, pyridine, steel, sulfoxides, sulfuric acid... 

Dehydration or Chemical Theory. In the dehydration or chemical theory, catalytic dehydration of ceUulose occurs. The decomposition path of ceUulose is altered so that flammable tars and gases are reduced and the amount of char is increased ie, upon combustion, ceUulose produces mainly carbon and water, rather than carbon dioxide and water. Because of catalytic dehydration, most fire-resistant cottons decompose at lower temperatures than do untreated cottons, eg, flame-resistant cottons decompose at 275—325°C compared with about 375°C for untreated cotton. Phosphoric acid and sulfuric acid [8014-95-7] are good examples of dehydrating agents that can act as efficient flame retardants (15—17). 

Organic Reactions. Nitric acid is used extensively ia iadustry to nitrate aHphatic and aromatic compounds (21). In many iastances nitration requires the use of sulfuric acid as a dehydrating agent or catalyst the extent of nitration achieved depends on the concentration of nitric and sulfuric acids used. This is of iadustrial importance ia the manufacture of nitrobenzene and dinitrotoluene, which are iatermediates ia the manufacture of polyurethanes. Trinitrotoluene (TNT) is an explosive. Various isomers of mononitrotoluene are used to make optical brighteners, herbicides (qv), and iasecticides. Such nitrations are generally attributed to the presence of the nitronium ion, NO2, the concentration of which iacreases with acid strength (see Nitration). 

Sulfonic acids may be subjected to a variety of transformation conditions, as shown in Figure 2. Sulfonic acids can be used to produce sulfonic anhydrides by treatment with a dehydrating agent, such as thionyl chloride [7719-09-7J. This transformation is also accomphshed using phosphoms pentoxide [1314-56-3J. Sulfonic anhydrides, particulady aromatic sulfonic anhydrides, are often produced in situ during sulfonation with sulfur trioxide. Under dehydrating conditions, sulfonic acids react with substituted aromatic compounds to give sulfone derivatives. 

Strong dehydrating agents such as phosphorous pentoxide or sulfur trioxide convert chlorosulfuric acid to its anhydride, pyrosulfuryl chloride [7791-27-7] S20 Cl2. Analogous trisulfuryl compounds have been identified in mixtures with sulfur trioxide (3,19). When boiled in the presence of mercury salts or other catalysts, chlorosulfuric acid decomposes quantitatively to sulfuryl chloride and sulfuric acid. The reverse reaction has been claimed as a preparative method (20), but it appears to proceed only under special conditions. Noncatalytic decomposition at temperatures at and above the boiling point also generates sulfuryl chloride, chlorine, sulfur dioxide, and other compounds. 

Butyl Ether. -Butyl ether is prepared by dehydration of -butyl alcohol by sulfuric acid or by catalytic dehydration over ferric chloride, copper sulfate, siUca, or alumina at high temperatures. It is an important solvent for Grignard reagents and other reactions that require an anhydrous, inert medium. -Butyl ether is also an excellent extracting agent for use with aqueous systems owing to its very low water-solubiUty. 

The use of dehydrating agents such as sulfuric or phosphoric acid on (555 X = OH) was also successful, and these closures may proceed via mixed anhydrides 67AHC(8)277, 75MIP41600). Carbonyldiimidazole effected the conversion of hydroxamic acid (557) into a 3-hydroxy-1,2-benzisoxazole derivative (79JHC1277). The mixed anhydride (558) where... 

The use of mixed acid (mixt of nitric and sulfuric acids) as a nitrating agent was first described in 1846 by Muspratt Hoffman (Ref 1). The sulfuric acid in their mixt was considered to be a dehydrating agent , a view first advanced by Spindler (Ref 4) and developed further by Markovnikov (Ref 7) Sapozhnikov (Ref 8). The modern concept of the role of sulfuric acid in mixed... 

Sulfur dioxide is the anhydride of sulfurous add, and sulfur trioxide is the anhydride of sulfuric acid. Sulfuric acid is a strong acid, a dehydrating agent, and an oxidizing agent. 

FIGURE 15.17 Sulfuric acid is a dehydrating agent. When concentrated sulfuric acid is poured on to sucrose (a), the sucrose, a carbohydrate, is dehydrated (b), leaving a frothy black mass of carbon (c). 

Although very corrosive, it can be stored and shipped in steel or common alloys at commercial concentrations. In some chemical processes, it is used simply as an acid while in others it is used as a dehydrating agent to remove water, as an agent to increase the rate of a chemical reaction (catalyst), or as a solvent for reactions in which it remains unchanged. It rarely ends up in the final product. Waste sulfuric acid can be recycled. 

This reaction is run by heating the alcohol in the presence of concentrated sulfuric acid, a good dehydrating agent. Diethyl ether, CH,CH2OCH2CH3, was formerly used as an anesthetic in surgery, but it caused undesirable side effects in many patients, and was replaced by other types of agents. 

Interaction of the anhydrous acid and sulfur trioxide is violent and highly exothermic, even in presence of chloroform as diluent, and explosions are frequent. See Dehydrating agents, above... 

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