Sulfuric acid as dehydrating agent

Flavins condense with activated aldehydes in the presence of sulfuric acid and dehydrating agents to yield deeply coloured condensation products 61). Similarly the dimerization of some substituted flavins to yield biflavins has been described 76). The loss of an 8a-proton has been proposed as the initial key step for both condensation reactions 76). 

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). 

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... 

The advance of sulfur trioxide as sulfating agent largely depended on advances in sulfonation/sulfation reactor development and changes in raw material quality. Undiluted sulfur trioxide cannot be used as a sulfating agent except in special cases where suitable equipment is used because of its violent nature. Sulfur trioxide diluted in an inert gas, usually air, when used in batch processes can cause excessive dehydration and dark-colored products. However, batch processes were used years ago and inert liquid solvents were often suggested or used to moderate the reaction. Inadequate reaction conditions lead to a finished product that can contain dialkyl sulfate, dialkyl ether, isomeric alcohols, and olefins whereas inadequate neutralization conditions can increase the content of the parent alcohol due to hydrolysis of the unstable acid sulfate accompanied by an increase of mineral sulfate. 

The last two compds serve as dehydrating agents. Mixed nitric-sulfuric acids are also used in the prepn of NC, which is an important component of many smokeless propellants and an occasional ingredient of some dynamites. Acids are also used to neutralize the residual basicity of some expls due to the use of alkalies in purification. 

Concentrated sulfuric acid and/or concentrated phosphoric acid are often used as reagents for dehydration because these acids act both as acidic catalysts and as dehydrating agents. Hydration of these acids is strongly exothermic. The overall reaction (using sulfuric acid) is... 

The Chemical Properties and Uses of Sulfuric Acid. The uses of sulfuric acid are determined by its chemical properties—as an acid, a dehydrating agent, and an oxidizing agent. 

Sulfuric acid as an acid, a dehydrating agent, an oxidizing agent. 

For the preparation of small amounts of olefin, sulfuric acid, phosphoric acid, or zinc chloride may be used as catalysts. For larger amounts of olefins the alcohol is dehydrated by passing it over alumina heated at 200-300°. The use of sulfuric acid as a dehydrating agent involves the possibility of a number of side reactions, of which oxidation is the most important, and causes troublesome frothing. Phosphoric acid is preferred when it is desired to keep oxidation at a minimum. Ordinary (syrupy) phosphoric acid contains 15 per cent of water hence it must be first dehydrated by heating in an open vessel. 

In 1884, C. Paal and L. Knorr almost simultaneously reported that 1,4-diketones upon treatment with strong mineral acids underwent dehydration to form substituted furans. This transformation soon became widely used and now it is referred to as the Paal-Knorr furan synthesis. The general features of the method are 1) virtually any 1,4-dicarbonyl compound (mainly aldehydes and ketones) or their surrogates are suitable substrates 2) the dehydration is affected by strong mineral acids such as hydrochloric acid or sulfuric acid, but often Lewis acids and dehydrating agents (e.g., phosphorous pentoxide, acetic anhydride, etc.) can be used and 3) the yields are usually moderate to good. The two major drawbacks of the reaction are the relative difficulty to obtain the 1,4-dicarbonyl substrates, and the sensitivity of many functionalities to acidic conditions. 

Acylations with carboxylic acids and anhydrides have been carried out with sulfuric acid as both solvent and catalyst, the reactive acylating agents from acyl halides probably being haloacyloxonium ions. Trifluoroacetic anhydride offers a rather milder reagent for reactions of carboxylic acids, with mixed anhydrides being likely intermediates. However, polyphosphoric acid remains the most widely used dehydrating agent for acylations by carboxylic acids. 

As also the oxides of phosphor, they form polymers willingly. Sulfuric acid is a strong acid, a dehydrating agent and an oxidizing agent. Most inorganic salts of the anion sulfate are water soluble. One exception is barium sulfate and the characteristic of this salt is used in the test. The identification test for sulfates is special since the purposes of the different steps of the analysis are explained in parentheses in the pharmacopoeial text. 

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