Purification, general anhydrides

Recovery and Purification. AH processes for the recovery and refining of maleic anhydride must deal with the efficient separation of maleic anhydride from the large amount of water produced in the reaction process. Recovery systems can be separated into two general categories aqueous- and nonaqueous-based absorption systems. Solvent-based systems have a higher recovery of maleic anhydride and are more energy efficient than water-based systems. 

This procedure is based upon a study 1 of the method outlined in the patent literature.2 The procedure is a general one and may be used for the condensation of succinic anhydride with naphthalene and with the mono- and dimethylnaphthalenes, although in no other case are the purification and separation of isomers so easily accomplished. In this particular type of condensation, as well as in certain other types of Friedel-Crafts reactions, nitrobenzene is far superior to the solvents which are more frequently employed. This is partly because of its great solvent power and partly because it forms a molecular compound with aluminum chloride, and so decreases the activity of the catalyst in promoting side reactions. 

Discussion In principle, acetals are cleaved by acid-catalyzed hydrolysis. In most cases aqueous acetic acid, aqueous trifluoracetic acid, dilute HC1 in THF or DOWEX 50W (H+) resin are used. Thus, treatment of 6 with DOWEX ion exchange resin in methanol rapidly furnishes the corresponding 1,2-diol without any further chromatographic purification steps. Generally, polymer supported reagents benefit from the ease of removal from the reaction mixture just by filtration of the insoluble resin. The resulting diol is acetylated by addition of acetic anhydride and pyridine. Final acetal exchange is achieved by acetic anhydride and catalytic amounts of concentrated sulfuric acid. A mixture (2 1) of anomers is obtained. 

Barton Esterification Reductive Decarboxylation. O-Acyl thiohydroxamates or Barton esters are useful precursors of carbon-centered radicals via thermolysis or photolysis. Several different methods are available for converting carboxylic acids into Barton esters (eq 1). These reactions generally proceed via the attack of a 2-mercaptopyridine-N-oxide salt on an activated carboxylic acid that has either been preformed (acid chloride, mixed anhydride) or generated in situ (with 1,3-dicyclohexylcarbodiimide or tri-n-butylphosphine + 2,2 -dithiodipyridine-l,r-dioxide). However, HOTT has the distinct advantages of (1) being easy to prepare and handle without the need for any special precautions, (2) facilitates efficient Barton esterification of carboxylic acids, and (3) simplifies subsequent work-up and purifications by avoiding the need to remove by-products like 1,3-dicyclohexylurea. 

Quinazoline-4(3//)-thiones 13 are prepared in a very simple, one-step synthesis by treatment of 2-[(ethoxymethylene)amino] derivatives 12 with alcoholic sodium hydrosulfide. Numerous other hetero-fused pyrimidinethiones are also available by this procedure (Method Cyclization proceeds uniformly in very high yields, generally in excess of 90%. In some cases, since isolation and purification of (ethoxymethylene)amino derivatives results in the lowering of the overall yield, the one-step procedure, consisting of treatment of a 2-aminonitrile 11, either with a 1 1 mixture of triethyl orthofoimate and acetic anhydride or with triethyl orthoformate alone, to give an intermediate ethoxymethyleneamino derivative which, without isolation, is treated with an ethanolic solution of sodium hydrosulfide, is preferred for preparative purposes (Method B). In a few cases, conversion of the 2-aminonitrile to a 2-aminothioamide followed by cyclization with triethyl orthoformate (see p 47) competes favorably with the above procedure. 

Additional versions of the earlier process [59-61] as well as processes based on phosphation of alcohol/water mixtures with phosphoric anhydride [62,63] have been published. The first group includes added intermediate or postreaction purification steps and steam stripping or other methods to remove residual alcohol. The products resulting from the water/phosphoric anhydride routes are generally not characterized by the combination of a high mono- to dialkyl ratio and low-residual phosphoric acid and alcohol levels associated with the phosphoric acid/phosphoric anhydride hybrid phosphation reagent processes. 

Acid anhydrides and chlorides are reactive as the electrophile for activation of dimethyl sulfoxide. Preparatively useful procedures based on acetic anhydride/ trifluoroacetic anhydride,and oxalyl chloride have come into general use. The pyridine-SOa complex is also useful.Scheme 10.3 gives some representative examples. Entry 4 is an example of the use of a water-soluble carbodiimide as the activating reagent. The modified carbodiimide facilitates product purification by providing for easy removal of the by-product urea formed from the carbodiimide. 

In general, chemical transformation processes can be exothermic or endothermic. The examples of catalytic selective oxidation presented by Leo Manzer in Chapter 9 are exothermic reactions, in which carbon oxides are formed as by-products. Increasing selectivity for the desired product would lower carbon emission because of less CO formation in the reaction and lower energy consumption needs for downstream separation and purification. One example mentioned in Chapter 9 is the production of maleic anhydride by selective oxidation of butane (Equation 10.1). The commercial yield is only about 50%. That is, about 4 moles of CO are formed as reaction by-products for every mole of maleic anhydride produced. There is substantial room for improvement ... 

Attempts to esterify these directly by the use of acrylic acid or acrylic anhydride were not successful. It was found that a convenient, high-yield synthesis could be carried out in fluorocarbon solvent by the reaction of acryloyl chloride and a tertiary amine acid acceptor. Product purification by distillation was generally not satisfactory because of the temperatures required, particularly for the difunctional compounds, but purification by percolation of the fluorocarbon solvent solutions over activated alumina resulted in colorless products of sufficient purity for effective polymerization. 

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