4- phthalic anhydride

Phthalic anhydride (melting point 131.6°C, boiling point 295°C with sublimation) can be made from the reaction of o-xylene with air and also from naphthalene (Fig. 1), which is isolated from coal tar and from petroleum. 

The use of naphthalene as the source of phthalic anhydride diminished but has become popular once again. 

Phthalic anhydride is used for the manufacture of plasticizers, such as dioctyl phthalate, unsaturated polyester resins, and alkyd resins. Phthalic anhydride reacts with alcohols such as 2-ethylhexanol, and the products are often liquids that, when added to plastics, impart flexibility without adversely affecting the strength of the plastic. Most of these plasticizers are used for poly(vinyl chloride) flexibility. Dioctyl phthalate is a common plasticizer. 

Phthalic Anhydride. Phthalic anhydride is manufactured by the catalytic vapor-phase oxidation of o-xylene [Eq. (9.182)] or naphthalene [Eq. (9.183)]  

Both oxidations are highly exothermic and carried out almost exclusively in tubular reactors cooled by a molten salt.1024 Supported vanadium oxide with additives to improve activity, selectivity, and stability usually serves as the catalyst.970 990 1025 Because of its more favorable stoichiometry (no carbon is lost in oxidation), most new plants use o-xylene as the starting material. 

Compressed air and o-xylene or naphthalene vapor are mixed in an approximately 20 1 ratio and oxidized at 400°C. The product phthalic anhydride is recovered by condensation and purified by vacuum distillation. Maleic anhydride, a byproduct in both oxidations may be recovered from the waste gases in the form of maleic acid after water scrubbing. 

The BASF process1026 was developed to use o-xylene exclusively. Most processes, however, such as the Wacker-von Hey den process1027 1028 are capable of operating on both feedstocks or may use a mixture of the two compounds. Fluidized-bed operations were developed in the 1960s (Badger process1029 1030) but have been replaced by improved, more economical fixed-bed processes. The Alusuisse low-air-ratio (LAR) process, for instance, allows the use of a 9.5 1 air o-xylene mixture and achieves an increased catalyst productivity.1031 1032 

Detailed mechanistic studies about both oxidations have been carried out.901,1006,1033 On the basis of the observed intermediates, a reaction network [Eq. (9.184)] was suggested in the oxidation of o-xylene indicating o-tolualdehyde as the first intermediate 1034,1035 

Currently, phthalic anhydride is mainly produced through catalyzed oxidation of o-xylene. A variety of metal oxides are used as catalysts. A typical one is V2O5 -1- Ti02/Sb203. Approximate conditions for the vapor-phase oxidation are 375-435°C and 0.7 atmosphere. The yield of phthalic anhydride is about 85%  

Liquid-phase oxidation of o-xylene also works at approximately 150°C. Cobalt or manganese acetate in acetic acid medium serves as a catalyst. 

The major by-products of this process are maleic anhydride, benzoic acid, and citraconic anhydride (methylmaleic anhydride). Maleic anhydride could be recovered economically.  

Phthalic anhydride s main use is for producing plasticizers by reactions with C4-C10 alcohols. The most important polyvinyl chloride plasticizer is formed by the reaction of 2-ethylhexanol (produced via butyraldehyde. Chapter 8) and phthalic anhydride  

The manufacture of phthalic anhydride (world installed capacity ca. 4.4 Mt/a) has several points of similarity to that of maleic anhydride in that there are two alternative feedstocks and a large amount of heat is released. The first process, introduced by BASF at the end of 19 century, was based on the liquid phase oxidation of naphthalene catalyzed by mercury salts. It was later replaced by the cleaner gas phase process, carried out over vanadium and molybdenum oxides. Naphthalene was supplied by coal tar distillation and was used exclusively until the end of 1950s when u-xylene, of petrochemical origin, became an abundantly available feedstock (Equation 36). A few production units however can use either feedstock, taking advantage of price fluctuations in coke plants (naphthalene) and in refineries (u-xylene). 

More than 90% of the phthalic anhydride is currently produced by fixed-bed processes. The concentration of u-xylene in the feed, ca. 60-70 g/m, is somewhat above the lower explosion limit ca. 40 g/m ). This can be raised further 

A third process was the liquid phase oxidation of o-xylene, catalyzed by Mn Co Br in acetic acid solution, via intermediate phthalic acid production. It was operated for a time in the 1970s and then discontinued. 

The major use of phthalic anhydride is for ester derivatives used as plasticizers in the manufacture of flexible poly(vinyl chloride), e.g., wallpaper. The largest volume plasticizer is the di(2-ethylhexyl)phthalate. Other uses of phthalic anhydride are for unsaturated polyester and alkyd resins, for dye intermediates, and for isatoic anhydride (for the production of saccharin). 

Alternatives to the use of hydroxylamine include the reaction of nit-rosylsulfuric acid with cyclohexanecarboxylic acid, obtained by the hydrogenation of benzoic acid, to produce s-caprolactam directly, and by the photochemical nitrosation of cyclohexane with NOCl, that yields cyclohexanone oxime hydrochloride. In both cases excess ammonium sulfate is also produced. 

The manufacturing method of making phthalic anhydride has been changing rapidly similar to the switchover in making maleic anhydride. In 1983 28% of phthalic anhydride came from naphthalene, 72% from o- 

The uses of phthalic anhydride include plasticizers (53%), unsaturated polyester resins (22%), and alkyd resins (15%). 

High doses of DEHP have been found to cause liver cancer in rats and mice and it is on the Reasonably Anticipated to Be Human Carcinogens list. In 2000 a report by the National Toxicology Program found serious concern that DEHP in vinyl medical devices may harm the reproductive organs of critically ill and premature male infants exposed during medical treatment. They also expressed concern that development of male unborn babies would be harmed by the pregnant mothers exposure to DEHP or that 

Szmant, Organic Building Blocks of the Chemical Industry, pp. 407-574. Wiseman, Petrochemicals, pp. 101-140. 

The oxidation of organic compounds to useful products was not reported extensively until the 1920s. Before then, any hot combustible material mixed with air passing over a catalyst produced mainly oxides of caibon. 

A number of low-activity catalysts that could control oxidation to some extent were eventually identified. These were metal oxides from Groups V and VI, such as vanadium, molybdenum, and tungsten, particulariy when mixed with phosphoric, arsenic, or boric acids. Oxidation was controlled by choosing the appropriate temperature and contact time. At first it was difficult to achieve reasonable selectivity when dealing with exothermic reactions. 

A review of the work up to 1920 was produced by Weiss and Downs/ of the Barrett Company, who were among the first to investigate the catalytic oxidation of naphthalene. Weiss remained an active consultant until at least 1946. 

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Example 13.1 Phthalic anhydride is an important intermediate for the plastics industry. Manufacture is by the controlled oxidation of o-xylene or naphthalene. The most common route uses o-xylene via the reaction... 

The reaction uses a fixed-bed vanadium pentoxide-titanium dioxide catalyst which gives good selectivity for phthalic anhydride, providing temperature is controlled within relatively narrow limits. The reaction is carried out in the vapor phase with reactor temperatures typically in the range 380 to 400°C. 

Figure 13.5 shows a flowsheet for the manufacture of phthalic anhydride by the oxidation of o-xylene. Air and o-xylene are heated and mixed in a Venturi, where the o-xylene vaporizes. The reaction mixture enters a tubular catalytic reactor. The heat of reaction is removed from the reactor by recirculation of molten salt. The temperature control in the reactor would be diflficult to maintain by methods other than molten salt. 

The crude phthalic anhydride is heated and held at 260 C to allow some byproduct reactions to go to completion. Purification is by continuous distillation in two columns. In the first column, maleic anhydride and benzoic and toluic acids are removed overhead. In the second column, pure phthalic anhydride is removed overhead. High boiling residues are removed from the bottom of the second column. 

Figure 13.6 The composite curves and grand composite curve for the phthalic anhydride process.

It was first described in 1608 when it was sublimed out of gum benzoin. It also occurs in many other natural resins. Benzoic acid is manufactured by the air oxidation of toluene in the liquid phase at 150°C and 4-6 atm. in the presence of a cobalt catalyst by the partial decarboxylation of phthalic anhydride in either the liquid or vapour phase in the presence of water by the hydrolysis of benzotrichloride (from the chlorination of toluene) in the presence of zinc chloride at 100°C. 

Most naphthalene produced is utilized in the manufacture of phthalic anhydride, for plasticizers, alkyd resins and polyesters. It is also used in the manufacture of 2-naphlhol and insecticides. Naphthalene derivatives are of importance, particularly as dyestufT intermediates. 

It is a dibasic acid, and forms stable metallic salts. Distillation with soda lime gives benzene. Readily dehydrated to phthalic anhydride. Its reactions are similar to phthalic anhydride in which form it is almost invariably used. 

Heating phthalic anhydride with urea (or ammonia) and a metallic salt. 

Used as fibres, particularly in textiles and film. Many other polyester polymers are of importance, e.g. unsaturated polyester resins from phthalic anhydride, propylene glycol and maleic anhydride used with reinforcement in boats, cars, etc. (alkyd resins). U.S. production 1983 1-7 megatonnes. 

It is extensively used in the preparation of dyestuffs. Combines with diazonium salts to form oxyazo-colouring matters. Gives rise to fluorescein dyes on fusion with phthalic anhydride. Used for production of plasticizers, resins, adhesives. 

B) Acetic, succinic anhydride phthalic anhydride (and substituted derivatives). 

Phthalein reaction. Place in a dry test-tube about 0 2 g. of the phenol and an equal quantity of phthalic anhydride (or acid), moisten with 2 drops (not more) of cone. H2SO4 and gently fuse together for about 1 minute. Allow to cool somewhat, and then add 10% NaOH solution in excess. 

Phthalein formation Fuse together carefully in a dry test-tube a few crystals of salicylic acid or of a salicylate with an equal quantity of phthalic anhydride rnoistened with 2 drops of cone. HjSO. Cool, dissolve in water and... 

To distinguish these anhydrides from the corresponding acids, note that succinic anhydride (m.p. 120°) is almost insoluble in cold water, whereas succinic acid (m.p. 185 ) is readily soluble. Phthalic anhydride has m.p. 132° and phthalic acid has m.p. 196-199° with decomposition. Each of these anhydrides when heated with water hydrolyses to the corresponding acids. 

Both succinic and phthalic anhydride respond to the hydroxamic acid test (see 5 above). 

The fluorescein test for succinic acid (p. 349) and the phthalein and fluorescein tests for phthalic acid (p. 351) are obviously given also by succinic anhydride and phthalic anhydride, as these tests depend upon the initial formation of the anhy dride in each case. 

Phthaleins (Phthalic anhydride TH SO.) red green fluorescence usually blue purple red blue- purple — green faint green fluorescence... 

Phthalic anhydride reacts similarly, but the acid phthalates are somewhat more difficult to isolate and the melting points are considerably lower. 

N-Substituted phthalimides. Phthalic anhydride reacts with primary amines only to yield N-substituted phthaUmides ... 

Phthalyl derivatives. Many amino acids condense with phthalic anhydride at 180-185° to yield crystaUine phthalyl derivatives ... 

Place 0-5 g. of the amino acid and 1 0 g. of phthalic anhdride in a Pyrex test-tube and immerse the lower part of the tube in an oil bath, which has previously been heated to 180-185°. Stir the mixture occasionally during the first 10 minutes and push down the phthalic anhydride which sublimes on the walls into the reaction mixture with a glass rod. Leave the mixture undisturbed for 5 minutes. After 15 minutes, remove the test-tube from the bath when the liquid mass solidifies, invert the test-tube and scrape out the excess of phthalic anhydride on the walls. RecrystaUise the residue from 10 per cent ethanol or from water. 

MC.-Octyl alcohol methyl n-hexyl carbinol CH3CH(OH)(CHj)jCH3 = CgH,OH is converted by heating with phthalic anhydride into sec.-octyl hydrogen phthalate ... 

If the presence of phthalic acid is suspected, it may be readily removed by mixing with cold chloroform j phthalic anhydride dissolves readily, but the acid is insoluble. 

Aroylbenzoic acids. Aromatic hydrocarbons condense with phthalic anhydride in the presence of anhydrous aluminium chloride producing aroylbenzoic acids in good yields ... 

Place a mixture of 1 0 g. of the hydrocarbon, 10 ml. of dry methylene chloride or ethylene dichloride or syw.-tetrachloroethane, 2 5 g. of powdered anhydrous aluminium chloride and 1-2 g. of pure phthalic anhydride in a 50 ml. round-bottomed flask fitted with a short reflux condenser. Heat on a water bath for 30 minutes (or until no more hydrogen chloride fumes are evolved), and then cool in ice. Add 10 ml. of concentrated hydrochloric acid cautiously and shake the flask gently for 5 min utes. Filter oflf the solid at the pump and wash it with 10-15 ml. of cold water. Boil the resulting crude aroylbenzoic acid with 10 ml. of 2 -5N sodium carbonate solution and 0 2 g. of decolourising carbon for 5 minutes, and filter the hot solution. Cool, add about 10 g. of crushed ice and acidify... 

Phthalic anhydride may be used as the carbonyl compound in the Perkin reaction see the preparation of phthalylacetic acid under Ninhytlrin (Section VIII,14),... 

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