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Phthalic anhydride is the organic compound with the formula C6H4(CO)2O. It is the anhydride of phthalic acid. This colourless solid is an important industrial chemical, especially for the large-scale production of plasticizers for plastics.

Phthalic anhydride is an important chemical intermediate in the plastics industry from which are derived numerous phthalate esters that function as plasticizers in synthetic resins. Phthalic anhydride itself is used as a monomer for synthetic resins such as glyptal, the alkyd resins, and the polyester resins.
Phthalic anhydride is also used as a precursor of anthraquinone, phthalein, rhodamine, phthalocyanine, fluorescein, and xanthene dyes.
Phthalic anhydride is used in the synthesis of primary amines, the agricultural fungicide phaltan, and thalidomide. Other reactions with phthalic anhydride yield phenolphthalein, benzoic acid, phthalylsulfathiazole (an intestinal antimicrobial agent), and orthophthalic acid.

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Phthalic Anhydride is moderately flammable, white solid (flake) or a clear, colorless, mobile liquid (molten) Characteristic, acrid, choking odor. It is very slightly soluble in H2O, soluble in alcohol, and slightly soluble in ether.

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Colorless to pale cream crystals with a characteristic, choking odor. Moisture sensitive. Odor threshold concentration is 53 ppb (quoted, Amoore and Hautala, 1983).

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ChEBI: Phthalic anhydride is the cyclic dicarboxylic anhydride that is the anhydride of phthalic acid. It has a role as an allergen. It is a cyclic dicarboxylic anhydride and a member of 2-benzofurans.

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The most important modifying component used in the manufacture of linear unsaturated polyesters is phthalic anhydride. The anhydride is generally obtained by the oxidation of o-xylene:

The reaction is carried out in the vapour phase by passing a mixture of o-xylene and air over a catalyst such as vanadium pentoxide supported on silica and promoted with titanium dioxide at about 400C. The exit gases are cooled and the phthalic anhydride is collected and purified by distillation under reduced pressure.

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A colorless to white lustrous solid in the form of needles with a mild distinctive odor. Moderately toxic by inhalation or ingestion and a skin irritant. Melting point 64°F Flash point 305°F. Forms a corrosive solution when mixed with water. Used in the manufacture of materials such as artificial resins.

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Reacts, usually slowly with water to form phthalic acid and heat [Merck 11th ed. 1989]. The phthalic acid is somewhat soluble in water.

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Phthalic anhydride reacts exothermically with water. The reactions are sometimes slow, but can become violent when local heating accelerates their rate. Acids accelerate the reaction with water. Incompatible with acids, strong oxidizing agents, alcohols, amines, and bases. Undergoes exothmeric nitration with fuming nitric acid-sulfuric acid and may give mixtures of the potentially explosive phthaloyl nitrates or nitrites or their nitro derivatives [Chem. & Ind. 20:790. 1972]. Phthalic anhydride reacts violently with CuO at elevated temperatures [Park, Chang-Man, Richard J. Sheehan. hthalic Acids and Other Benzenepolycarboxylic Acids Kirk-Othmer Encyclopedia of Chemical Technology. John Wiley & Sons, Inc. 2005]. Mixtures of Phthalic anhydride and anhydrous CO2 explode violently if heated [eaflet No. 5, Inst. of Chem., London, 1940].

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Solid irritates skin and eyes, causing coughing and sneezing. Liquid causes severe thermal burns.

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Combustible material: may burn but does not ignite readily. Substance will react with water (some violently) releasing flammable, toxic or corrosive gases and runoff. When heated, vapors may form explosive mixtures with air: indoors, outdoors and sewers explosion hazards. Most vapors are heavier than air. They will spread along ground and collect in low or confined areas (sewers, basements, tanks). Vapors may travel to source of ignition and flash back. Contact with metals may evolve flammable hydrogen gas. Containers may explode when heated or if contaminated with water.

Pharmaceutical Applications

Phthalic anhydride reacted with cellulose acetate forms cellulose acetate phthalate (CAP), a common enteric coating excipient that has also been shown to have antiviral activity. Phthalic anhydride is a degradation product of CAP.

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Phthalic anhydride is used in the manufacture of unsaturated polyesters and as a curing agent for epoxy resins. When used as a pigment, it can be responsible for sensitization in ceramic workers. Phthalic anhydride per se is not responsible for the sensitization to the resin used in nail varnishes phthalic anhydride/trimellitic anhydride/ glycols copolymer.

Safety Profile

Poison by ingestion. Experimental teratogenic effects. A corrosive eye, skin , and mucous membrane irritant. A common air contaminant. Combustible when exposed to heat or flame; can react with oxidzing materials. Moderate explosion hazard in the form of dust when exposed to flame. The production of ths material has caused many industrial explosions. Mixtures with copper oxide or sodium nitrite explode when heated. Violent reaction with nitric acid + sulfuric acid above 80℃. To fight fire, use CO2, dry chemical. Used in plasticizers, polyester resins, and alkyd resins, dyes, and drugs. See also ANHYDRIDES.

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Phthalic anhydride is used in plasticizers; in the manufacture of phthaleins; benzoic acid; alkyd and polyester resins; synthetic indigo; and phthalic acid;which is used as a plasticizer for vinyl resins. To a lesser extent, it is used in the production of alizarin, dye, anthranilic acid; anthraquinone, diethyl phthalate; dimethyl phthalate; erythrosine, isophthalic acid; methylaniline, phenolphthalein, phthalamide, sulfathalidine, and terephthalic acid. It has also found uses as a pesticide intermediate.

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UN2214 Phthalic anhydride with＞.05 % maleic anhydride, Hazard class: 8; Labels: 8-Corrosive material.

Purification Methods

Distil the anhydride under reduced pressure. Purify it from the acid by extracting with hot CHCl3, filtering and evaporating. The residue is crystallised from CHCl3, CCl4 or *benzene, or sublimed. Fractionally crystallise it from its melt. Dry it under vacuum at 100o. [Saltiel J Am Chem Soc 108 2674 1986, Beilstein 17/11 V 253.]

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Dust forms an explosive mixture with air. Phthalic anhydride reacts exothermically with water. The reactions are sometimes slow, but can become violent when local heating accelerates their rate. Acids accelerate the reaction with water. Incompatible with acids, strong oxidizing agents, alcohols, amines, and bases. Converted to phthalic acid in hot water. Incompatible with oxidizers (chlorates, nitrates, peroxides, permanganates, perchlorates, chlorine, bromine, fluorine, etc.); contact may cause fires or explosions. Keep away from alkaline materials, strong bases, strong acids, oxoacids, epoxides. caustics, ammonia, amines, water. Reacts violently with copper oxide or sodium nitrite 1 heat.

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Use a licensed professional waste disposal service to dispose of this material. Dissolve or mix the material with a combustible solvent and burn in a chemical incinerator equipped with an afterburner and scrubber. All federal, state, and local environmental regulations must be observed. Consult with environmental regulatory agencies for guidance on acceptable disposal practices. Generators of waste containing this contaminant (≥100 kg/mo) must conform with EPA regulations governing storage, transportation, treatment, and waste disposal.

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