PHTHALIC ANHYDRIDE

Phthalic anhydride is an industrially important raw material for the production of anthraquinone used in the manufacture of many vat dyes and in alizarin and alizarin derivatives. It is used directly for the fluorescein, eosine, and rhodamine dyes. Several esters are made from phthalic anhydride and are largely used in the acquer industry as plasticizers. It is also used to manufacture alkyd resins, the glyptal and rezyl resins, dioctyl phthalate and the poly-vinyl resins.

Phthalic anhydride, first discovered by Laurent in 1863, was originally prepared by the oxidation of naphthalene with chromic acid. The early methods of manufacture of phthalic anhydride involved liquid phase processes in which expensive nitric and chromic acids were used as the oxidizing agents. The growing demand for phthalic anhydride as an intermediate for dye manufacture in the latter part of the nineteenth century made it imperative that cheaper means for its production be obtained. Consequently, a method of oxidizing naphthalene by sulfuric acid in the presence of mercury salts to form phthalic anhydride was developed .

The discovery of the effectiveness of the oxides of the metals of the fifth and sixth groups of the periodic table, especially of vanadium and molybdenum oxides , in the vapor phase oxidation o£ naphthalene by air led to the present productior~. on a large commercial scale, of phthalic anhydride in either fixed or fluidized bed reactors.

The ever-increasing demand for phthalic anhydride has stimulated search for alternative raw materials. Ortho-xylene, which is available in abundant quantities from petroleum refineries, appears to be the most suitable. Phthalic anhydride may be produced from o-xylene according to the following chemical equation.

Phthalic anhydride is a white crystalline solid that is the commercial form of phthalic acid. The largest markets for phthalic anhydride are phthalate plasticizers, unsaturated polyester resins, and alkyd resins for surface coatings. Commercial phthalic anhydride is 99.8–99.9% pure (99.5% is generally guaranteed) and is available in two forms – flake and molten. Most worldwide consumption of phthalic anhydride is molten.

As a raw material for the production of phthalic anhydride, o -xylene has several advantages . The theoretical amount of air required for oxidizing o-xylene is only 2/3 of that required for the oxidation of naphthalene; the heat given off during the reaction is 121 Kcal less than that of naphthalene; the product is of higher purity; theoretical yield per cent is higher than that of naphthalene. Furthermore, since o-xylene is a liquid at ordinary temperature, its use permits a simpler feed system.

Phthalic anhydride forms long needles, m.p. 128oC, b.p. 285oC. It is used extensively for the manufacture of anthraquinone, anthranilic acid, indigo, butyl phthalate (nitrocellulose plasticiser) etc. with phosphorous penta chloride it yields phthaloyl chloride, which exhibits ring-chain isomerism by reacting chemically in both structures I and II. For instance phthalolyl chloride with benzene and Aluminium chloride forms both anthraquinone and diphenyl phthalide (Phthalophenone) (v), with hydrazine it reacts in form II to give a cyclic hydrazole, where-as with zinc dust and acetic acid it reacts in form II to yield phthalide (IV) , a carbo-cyclic lactone. Final proof of the existence of the two tautomeric forms is afforded by their isolation. The symimetrical form (I) is prepared by the action of Phosphorus pentachloride on phthalic anhydride and on treatment with aluminium chloride is transformed into the unsymmetrical form (II). This type of ring-chain isomerism is observed in the diacid halids of those dicarboxylic acids whose carboxylic groups are in close proximity.

Phthalal al dehyde (III) and phthalide (IV) also finish examples of ring-chain tautomerism.

CCl2
/COCl /\ /CHO
C6H4 / C6H4 / \O C6H4 /
\ \ / \
\COCl \/ \CHO
CO
(I) (II) (III)

C6H5
|
CH2 C-C6H5
/\ /\
C6H4 / \O C6H4 / \O
\ / \ /
\/ \/
CO CO
(IV) (V)

Phthalic anhydride condenses with phenols to form triphenyl methane dyestuffs. The reaction proceeds by the P-hydrogen atoms of two molecules of phenol uniting with a carbongyl oxygen atom of anhydride to give water. The simplest of these compounds is phenolphthalein which is prepared by heating phthalic anhydride and phenol at 120o in the presence of sulphuric acid. It forms colourless crystals and is a lactone which dissolves in alkali to give an intense red solution from which the compound is precipitated in the colourless state by the addition of acids.

HC CH CH
//\ C /\\ /\ //\ CO
HC // \/ \\CH 450o-520o, HC/ \C-COOH // \/ \
| | | ————> | |-heat–> | | O
HC \\ /\ //CH Air Vana- HC\ /C-COOH \\ /\ CO/
\\/ C \// dium pentaoxide \/ \\/
HC HC CH

Naphthalene Phthalic Acid Phthalic Anhydride

Phthalicanhydride is an exceedingly important compound technically. It ia used for the synthesis of many dyes of the odamine and fluorescein series, vat dyes, phenolphthalein etc.

Indigo is prepared artificially from phthalic anhydride through thalimide and anthranilic acid.

CO
//\ /\ //\ /CO\
// \/ \ NH3 // \/ \NH + H2O
| | O ————> | | /
\\ /\ / \\ /\ /
\\/ \/ \\/ \CO
CO
Phthalic anhydride Phthalimide

On heating, or when submitted to the action of dehydrating agents phthalic acids is very readily converted into phthalic anhydride. It crystallizes in daggling white, long needles which melt at 128o.
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