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Phthalic anhydride

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Phthalic anhydride
Skeletal formula of phthalic anhydride Space-filing model
Sample of phthalic anhydride.jpg
CAS number 85-44-9 YesY
ChemSpider 6552 YesY
ChEBI CHEBI:36605 YesY
RTECS number TI3150000
Jmol-3D images Image 1
Molecular formula C8H4O3
Molar mass 148.1 g/mol
Appearance white flakes
Density 1.53 g/cm3, solid
Melting point 131 °C
Boiling point 295 °C subl.
Solubility in water 0.62 g/100g (20—25 °C);
19.0 g/100g (100 °C);
reacts slowly
R-phrases R22, R37/38, R41, R42/43
S-phrases (S2), S23, S24/25, S26, S37/39, S46
NFPA 704
Flammability code 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g., canola oil Health code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gas Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
Flash point 152 °C (306 °F; 425 K)
Related compounds
Related compounds Phthalic acid
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Phthalic anhydride is the organic compound with the formula C6H4(CO)2O. It is the anhydride of phthalic acid. Phthalic anhydride is a principal commercial form of phthalic acid. It was the first anhydride of a dicarboxylic acid to be used commercially. This colourless solid is an important industrial chemical, especially for the large-scale production of plasticizers for plastics. In 2000, the world wide production volume was estimated to be about 3M tonnes per year.1

Synthesis and production

Phthalic anhydride was first reported in 1836 by Auguste Laurent. More contemporary routes include catalytic oxidation of ortho-xylene and naphthalene ("Gibbs phthalic anhydride process"), although use of naphthalene has declined. Starting from o-xylene, the oxidation follows the following stoichiometry:

C6H4(CH3)2 + 3 O2 → C6H4(CO)2O + 3 H2O

The reaction proceeds with about 70% selectivity. About 10% of maleic anhydride is also produced:

C6H4(CH3)2 + 7.5 O2 → C4H2O3 +4 H2O + 4 CO2

A modified vanadium pentoxide (V2O5) serves as the catalysts, being active in the range 320 - 400 °C. The phthalic anhydride is separated from byproducts by a series of "switch condensers". Phthalic anhydride and maleic anhydride are recovered by distillation. Phthalic anhydride can also be prepared from phthalic acid:1
Large scale industrial production of maleic anhydride is achieved by unification of following unit processes:
• Catalyst packed bed reactor
• Switch Condensers
• Distillation column
The reactant stream of o-xylene and air mixture enters the packed bed reactor at a temperature of 300-420 ⁰C and a pressure slightly less than 1 atm. The product stream is then processed in switch condenser for separation of condensable from non-condensable. Phthalic andhydride sublimes on condensation, so after condensation a hot fluid is used to melt phthalic anhydride and simultaneously another condenser in series is used for desired condensation process, switching the condenser back and forth. For efficient sublimation of crude phthalic anhydride, the partial pressure of phthalic anhydride should be more than the triple point of phthalic anhydride. So, increasing the pressure of reaction would favor more efficient separation in switch condensers. Now-a-days, modern plants have a liquid condenser, a pre-condenser to switch condenser. Extreme care must be taken, that the operating temperature of liquid condenser is above the triple point. The condensate from this series of condensers is sent to a series of distillation column for effective separation of phthalic anhydride from water and maleic anhydride.


The primary use of Phthalic Anhydride is as a chemical intermediate in the production of plastics from vinyl chloride. Phthalate esters, which function as plasticizers, are derived from Phthalic Anhydride. Phthalate plasticizers are used for the production of flexible PVC products such as cables, pipes and hoses, leather cloth, shoes, film for packaging etc. Phthalic Anhydride has another major use in the production of polyester resins and other minor uses in the production of alkyd resins used in paints and lacquers; certain dyes (anthraquinone, phthalein, rhodamine, phthalocyanine, fluorescin, and xanthene dyes); insect repellents; and polyester polyols for polyurethanes. Besides, it is also utilized as a rubber scorch inhibitor and retarder.


Phthalic anhydride is a versatile intermediate in organic chemistry, in part because it is bifunctional and cheaply available. Hydrolysis by hot water forms ortho-phthalic acid.2 Hydrolysis of anhydrides is not typically a reversible process. Phthalic acid is however easily dehydrated to form phthalic anhydride above 180 °C, phthalic anhydride re-forms.

Preparation of phthalate esters

As with other anhydrides, the alcoholysis reaction is the basis of the manufacture of phthalate esters, which are widely used (and controversial - see endocrine disruptor) plasticizers.1 In the 1980s, approximately 6.5×109 kg of these esters were produced annually, and the scale of production was increasing each year, all from phthalic anhydride. The process begins with the reaction of phthalic anhydride with alcohols, giving the monoesters:

C6H4(CO)2O + ROH → C6H4(CO2H)CO2R

The second esterification is more difficult and requires removal of water:

C6H4(CO2H)CO2R + ROH \overrightarrow{\leftarrow} C6H4(CO2R)2 + H2O

The most important diester is bis(2-ethylhexyl) phthalate ("DEHP"), used in the manufacture of polyvinyl chloride.

Organic synthesis

Phthalic anhydride is a precursor to a variety of reagents useful in organic synthesis. Important derivatives include phthalimide and its many derivatives. Chiral alcohols form half-esters (see above), and these derivatives are often resolvable because they form diastereomeric salts with chiral amines such as brucine.3 A related ring-opening reaction involves peroxides to give the useful peroxy acid:4

C6H4(CO)2O + H2O2 → C6H4(CO3H)CO2H

Preparation of aliphatic nitroalkenes

Phthalic anhydride is used to dehydrate short-chain nitro-alcohols to yield nitroalkenes, compounds with a high tendency to polymerize.5

Precursor to dyestuffs

Phthalic anhydride is widely used in industry for the production of certain dyes. A well-known application of this reactivity is the preparation of the anthroquinone dye quinizarin by reaction with para-chlorophenol followed by hydrolysis of the chloride.6


Phthalic anhydride treated with cellulose acetate gives cellulose acetate phthalate (CAP), a common enteric coating excipient that has also been shown to have antiviral activity.7 Phthalic anhydride is a degradation product of CAP.8


The most probable human exposure to phthalic anhydride is through skin contact or inhalation during manufacture or use. Studies show that exposure to phthalic anhydride can cause rhinitis, chronic bronchitis, and asthma. Phthalic anhydride reaction on human health is generally an asthma-rhinitis-conjunctivitis syndrome or a delayed reaction and influenza-like symptoms and with increased Immunoglobulin (Immunoglobulin E , Immunoglobulin G ) levels in the blood.


  1. ^ a b c Peter M. Lorz, Friedrich K. Towae, Walter Enke, Rudolf Jäckh, Naresh Bhargava, Wolfgang Hillesheim "Phthalic Acid and Derivatives" in Ullmann's Encyclopedia of Industrial Chemistry, 2007, Wiley-VCH, Weinheim. doi:10.1002/14356007.a20_181.pub2
  2. ^ Noller, Carl R. (1965). Chemistry of Organic Compounds, 3rd ed. Philadelphia: W. B. Saunders. p. 602. 
  3. ^ Joseph Kenyon (1941), "d- and l-Octanol-2", Org. Synth. ; Coll. Vol. 1: 418 
  4. ^ George B. Payne (1973), "Monoperphthalic acid", Org. Synth. ; Coll. Vol. 5: 805 
  5. ^ Ranganathan, Darshan; Rao, Bhushan; Ranganathan, Subramania; Mehrotra, Ashok; and Iyengar, Radha (1980). "Nitroethylene: a stable, clean, and reactive agent for organic synthesis". The Journal of Organic Chemistry 45 (7): 1185–1189. doi:10.1021/jo01295a003. Retrieved 5 January 2014. 
  6. ^ L. A. Bigelow and H. H. Reynolds (1941), "Quinizarin", Org. Synth. ; Coll. Vol. 1: 476 
  7. ^ Neurath AR. (2000). "Microbicide for prevention of sexually transmitted diseases using a pharmaceutical excipient.". AIDS Patient Care STDS. 14 (4): 215–9. doi:10.1089/108729100317830. PMID 10806641. 
  8. ^ Mayhew JW, Gideon LT, Ericksen B, Hlavaty JJ, Yeh SM, Chavdarian CG, Strick N, Neurath AR. (2009). "Development of a gel permeation chromatographic assay to achieve mass balance in cellulose acetate phthalate stability studies.". J Pharm Biomed Anal. 49 (2): 240–6. doi:10.1016/j.jpba.2008.10.039. PMC 2859192. PMID 19070984. 

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