Phthalic anhydride

Phthalic anhydride
IUPAC name 2-benzofuran-1,3-dione [1]
Other names Isobenzofuran-1,3-dione
Identifiers
CAS number	85-44-9 Y
ChemSpider	6552
ChEBI	CHEBI:36605
RTECS number	TI3150000
Jmol-3D images	Image 1
SMILES O=c1oc(=O)c2ccccc12
InChI InChI=1S/C8H4O3/c9-7-5-3-1-2-4-6(5)8(10)11-7/h1-4H Key: LGRFSURHDFAFJT-UHFFFAOYSA-N
Properties
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
Hazards
R-phrases	R22, R37/38, R41, R42/43
S-phrases	(S2), S23, S24/25, S26, S37/39, S46
NFPA 704	1 3 0
Flash point	152 °C
Related compounds
Related compounds	Phthalic acid Phthalimide Phthalide
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

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. In 2000, the world wide production volume of phthalic anhydride is estimated to be about 3 232 000 tonnes per year.

Synthesis and production

Phthalic anhydride was first reported in 1836 by Auguste Laurent. It is presently obtained by catalytic oxidation of ortho-xylene and naphthalene ("Gibbs phthalic anhydride process"):

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

C₁₀H₈ + 4.5 O₂ → C₆H₄(CO)₂O + 2 H₂O + 2 CO₂

The catalyst that is used for the oxidation of xylene is a modified vanadium pentoxide (V₂O₅). When separating the phthalic anhydride from byproducts such as o-xylene in water, or maleic anhydride, a series of “switch condensers” is required. Phthalic anhydride can also be prepared from phthalic acid:

Applications in industry and organic synthesis

Phthalic anhydride is a versatile intermediate in organic chemistry, in part because it is bifunctional and cheaply available. It undergoes hydrolysis and alcoholysis. Hydrolysis by hot water forms ortho-phthalic acid. This process is reversible: Phthalic anhydride re-forms upon heating the acid above 180 °C. Hydrolysis of anhydrides is not typically a reversible process. However, phthalic acid is easily dehydrated to form phthalic anhydride due to the creation of a thermodynamically favorable 5-membered ring.

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. 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. A related ring-opening reaction involves peroxides to give the useful peroxy acid:

C₆H₄(CO)₂O + H₂O₂ → C₆H₄(CO₃H)CO₂H

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.

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.

External links

• International Chemical Safety Card 0315
• NIOSH Pocket Guide to Chemical Hazards