Phthalic anhydride is the
organic compoundAn organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
with the
formulaA chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
C
6H
4(CO)
2O. It is the anhydride of
phthalic acidPhthalic acid is an aromatic dicarboxylic acid, with formula C6H42. It is an isomer of isophthalic acid and terephthalic acid. Although phthalic acid is of modest commercial importance, the closely related derivative phthalic anhydride is a commodity chemical produced on a large...
. This colourless solid is an important industrial chemical, especially for the large-scale production of
plasticizerPlasticizers or dispersants are additives that increase the plasticity or fluidity of the material to which they are added; these include plastics, cement, concrete, wallboard, and clay. Although the same compounds are often used for both plastics and concretes the desired effects and results are...
s for plastics. In 2002, approximately 4.6 billion kilograms were produced.
Synthesis and production
Phthalic anhydride was first reported in 1836 by Laurent. It is presently obtained by catalytic oxidation of
ortho-
xyleneXylene encompasses three isomers of dimethylbenzene. The isomers are distinguished by the designations ortho- , meta- , and para- , which specify to which carbon atoms the two methyl groups are attached...
and
naphthaleneNaphthalene is an organic compound with formula . It is a white crystalline solid with a characteristic odor that is detectable at concentrations as low as 0.08 ppm by mass. As an aromatic hydrocarbon, naphthalene's structure consists of a fused pair of benzene rings...
(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
When separating the phthalic anhydride from byproducts such as
o-xylene in water, or
maleic anhydrideMaleic anhydride is an organic compound with the formula C2H22O. It is the acid anhydride of maleic acid and in its pure state it is a colourless or white solid with an acrid odour....
, a series of “switch condensers” is required. Phthalic anhydride can also prepared from phthalic acid:
Applications in industry and organic synthesis
Phthalic anhydride is a versatile intermediate in
organic chemistryOrganic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
, in part because it is bifunctional and cheaply available. It undergoes hydrolysis and alcoholysis.
HydrolysisHydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
by hot water forms
ortho-
phthalic acidPhthalic acid is an aromatic dicarboxylic acid, with formula C6H42. It is an isomer of isophthalic acid and terephthalic acid. Although phthalic acid is of modest commercial importance, the closely related derivative phthalic anhydride is a commodity chemical produced on a large...
. 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)
plasticizerPlasticizers or dispersants are additives that increase the plasticity or fluidity of the material to which they are added; these include plastics, cement, concrete, wallboard, and clay. Although the same compounds are often used for both plastics and concretes the desired effects and results are...
s. In the 1980s, approximately 6.5×10
9 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
C6H4(CO2R)2 + H2O
The most important diester is
bis(2-ethylhexyl) phthalateBisphthalate, commonly abbreviated DEHP, is an organic compound with the formula C6H42. It is sometimes called dioctyl phthalate and abbreviated DOP. It is the most important "phthalate," being the diester of phthalic acid and the branched-chain 2-ethylhexanol. This colourless viscous liquid is...
("DEHP"), used in the manufacture of
polyvinyl chloridePolyvinyl chloride, commonly abbreviated PVC, is a thermoplastic polymer. It is a vinyl polymer constructed of repeating vinyl groups having one hydrogen replaced by chloride. Polyvinyl chloride is the third most widely produced plastic, after polyethylene and polypropylene. PVC is widely used in...
.
Organic synthesis
Phthalic anhydride is a precursor to a variety of reagents useful in
organic synthesisOrganic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...
. 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
brucineBrucine is a bitter alkaloid closely related to strychnine. It occurs in several plant species, the most well known being the Strychnos nux-vomica tree, found in South-East Asia.While brucine is related to strychnine, it is not as poisonous...
. A related ring-opening reaction involves
peroxideA peroxide is a compound containing an oxygen–oxygen single bond or the peroxide anion .The O−O group is called the peroxide group or peroxo group. In contrast to oxide ions, the oxygen atoms in the peroxide ion have an oxidation state of −1.The simplest stable peroxide is hydrogen peroxide...
s to give the useful
peroxy acidA peroxy acid is an acid which contains an acidic -OOH group. The two main classes are those derived from conventional mineral acids, especially sulfuric acid, and the organic derivatives of carboxylic acids...
:
- 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
quinizarinQuinizarine Green SS, also called Solvent Green 3, C.I. 61565, Oil Green G, D&C Green #6, is a green dye, an anthraquinone derivative. It has the appearance of a black powder with melting point of 220-221 °C. It is insoluble in water. It is used for adding greenish coloring to materials. It is used...
by reaction with para-chlorophenol followed by hydrolysis of the chloride.
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