The anhydride is the chemical compound which is obtained, either in principle or in practice, by the elimination of water from any other compound. An example of inorganic anhydrides is given as sulfur trioxide, SO3, which has been derived from the sulfuric acid, and the calcium oxide, CaO, derived from calcium hydroxide. Sulfur trioxide including other oxides produced by the removal of water from acid is often known as acid anhydrides, whereas the ones such as calcium oxide, are produced by a base upon the loss of water are designated as basic anhydrides.
Importance of Anhydride
The most important compound of the organic anhydrides is acetic anhydride, with the chemical formula (CH3CO)2O. It can be prepared industrially in either two ways: and from acetic acid by the reaction with ketene or acetylene or by acetaldehyde's atmospheric oxidation in the presence of a metal acetate. Other organic anhydrides are prepared from the carboxylic acids by reaction with ketene, acetic anhydride, isopropenyl acetate or methoxy-acetylene. Also, anhydrides can be produced when acyl halides react with carboxylic acid and pyridine or acetic anhydride.
The organic anhydrides can be used to introduce the acyl group (RCO) in organic synthesis. They react with water to produce carboxylic acids, either with alcohols or phenols to form esters and with ammonia and amines to form amides. Acetic anhydride can be employed in cellulose acetate manufacturing, which is widely used as a base for the magnetic tape and in textile fibre manufacturing. Also, it can be heated with salicylic acid to form the medicinal chemical acetylsalicylic acid (which is called aspirin).
Organic acid anhydrides can be prepared in the industry by various means. Mainly, acetic anhydride is produced by the carbonylation of methyl acetate. Maleic anhydride can be prepared by the oxidation of butane or benzene. Laboratory routes emphasize the corresponding acid's dehydration. The conditions change from acid to acid, but the phosphorus pentoxide remains as a common dehydrating agent:
2 CH3COOH + P4O10 → CH3C(O)OC(O)CH3 + P4O9(OH)2
Mixed anhydrides, which contains the acetyl group can be prepared from ketene:
RCO2H + H2C=C=O → RCO2C(O)CH3
Acid chlorides are also effective precursors:
CH3C(O)Cl + HCO2Na → HCO2COCH3 + NaCl
Acid anhydrides are the reactive acyl group source, and the reaction and use of acetic anhydride resemble those of acyl halides. With the protic substrate reactions, they afford similar amounts of the acylated product and the carboxylic acid:
RC(O)OC(O)R + HY → RC(O)Y + RCO2H
For HY = HOR (alcohols), aromatic ring (see Friedel-Crafts acylation), and HNR'2 (ammonia, primary, and secondary amines).
Acid anhydrides tend to be less electrophilic compared to acyl chlorides, and only one acyl group can be transferred per molecule of the acid anhydride, which can lead to a lower efficiency atom. However, the low cost of acetic anhydride makes it a common choice for the acetylation reactions.
Applications and Occurrence of Acid Anhydrides
Acetic anhydride is a primary industrial chemical, which can be widely used in the preparation of acetate esters. For example, cellulose acetate is the one. Maleic anhydride is also the precursor to multiple resins by copolymerization with styrene. And, maleic anhydride is a dienophile in the reaction of Diels-Alder.
Dianhydrides, the molecules containing two acid anhydride functions can be used to synthesize polyimides and at times, polyamides and polyesters. Examples of dianhydrides can be given as 3,3', 4,4’-benzophenone tetracarboxylic dianhydride (BTDA), pyromellitic dianhydride (PMDA), 3,3', 4,4' - oxydiphthalic dianhydride (ODPA), benzoquinone tetracarboxylic dianhydride, ethylene tetracarboxylic dianhydride, and 4,4'-diphthalic (hexafluoroisopropylidene) anhydride (6FDA). Polyanhydrides are given as a class of polymers characterized by the anhydride bonds, which connect to the repeat units of the polymer backbone chain.
Natural products that contain acid anhydrides have been isolated from the bacteria, fungi, and animals. Some of the examples include cantharidin from the blister beetle species, including the Lytta vesicatoria, tautomycin, and Spanish fly, from the bacterium Streptomyces spiro verticillatus. The maleidride family of the fungal secondary metabolites that possess a wide range of antifungal and antibiotic activity are the alicyclic compounds with the functional groups of maleic anhydride.
A molecule where two acid groups which contain more than one oxygen, bonded to each other, generally having displaced water in the process. (Thus the term "anhydride," strongly implies an unspecified absence of the water at someplace). The resulting chemical structure can be represented as follows:
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Where R and R1 would be anything, and in some special cases, the carbon lying next to them can also be something. In other terms, the oxygens are representative and the bonds leading off them are the strict requirements, but everything else can be shown as something else.
In other special cases, the oxygen compounds are also swapped out for sulfurs, but this case is very rare, and the bond structure remains similar in this case.