CHEMICAL NAME = hexanedioic acid CAS NUMBER = 124–04–9
MOLECULAR FORMULA = C6H10O4 MOLAR MASS = 146.1 g/mol
COMPOSITION = C(49.3%) H(6.9%) O(43.8%) MELTING POINT = 152°C
BOILING POINT = 337°C DENSITY = 1.36g/cm3
Adipic acid is a straight-chain dicarboxylic acid that exists as a white crystalline compound at standard temperature and pressure. Adipic acid is one of the most important industrial chemicals and typically ranks in the top 10 in terms of volume used annually by the chemical industry. Worldwide, approximately 2.5 million tons of adipic acid are produced annually.
Adipic acid’s main use is in the production of 6,6 nylon. It is also used in resins, plasticizers, lubricants, polyurethanes, and food additives.
Adipic acid can be manufactured using several methods, but the traditional and main route of preparation is by the two-step oxidation of cyclohexane (C6H12) displayed in Figure 7.1. In the fi rst step, cyclohexane is oxidized to cyclohexanone and cyclohexanol with oxygen or air.
Th is occurs at a temperature of approximately 150°C in the presence of cobalt or manganese catalysts. Th e second oxidation is done with nitric acid and air using copper or vanadium cata-lysts. In this step, the ring structure is opened and adipic acid and nitrous oxide are formed.
Other feedstocks such as benzene and phenol may be use to synthesize adipic acid. Adipic acid production used to be a large emitter of nitrous oxide, a greenhouse gas, but these have been controlled in recent years using pollution abatement technology.
Th e vast majority of adipic acid production is used to produce nylon, accounting for approximately 90% of its use. Nylon was the fi rst truly synthetic fi ber produced and capped a long search for such a material. Th roughout human history, a limited number of fi bers provided the fabrics used for clothing and other materials: wool, leather, cotton, fl ax, and silk. As early as 1664, Robert Hooke (1635–1703) speculated that production of artifi cial
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silk was possible, but it took another 200 years before synthetic fi bers started to be produced.
Th e production of synthetic fi bers took place in two stages. Th e fi rst stage began in the last decades of the 19th century and involved chemical formulations using cellulose as a raw material. Because the cellulose used in these fi bers came from cotton or wood, the fi bers were not truly synthetic. Charles Topham was searching for a suitable fi lament for light bulbs in 1883 when he produced a nitrocellulose fi ber. Louis Marie Hilaire Bernigaut (1839–1924) applied Topham’s nitrocellulose to make artifi cial silk in 1884. Bernigaut’s artifi cial silk was the fi rst rayon. Rayon is a generic term that includes several cellulose-derived fi bers produced by diff erent methods. Bernigaut’s rayon was nitrocellulose produced from cellulose; cellulose obtained from cotton was reacted with a mixture of sulfuric and nitric acid. A general reaction to represent the nitration of cellulose is: [C6H7O2(OH)3]n + HNO3 ↔ [C6H7O2(NO3)3]n + H2O. Th e sulfuric acid acts to take up the water formed in the reaction. After the acid converts the cellulose into nitrocellulose, the liquid is removed from the solution, and the nitrocellu-lose is forced through a spinneret to produce fi bers. Bernigaut’s rayon was highly fl ammable and he spent several years reducing the fl ammability of his nitrocellulose rayon before start-ing commercial production in 1891. Rayon was fi rst produced commercially in the United States in 1910. By this time, several other methods of treating cellulose had been developed to replace nitrocellulose rayon. One involved dissolving cellulose in a copper-ammonium hydroxide solution and is called the cupraammonium process. Th e third method, known as the viscose process, involves reacting cellulose that has been soaked in an alkali solution with carbon disulfi de, CS2, to produce a cellulose xanthate solution called viscose. Th e viscose process became the most widely accepted method for producing rayon and accounts for most of today’s rayon production.
By the 1920s, many companies were producing cellulose-based synthetic materials, and the stage had been set for the production of truly synthetic materials. DuPont had diversifi ed from gunpowder and munitions production, which used large quantities of nitrocellulose, into a comprehensive chemical company. Th e company began producing rayon in the 1920s and Figure 7.1 Production of adipic acid.
22 | Th e 100 Most Important Chemical Compounds
started to invest heavily in research on synthetics. In 1928, DuPont hired the Harvard organic chemist, Wallace Hume Carothers (1896–1937), whose specialty was polymers, to lead a team of highly trained chemists in basic research. Carothers’s team quickly started to develop com-mercially viable products such as the synthetic rubber neoprene. Carothers’s group also devel-oped equipment to carry out polymerization reactions. One of these was a molecular still to polymerize compounds under a high vacuum. In 1930, a member of Carothers’s team named Julian Hill (1904–1996) reacted propylene glycol with hexadecamethylene dicarboxylic acid to produce a material called polyester 3–16. Its name derived from the fact that propylene glycol has 3 carbon atoms, and hexadecamethylene dicarboxylic acid has 16 carbon atoms.
Polyester 3–16 and other synthetic materials developed in the early 1930s by the DuPont team could be drawn into fi ber, but these had specifi c problems such as lacking heat resistance.
In February 1935, a fi ber known in the laboratory as fi ber 66 was produced that held promise for commercialization. Th e 66 refers to the number of carbon atoms in the reactants used to produce it. In the case of fi ber 66, the two sixes refer to the six carbon atoms in adipic acid and six carbon atom in hexamethylenediamine, H2N(CH2)6NH2. Fiber 66 was the fi rst nylon produced. Like rayon, nylon is a generic term used for a group of synthetically produced polyamides. Th e name nylon was not introduced until 1938 after an extensive discussion by DuPont on what to call fi ber 66. Th ere are several versions of how the name nylon was coined, but one claims that nylon was a modifi cation of norun (no run), which was modifi ed into a unique name that could be used to market the product. DuPont offi cials had hoped to keep the name secret until the 1939 World’s Fair, but leaks and patent preparation forced them to reveal the name early. DuPont did not trademark the name, but promoted the material generically as nylon.
Fiber 66 became known as nylon 66. It is produced when adipic acid and hexamethylene-diamine are combined under the proper conditions:
HOOC(CH2)4COOH + H2N(CH2)6NH2 → nylon salt
From 1936 to 1939, DuPont developed the production methods and marketing strategies for nylon. Unfortunately, Carothers never lived to reap the rewards as the inventor of nylon.
He committed suicide in April 1937. Nylon’s fi rst popular use was as replacement for silk in women hosiery. It was introduced as a great technological advance at the 1939 World’s Fair in New York City, although it was being used in toothbrush bristles more than a year before then. By 1941, nylon was being used in neckties, toothbrushes, thread, and some garments.
During World War II, the U.S. government requisitioned the production of nylon solely for the war eff ort. Nylon replaced silk in military items such as parachutes, tents, rope, and tires. After the war ended, nylon’s use in civilian products, such as nylon stockings, resumed.
In addition to its use as a hosiery fabric, nylon was used in upholstery, carpet, nets, and clothing.
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Adipic acid can also polymerize with alcohols such as ethylene glycol to form polyesters, which can combine with isocyanates to form polyurethanes. Smaller esters of adipic acid pro-duced with alcohols in the C-8 to C-10 range are called adipates. Th ese are used as softeners in plastic (such as polyvinyl chloride) and as synthetic grease base oils. Adipic acid is also used in the food industry. Food grade adipic acid is prepared synthetically or extracted from beet juice as a natural source. It is used as a gelling agent, as an acidulant to provide tartness, and as a preservative.