6-Caprolactam

The common name caprolactam comes from the original name for the Ce carboxylic acid, caproic acid. Caprolactam is the cyclic amide (lactam) of 6-aminocaproic acid. Its manufacture is from cyclohexanone, made usually from cyclohexane (58%), but also available from phenol (42%). Some of the cyclohexanol in cyclohexanone/cyclohexanol mixtures can be converted to cyclohexanone by a ZnO catalyst at 400°C. Then the cyclohexanone is converted into the oxime with hydroxylamine. The oxime undergoes a very famous acid-catalyzed reaction called the Beckmann rearrangement to give caprolactam. Sulfuric acid at 100-120°C is common but phosphoric acid is also used, since after treatment with ammonia the by-product becomes 

The first reaction, formation of the oxime, is a good example of a nucleophilic addition to a ketone followed by subsequent dehydration. Oximes are common derivatives of aldehydes and ketones because they are solids that are easily purified. 

The student should adapt this general mechanism and work through the specific cyclic example of cyclohexanone oxime to caprolactam. Note that the result of the shift is an expansion of the ring size in the final amide product with the incorporation of the nitrogen atom as part of the ring. 

All of the caprolactam goes into nylon 6 manufacture, especially fibers (80%) and plastic resin and film (20%). Although nylon 6,6 is still the more important nylon in this country (about 2 1) and in the U.K., nylon 6 is growing rapidly, especially in certain markets such as nylon carpets. In other countries, for example, Japan, nylon 6 is more predominant. Nylon 6 is made directly from caprolactam by heating with a catalytic amount of water. 

Several processes are used for the industrial production of caprolactam. Generally cyclohexanone is the key intermediate and it is produced by catalytic hydrogenation of phenol (ex benzene or toluene) or the catalytic autoxidation of cyclohexane (from benzene hydrogenation) as shown in Fig. 2.27. 

In the cyclohexane oxidation route cyclohexane is oxidized with air at 125-126°C and 8-15 bar in the liquid phase using Co or Mn naphthanates as the catalyst. This affords a mixture of cyclohexanol and cyclohexanone via a classical free radical autoxidation mechanism. Cyclohexane conversion is limited to 10-12% in order to minimize by-product formation via further oxidation. The selectivity to cyclohexanol/cyclohexanone is 80-85%. 

The unreacted cyclohexane is distilled off and recycled. The ca. 1 1 mixture of cyclohexanol and cyclohexanone is then subjected to dehydrogenation over a palladium catalyst (the same catalyst as is used in phenol hydrogenation) to give pure cyclohexanone. 

In classical processes cyclohexanone is converted to the corresponding oxime by reaction with hydroxylamine (see Fig. 2.27). The oxime subsequently affords caprolactam via the Beckmann rearrangement with sulphuric or phosphoric acid. Alternatively, in a more recent development, not yet commercialized, a mixture of cyclohexanone, ammonia and hydrogen peroxide is directly converted to cyclohexanone oxime over a titanium(IV)-silicalite (TS-1) catalyst. This route is more direct than the classical route and reduces the amount of salt formation but it involves the use of a more expensive oxidant (H2O2 rather than O2). 

The alternative route involves the air oxidation of cyclohexane and proceeds via the production of a mixture of cyclohexanol and cyclohexanone often known as KA oil. It was in the cyclohexane oxidation section of the caprolactam plant of Nypro Ltd that the huge explosion occurred at Flixborough, England in 1974. 

In one process the resulting solution is continuously withdrawn and cooled rapidly to below 75°C to prevent hydrolysis and then further cooled before being neutralised with ammonia. After phase separation, the oil phase is then treated with trichlorethylene to extract the caprolactam, which is then steam distilled. Pure caprolactam has a boiling point of 120°C at 10 mmHg pressure. In the above process 5.1 tons of ammonium sulphate are produced as a by-product per ton of caprolactam. 

Of the other routes the photonitrosation process involving nitrosyl chloride is in use in Japan. This avoids, at the expense of complicated purification processes, the high yields of ammonium sulphate unavoidably produced in the route involving the Beckmann rearrangement. 

Nylon was such a success that other companies already woildng on polymerization began to search for competitive polyamides not covered by the DnPont patents. Paul Schleck of LG. Faiben quickly developed a type of itylon based on 8-caprolactam. The pure dry e-caprolactam was difficult to polymerize, but the problem was solved by the addition of catalytic amounts of an acid and water. Production details were soon developed. Production of polycaprolactam (trademark Perlon L) began in 1940. Nylons based on e-caprolactam were soon being produced by many other companies because after the war, details of the process were released as intelligence reports and were fiee of patent restrictions.  

The DuPont polymer then became known as nylon 66 because it was made from two Ce monomers, while the I.G. Faiben polymer, from a single Ce monomer, was nylon 6. 

The preparation of s-caprolactam via the Beckman rearrangement of cyclohexanone oxime was first described by Wallach in 1900. In the I.G. Faiben process, phenol was hydrogenated to cyclohexanol, at 140°-160°C and 15 bar, with a nickel oxide/silica catalyst. The cyclohexanol was del drogenated to cyclohexanone using a zinc/iron catalyst at 400°C, and the cyclohexanone oxime was formed by reaction with hydroxylamine monosidfonate. Conversion to cyclohexanone oxime was maximized at pH 7 by neutralizing the solution with ammonia. The organic layer of cmde oxime was crystallized and then isomerized to s-caprolactam with 20% oleum at 120°C. e-Caprolactam was purified and the ammonium sulfate recovered. 

The basic industrial process is still essentially the same except that cyclohexane is now used to provide more than 60% of the e-caprolactam produced, with phenol used for the balance. BASF switched to cyclohexane in feedstock 1963. A big disadvantage of the process has been the need to sell about 2.3 lb of by-product ammonium sulfate for every poimd of e-caprolactam produced, that is, about 5 kg of ammoniirm sulfate for each kilogram of s- caprolactam. However, there is a ready market for ammoniirm sulphate in the fertiUzer business. 

Usually, the cyclohexanone intermediate is made from the oxidation of cyclohexane. However, cyclohexanone is also made from phenol (Honeywell) or toluene (BASF, DSM). With the new processes, ammonia is oxidized to nitrous oxide (NjO), which is hydrogenated in the presence of sulfuric acid into hydroxylamine sulfate, which in turn is reacted with cyclohexanone to form cyclohexanone oxime. This chemical product is subjected to a Beckmann rearrangement with oleum to produce caprolactam. 

In the United States, caprolactam is produced by BASF at Freeport, TX, by Honeywell at Hopewell, VA, and by DSM Chemicals at Augusta, GA. 

The United States produces about 2 billion pounds of caprolactam annually, and the world a total of 4.5 billion pounds. 

Over 70% of caprolactam production is used as the principal feedstock for nylon-6 production, some of which is used to make tire cord. 

Caprolactam is the chief feedstock to produce nylon-6, which is used to make the thermoplastic vulcanizate (TPV) based on nylon-6 and ACM rubber. 

The development of new reaction paths to nylon monomer intermediates is an active industrial research area. Millions of kilograms of these monomers are produced each year and even small fractional improvements can have large economic impact. 

The results of the study indicated several new research directions which the research and development teams are now pursuing. 

ACS Symposium Series American Chemical Society Washington, DC, 1977. 

The ability of programs like REACT to generate reasonable and potentially Interesting reaction paths depends on the number and quality of reactions In the data base. A strong commitment to reaction doctimentatlon needs to be Initiated within a company If the conq uter program Is going to be useful. The spe-ical reactions known to the company as well as reactions reported In the open literature need to be considered. 

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Much of the benzoic acid produced is converted to sodium benzoate, which is used as a food preservative (as is the acid) and a corrosion inhibi tor. Other important uses of the acid are in the manufacture of alkyd resins, plasticizers, caprolactam, dyestuffs and pharmaceuticals. 

Colourless liquid with a strong peppermintlike odour b.p. 155" C. Manufactured by passing cyclohexanol vapour over a heated copper catalyst. Volatile in steam. Oxidized to adipic acid. Used in the manufacture of caprolactam. Nylon, adipic acid, nitrocellulose lacquers, celluloid, artificial leather and printing inks. 

C, b.p. 81"C. Manufactured by the reduction of benzene with hydrogen in the presence of a nickel catalyst and recovered from natural gase.s. It is inflammable. Used as an intermediate in the preparation of nylon [6] and [66] via caprolactam and as a solvent for oils, fats and waxes, and also as a paint remover. For stereochemistry of cyclohexane see conformation. U.S. production 1980 1 megatonne. 

Nylon A class of synthetic fibres and plastics, polyamides. Manufactured by condensation polymerization of ct, oj-aminomonocarboxylic acids or of aliphatic diamines with aliphatic dicarboxylic acids. Also rormed specifically, e.g. from caprolactam. The different Nylons are identified by reference to the carbon numbers of the diacid and diamine (e.g. Nylon 66 is from hexamethylene diamine and adipic acid). Thermoplastic materials with high m.p., insolubility, toughness, impact resistance, low friction. Used in monofilaments, textiles, cables, insulation and in packing materials. U.S. production 1983 11 megatonnes. 

The polyamides poly(hexamethylene sebacamide) and poly(hexamethylene adipamide) are also widely known as nylon-6,10 and nylon-6,6, respectively. The numbers following the word nylon indicate the number of carbon atoms in the diamine and dicarboxylic acid, in that order. On the basis of this same system, poly (e-caprolactam) is also known as nylon-6. 

A linear molecule has a carboxyl group at one end and an amino group at the other, such as poly(e-caprolactam) ... 

Aqueous caprolactam is polymerized alone and in the presence of sebacic acid (S) or hexamethylenediamine (H).t After a 24-hr reaction time, the polymer is isolated and the end groups are analyzed by titrating the carboxyl groups with KOH in benzyl alcohol and the amino groups with p-toluenesulfonic acid in trifluoroethanol. The number of milliequivalents of carboxyl group per mole caprolactam converted to polymer, [A ], and the number of milliequivalents of amino groups per mole caprolactam converted to polymer, [B ], are given below for three different runs ... 

The polymerization of j3-carboxymethyl caprolactam has been observed to consist of initial isomerization via a second-order kinetic process followed by condensation of the isomer to polymer ... 

Reimschuessel and Deget polymerized caprolactam in sealed tubes containing about 0.0205 mol HjO per mole caprolactam. In addition, acetic acid (V), sebacic acid (S), hexamethylene diamine (H), and trimesic acid (T) were introduced as additives into separate runs. The following table lists (all data per mole caprolactam) the amounts of additive present and the analysis for end groups in various runs ... 

In the study described in the last problem, caprolactam was polymerized for 24 hr at 225°C in sealed tubes containing various amounts of water. M and were measured for the resulting mixture by osmometry and light scattering, respectively, and the following results were obtained ... 

Use the molecular weight ratio to calculate the apparent extent of reaction of the caprolactam in these systems. Is the variation in p qualitatively consistent with your expectations of the effect of increased water content in the system Plot p versus moisture content and estimate by extrapolation the equilibrium moisture content of nylon-6 at 255 C. Does the apparent equilibrium moisture content of this polymer seem consistent with the value given in Sec. 5.6 for nylon-6,6 at 290°C ... 

Table 9.3 lists the intrinsic viscosity for a number of poly(caprolactam) samples of different molecular weight. The M values listed are number average figures based on both end group analysis and osmotic pressure experiments. Tlie values of [r ] were measured in w-cresol at 25°C. In the following example we consider the evaluation of the Mark-Houwink coefficients from these data. 

Table 9.3 Intrinsic Viscosity as a Function of Molecular Weight for Samples of Poly(caprolactam) ...

Evaluate the Mark-Houwink coefficients for poly (caprolactam) in w-cresol at 25°C from the data in Table 9.3. 

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