The Synthesis of Caprolactam

Since light is a costly form of energy its use can be acceptable for industrial applications only when the overall reactions have very high quantum yields, or when the final products are in any case expensive. This applies in particular to pharmaceutical and cosmetic products, and here photochemical processes can be used within the requirements of commercial considerations. 

The industrial synthesis of vitamin D is a perfect replica of the biosynthesis which relies on a key photochemical step of electrocyclic ring closure/ring opening (section 5.6). In this case the photochemical process is essential, since the dark reaction is forbidden by reasons of orbital symmetry considerations. 

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The discovery of the new titanium silicates and of their catalytic properties in H2O2 oxidation reactions has had a major impact in catalytic science and its industrial applications. One 10,000 ton/year plant for the production of catechol and hydroquinone has been operating since 1986 with excellent results. Moreover, successful tests conducted on a 12,000-ton/year pilot plant for cyclohexanone ammoximation (Notari, 1993b) could be followed soon by an industrial-size plant that would greatly simplify the synthesis of caprolactam. Both these examples are clear indications of the potentials of the new oxidation chemistry made possible by the new materials. 

Benzoic Acid. Benzoic acid can be produced by the LPO of toluene using a catalyst such as cobalt or manganese. Domestic production of benzoic acid was about 130 million lb in 2000. Of this amount, about one half went to make phenol or phenolic derivatives. Other uses are in the synthesis of caprolactam and terephthalic acid, and as food additive, and as a plasticizer and resin intermediate. 

Chromium substituted aluminophosphate-5 is an active and recyclable catalyst for the selective decomposition of cyclohexenyl hydroperoxide to 2-cyclohexen-l-one. The product is of potential industrial interest for the synthesis of caprolactam. 

Aliphatic and alicyclic molecules such as cyclohexane undergo photosub-stitution with nitrosyl chloride (Pape, 1%7). The reaction is of considerable industrial importance in the synthesis of -caprolactam, an intermediate in the manufacture of polyamides (nylon 6). (Cf. Fischer, 1978.) At long wavelengths a cage four-center transition state between alkane and an excited nitrosyl chloride molecule is involved, as indicated in Scheme 63. In contrast to light-induced halogenation, photonitrosation has a quantum yield smaller than unity, and is not a chain reaction. 

Since its discovery some 55 years ago, the synthesis of caprolactam has been the subject of intense research and development. Interest in alternative routes continues today and current activities receiving a lot of attention are carbon monoxide-based routes under development by DSM, EniChem and DuPont [32]. Numerous routes using a variety of feedstocks have been patented and many have been piloted, however, only seven have actually been commercialized. The first was the process developed by I. G. Farben based on Schlack s chemistry known today as the Rashig or conventional route. Other commercial routes are the CAPROPOL process, the BASF process, the DSM-HPO process, the Allied process, the Toray PNC process, and the SNIA Viscosa process. 

We are particularly interested in the Wacker oxidation of cyclohexene as the product, cyclohexanone, is a starting material in the synthesis of caprolactam, which is an intermediate in nylon production. Furthermore, we have strong interest in oxidation of acrolein in particular and acryhc compounds in general. Acrolein oxidation leads to a convenient route to 1,3-propanediol, while methyl acrylate oxidation leads to a starting material for adhesives. 

Figure 2.11 shows schematically the individual processes for the synthesis of caprolactam. The solid lines indicate processes that have been practiced commercially. As can be seen, all processes start from materials that belong to the group consisting of phenol, benzene, toluene, and cyclohexane. The chemistry of different processes has been reviewed [27,90,91]. Commercially, processes 1, 2, and 3 as shown in Figure 2.11 are important. The principal intermediates are cyclohexanone and cyclohexanone oxime for process 1, cyclohexanone oxime for process 2 [92-95], and cyclohexane carboxylic add for process 3. 

Developments in aliphatic isocyanates include the synthesis of polymeric aliphatic isocyanates and masked or blocked diisocyanates for appflcafions in which volatility or reactivity ate of concern. Polymeric aliphatic isocyanates ate made by copolymerizing methacrylic acid derivatives, such as 2-isocyanatoethyl methacrylate, and styrene [100-42-5] (100). Blocked isocyanates ate prepared via the reaction of the isocyanate with an active hydrogen compound, such as S-caprolactam, phenol [108-95-2] or acetone oxime. 

The preparation of nylon resins from lactam precursors involves ring opening, which is facihtated by a controlled amount of water in the reaction mixture. The salt complex condenses internally to produce the polyamide (57). The synthesis of nylon-6 [25038-54-4] from S-caprolactam is as follows ... 

Caprolactam, a white solid that melts at 69°C, can be obtained either in a fused or flaked form. It is soluble in water, ligroin, and chlorinated hydrocarbons. Caprolactam s main use is to produce nylon 6. Other minor uses are as a crosslinking agent for polyurethanes, in the plasticizer industry, and in the synthesis of lysine. 

Synthesis of PDMS-b-(e-caprolactam) ABA block copolymers was reported 343 In these reactions, anhydride-terminated PDMS oligomers were used to initiate the polymerization of -caprolactam in the presence of a catalytic amount of sodium hydride in melt at 130 °C. Under these conditions, the reaction was reported to be completed in... 

Dutton reported on the synthesis of an e-caprolactam analog of an anthelmintic cyclic peptide. The a-hydroxy-e-caprolactam 44 was generated in an ex chiral pool synthesis staring from malic acid. The a-hydroxy carboxylic acid unit was protected as a dioxolanone in 43. The protective group served simultaneously as the reactive function during cyclization lactam 44 formation succeeded by ring opening of the dioxolanone 43 by the nucleophilic attack of the amino function, Eq. (8) [14]. 

There are nine chemicals in the top 50 that are manufactured from benzene. These are listed in Table 11.1. Two of these, ethylbenzene and styrene, have already been discussed in Chapter 9, Sections 5 and 6, since they are also derivatives of ethylene. Three others—cumene, acetone, and bisphenol A— were covered in Chapter 10, Sections 3-5, when propylene derivatives were studied. Although the three carbons of acetone do not formally come from benzene, its primary manufacturing method is from cumene, which is made by reaction of benzene and propylene. These compounds need not be discussed further at this point. That leaves phenol, cyclohexane, adipic acid, and nitrobenzene. Figure 11.1 summarizes the synthesis of important chemicals made from benzene. Caprolactam is the monomer for nylon 6 and is included because of it importance. 

Table 11.2 outlines the uses of phenol. We will consider the details of phenol uses in later chapters. Phenol-formaldehyde polymers (phenolics) have a primary use as the adhesive in plywood formulations. We have already studied the synthesis of bisphenol A from phenol and acetone. Phenol s use in detergent synthesis to make alkylphenols will be discussed later. Caprolactam and aniline are mentioned in the following sections in this chapter. 

Several substituted caprolactams were produced as intermediates for the synthesis of 2-iminohomopiperidinium salts 390 0,243 (equation 157). The privileged migration of the more bulky group was observed once again, as the 2-substituted cyclohexanones 388 gave preferentially the 7-substituted caprolactams 389. 

Apart from the uses in the production of e-caprolactam, the Beckmann rearrangement has been used industrially in the synthesis of various APIs (Active Pharmaceutical Ingredient) or other compounds with economical value (essentially monomers for the production of polymers). A survey of the bulk reaction scaled in the GMP facilities at... 

A general route to azepinones is by intramolecular cyclization of e-aminohexanoic acids or their derivatives (80TL2443). For caprolactam, however, yields are low and superior preparative methods are available (70MI51600, B-75MI51601). Surprisingly few methods are known for the synthesis of C-substituted caprolactams. A useful summary of existing... 

Caprolactam is used primarily in the manufacture of synthetic fibres and resins (especially nylon 6), bristles, film, coatings synthetic leather, plasticizers and paint vehicles as a cross-linking agent for polyurethanes and in the synthesis of the amino acid lysine (Lewis, 1993). 

A similar mechanism has been proposed for photonitrosylations, with the difference that the reaction of the alkyl radical with NOC1 (Eq. 5) is not competitive (absence of chain reaction) and that the rate of reaction 6 cannot prevent dismutation (Eq. 4) and subsequent radical polymerization of unsaturated hydrocarbons (e.g., cyclohexene in the case of caprolactam synthesis). 

Figure 17.17. Examples of reactors for specific liquid-gas processes, (a) Trickle reactor for synthesis of butinediol 1.5 m dia by 18 m high, (b) Nitrogen oxide absorption in packed columns, (c) Continuous hydrogenation of fats, (d) Stirred tank reactor for batch hydrogenation of fats, (e) Nitrogen oxide absorption in a plate column, (f) A thin film reactor for making dodecylbenzene sulfonate with S03. (g) Stirred tank reactor for the hydrogenation of caprolactam, (h) Tubular reactor for making adiponitrile from adipic acid in the presence of phosphoric acid.

The reaction is very important because cyclohexane is used widely as a solvent and also is oxidized to cyclohexanone, an important intermediate in the synthesis of hexanedioic (adipic) and azacycloheptan-2-one (caprolactam), which are used in the preparation of nylon (Section 24-3C). 

The synthesis of aza-2-cycloheptanone (e-caprolactam) by the Beckmann rearrangement of the oxime of cyclohexanone is of commercial importance because the lactam is an intermediate in the synthesis of a type of nylon (a polyamide called nylon-6 2) ... 

Homolytic liquid-phase processes are generally well suited to the synthesis of carboxylic acids, viz. acetic, benzoic or terephthalic acids which are resistant to further oxidation. These processes operate at high temperature (150-250°C) and generally use soluble cobalt or manganese salts as the main catalyst components. High conversions and selectivities are usually obtained with methyl-substituted aromatic hydrocarbons such as toluene and xylenes.95,96 The cobalt-catalyzed oxidation of cyclohexane by air to a cyclohexanol-cyclohexanone mixture is a very important industrial process since these products are intermediates in the manufacture of adipic acid (for nylon 6,6) and caprolactam (nylon 6). However, the conversion is limited to ca. 10% in order to prevent consecutive oxidations, with roughly 70% selectivity.97... 

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