Chapter 11

Derivatives of the Basic Aromatics

1. BENZENE DERIVATIVES

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

Table 11.1 Benzene Derivatives in the Top 50

EthylbenzeneStyreneCumenePhenolAcetoneBisphenol ACyclohexaneAdipic acidNitrobenzene

benzene cumene phenol acetone

bisphenol A

ethylbenzene styrene

cyclohexaneadipic acid

caprolactam

nitrobenzene aniline

Figure 11.1 Synthesis of benzene derivatives.

2. PHENOL (CARBOLIC ACID)

The major manufacturing process for making phenol was discussed inChapter 10, Section 4, since it is the co-product with acetone from the acid-catalyzed rearrangement of cumene hydroperoxide. The student shouldreview this process. It accounts for 95% of the total phenol production andhas dominated phenol chemistry since the early 1950s. But a few othersyntheses deserve some mention.

A historically important method, first used about 1900, is sulfonation ofbenzene followed by desulfonation with caustic. This is classic aromaticchemistry. In 1924 a chlorination route was discovered. Both thesulfonation and chlorination reactions are good examples of electrophilicaromatic substitution on an aromatic ring. Know the mechanism of thesereactions. These routes are no longer used commercially.

high pressure

A minor route, which now accounts for 2% of phenol, takes advantage ofthe usual surplus of toluene from petroleum refining. Oxidation with anumber of reagents gives benzoic acid. Further oxidation to p-hydroxybenzoic acid and decarboxylation yields phenol. Here phenolcompetes with benzene manufacture, also made from toluene when thesurplus is large. The last 2% of phenol comes from distillation of petroleumand coal gasification.

Cu benzoate

Table 11.2 Uses of Phenol

Bisphenol A 35%

Phenolic resins 34

Caprolactam 15

Aniline 5

Xylenols 5

Alkylphenols 5

Miscellaneous 1

Source: Chemical Profiles

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

Although phenol ranked thirty-fourth in 1995, it is still the highest rankedderivative of benzene other than those using ethylene or propylene alongwith benzene. Its 2000 price was 38C/lb. That gives a total commercialvalue of $1.6 billion for the 4.2 billion Ib produced.

3. CYCLOHEXANE (HEXAHYDROBENZENE,HEXAMETHYLENE)

Benzene can be quantitatively transformed into cyclohexane byhydrogenation over either a nickel or platinum catalyst. This reaction iscarried out at 21O0C and 350-500 psi, sometimes in several reactors placedin series. The yield is over 99%.

Although many catalytic reactions are not well understood, a largeamount of work has been done on hydrogenations of double bonds. Themetal surface acts as a source of electrons. The TT bonds as well as hydrogenatoms are bound to this surface. Then the hydrogen atoms react with thecomplexed carbons one at a time to form new C—H bonds. No reactionoccurs without the metal surface. The metal in effect avoids what wouldotherwise have to be a free radical mechanism that would requireconsiderably more energy. The mechanism is outlined as follows.

Table 11.3 shows the main uses of cyclohexane. Adipic acid is used tomanufacture nylon 6,6, the major nylon used currently in the U.S.Caprolactam is the monomer for nylon 6, for which there is a growingmarket.

4. ADIPIC ACID (1,6-HEXANDIOIC ACID)

Nearly all the adipic acid manufactured, 98%, is made from cyclohexaneby oxidation. Air oxidation of cyclohexane with a cobalt or manganese (II)naphthenate or acetate catalyst at 125-16O0C and 50-250 psi pressures givesa mixture of cyclohexanone and cyclohexanol. Benzoyl peroxide is another

Table 11.3 Uses of Cyclohexane

Adipic acid 55%Caprolactam 26Miscellaneous 19

Source: Chemical Profiles

possible catalyst. The yield is 75-80% because of some ring opening andother further oxidation that takes place. The cyclohexanone/cyclohexanolmixture (sometimes referred to as ketone-alcohol, KA mixture, or "mixedoil") is further oxidized with 50% nitric acid with ammonium vanadate andcopper present as catalysts at 50-9O0C and 15-60 psi for 10-30 min.

1:3 mixed oil

The mechanism of cyclohexane oxidation involves cyclohexanehydroperoxide as a key intermediate.

then (2), (3), (2), (3), etc.

The cyclohexane hydroperoxide then undergoes a one-electron transferwith cobalt or manganese (II). Chain transfer of the cyclohexyloxyl radicalgives cyclohexanol or p-scission gives cyclohexanone.

to step (6) or (7)

Figure 11.2 shows a cyclohexane oxidation reactor. The furtheroxidation of the ketone and alcohol to adipic acid is very complex but occursin good yield, 94%, despite some succinic and glutaric acid by-productsbeing formed because the adipic acid can be preferentially crystallized andcentrifuged.

A small amount of adipic acid, 2%, is made by hydrogenation of phenolwith a palladium or nickel catalyst (15O0C, 50 psi) to the mixed oil, thennitric acid oxidation to adipic acid. If palladium is used, morecyclohexanone is formed. Although the phenol route for making adipic acidis not economically advantageous because phenol is more expensive thanbenzene, the phenol conversion to greater cyclohexanone percentages can beused successfully for caprolactam manufacture (see next section), wherecyclohexanone is necessary.

to step (3)

to step (2)

caprolactam

Figure 11.2 The large tower on the right is the cyclohexane oxidation chamber and

purification unit to convert cyclohexane to the hydroperoxide and then to

cyclohexanone/cyclohexanol. An elevator leads to the top platform of this narrow tower,

where an impressive view of this and other surrounding plants can be obtained.

(Courtesy of Du Pont)

Table 11.4 gives the uses of adipic acid. As will be seen later, nylon 6,6has large markets in textiles, carpets, and tire cords. It is made by reactionof HMDA and adipic acid.

Table 11.4 Uses of Adipic Acid

Nylon 6,6 fibers 72%

Nylon 6,6 resins 18

Polyurethanes 5Plasticizer 3

Miscellaneous 2

Source: Chemical Profiles

adipic acid HMDA

nylon 6,6

5. CAPROLACTAM

The common name caprolactam comes from the original name for the Cecarboxylic acid, caproic acid. Caprolactam is the cyclic amide (lactam) of 6-aminocaproic acid. Its manufacture is from cyclohexanone, made usuallyfrom cyclohexane (58%), but also available from phenol (42%). Some ofthe cyclohexanol in cyclohexanone/cyclohexanol mixtures can be convertedto cyclohexanone by a ZnO catalyst at 40O0C. Then the cyclohexanone isconverted into the oxime with hydroxylamine. The oxime undergoes a veryfamous acid-catalyzed reaction called the Beckmann rearrangement to givecaprolactam. Sulfuric acid at 100-12O0C is common but phosphoric acid isalso used, since after treatment with ammonia the by-product becomes

ammonium phosphate, which can be sold as a fertilizer. The caprolactamcan be extracted and vacuum distilled, bp 1390C at 12 mm. The overallyield is 90%.

cyclohexaneor

phenol

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

In the rearrangement of cumene hydroperoxide we saw an industrialexample of a rearrangement of electron-deficient oxygen. The Beckmannrearrangement of caprolactam is a successful large-scale example of arearrangement to electron-deficient nitrogen. Protonation of the hydroxylfollowed by loss of a water molecule forms the positive nitrogen, but the Rgroup can migrate while the water leaves, so the nitrenium ion may not be adiscreet intermediate. Attack of water on the rearranged ion and a protonshift to form the amide completes the process.

The student should adapt this general mechanism and work through thespecific cyclic example of cyclohexanone oxime to caprolactam. Note thatthe result of the shift is an expansion of the ring size in the final amideproduct 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 moreimportant nylon in this country (about 2:1) and in the U.K., nylon 6 isgrowing rapidly, especially in certain markets such as nylon carpets. Inother countries, for example, Japan, nylon 6 is more predominant. Nylon 6is made directly from caprolactam by heating with a catalytic amount ofwater.

6. NITROBENZENE

Aniline is an important derivative of benzene that can be made in twosteps by nitration to nitrobenzene and either catalytic hydrogenation oracidic metal reduction to aniline. Both steps occur in excellent yield.Almost all nitrobenzene manufactured (97%) is directly converted intoaniline. The nitration of benzene with mixed acids is an example of anelectrophilic aromatic substitution involving the nitronium ion as theattacking species. The hydrogenation of nitrobenzene has replaced the iron-

Reaction:

catalyst

aniline

acid reduction process. At one time the special crystalline structure of theFe3O4 formed as a by-product in the latter process made it unique for use inpigments. But the demand for this pigment was not great enough to justifycontinued use of this older method of manufacturing aniline.

The uses of aniline obtained from nitrobenzene are given in Table 11.5.Aniline's use in the rubber industry is in the manufacture of variousvulcanization accelerators and age resistors. By far the most important andgrowing use for aniline is in the manufacture of/7,^-methylene diphenyldiisocyanate (MDI), which is polymerized with a diol to give a polyurethane.

Mechanism:

Table 11.5 Uses of Aniline

MDI 80%Rubber-processing chemicals 11Herbicides 3Dyes and pigments 3Specialty fibers 2

Miscellaneous 1

Source: Chemical Profiles

MDA

MDI

Two moles of aniline react with formaldehyde to give p,p-methylenedianiline (MDA). MDA reacts with phosgene to give MDI. Thestudent should develop the mechanism of this electrophilic aromaticsubstitution.

We have already been introduced to polyurethane chemistry in Chapter10, Section 2, where we used toluene diisocyanate (TDI) reacting with a diolto give a polyurethane. Polyurethanes derived from MDI are more rigid thanthose from TDI. New applications for these rigid foams are in homeinsulation and exterior autobody parts. The intermediate MDA is now on the"Reasonably Anticipated to Be Human Carcinogens" list and the effect ofthis action on the market for MDI remains to be seen. The TLV-TWAvalues for MDA and MDI are some of the lowest of the chemicals we havediscussed, being 0.1 and 0.005 ppm respectively.

7. TOLUENE DERIVATIVES

Other than benzene, 30% of which is made from toluene by thehydrodealkylation process, there are no other top 50 chemicals derived from

para only

Figure 11.3 Conversion of toluene to other aromatic compounds.

zeolites

para only

catalyst

toluene in large amounts. However, a few important chemicals are madefrom toluene. As we learned earlier in this chapter, Section 2, a very smallamount of phenol is made from toluene. Toluene also provides an alternatesource that is becoming more popular for the xylenes, especially /?-xylene.These routes are indicated in Fig. 11.3. The first example, thedisproportionation of toluene to benzene and the xylenes, is being used in theU.S. to the extent of 3-4 billion Ib of benzene and xylenes. The last twoexamples provide routes respectively to terephthalic acid and /?-xylenewithout the need for an isomer separation, a very appealing use for toluenethat is often in excess supply as compared to the xylenes.

Two other derivatives of toluene are the important explosivetrinitrotoluene (TNT) and the polyurethane monomer toluene diisocyanate(TDI). TNT requires complete nitration of toluene. TDI is derived from amixture of dinitrotoluenes (usually 80% o,p and 20% 0,0) by reduction to thediamine and reaction with phosgene to the diisocyanate. TDI is made intoflexible foam polyurethanes for cushioning in furniture (35%), transportation(25%), carpet underlay (20%), and bedding (10%). A small amount is usedin polyurethane coatings, rigid foams, and elastomers.

TNT TDI

Finally, benzaldehyde, an ingredient in flavors and perfumes, is made bydichlorination of toluene (free radically via the easily formed benzyl radical)followed by hydrolysis.

benzaldehyde

8. TEREPHTHALIC ACID AND DIMETHYLTEREPHTHALATE

TA, TPA5 or PTA DMT

There are only two top 50 chemicals, terephthalic acid and dimethylterephthalate, derived from /?-xylene and none from o- or w-xylene. Butphthalic anhydride is made in large amounts from o-xylene.

Terephthalic acid is commonly abbreviated TA or TPA. Theabbreviation PTA (P = pure) is reserved for the product of 99% purity forpolyester manufacture. For many years polyesters had to be made fromdimethyl terephthalate (DMT) because the acid could not be made pureenough economically. Now either can be used. TA is made by air oxidationof/7-xylene in acetic acid as a solvent in the presence of cobalt, manganese,and bromide ions as catalysts at 20O0C and 400 psi. TA of 99.6% purity isformed in 90% yield. This is called the Amoco process.

A partial mechanism with some intermediates is given on the next page.Details are similar to the cyclohexane to cyclohexanonexyclohexanolprocess discussed in this chapter, Section 4.

The crude TA is cooled and crystallized. The acetic acid and xylene areevaporated and the TA is washed with hot water to remove traces of thecatalyst and acetic acid. Some /7-formylbenzoic acid is present as animpurity from incomplete oxidation. This is most easily removed byhydrogenation to /7-methylbenzoic acid and recrystallization of the TA togive 99.9% PTA, which is a polyester-grade product, mp > 30O0C.

p-formylbenzoic acid /?-methylbenzoic acid

DMT can be made from crude TA or from /?-xylene directly.Esterification of TA with methanol occurs under sulfuric acid catalysis.Direct oxidation of /?-xylene with methanol present utilizes copper andmanganese salt catalysis.

Table 11.6 Uses of TA/DMT

Polyester fiber 50%Polyester resin 33Polyester film 8Miscellaneous 9

Source: Chemical Profiles

DMT must be carefully purified via a five-column distillation system, bp2880C, mp 1410C. The present distribution of the TA/DMT market in theU.S. is 44:56. All new plants will probably make terephthalic acid.

Table 11.6 shows the uses of TA/DMT. TA or DMT is usually reactedwith ethylene glycol to give poly(ethylene terephthalate) (90%) butsometimes it is combined with 1,4-butanediol to yield poly(butyleneterephthalate). Polyester fibers are used in the textile industry. Films findapplications as magnetic tapes, electrical insulation, photographic film, andpackaging. Polyester bottles, especially in the soft drink market, aregrowing rapidly in demand.

9. PHTHALIC ANHYDRIDE

The manufacturing method of making phthalic anhydride has beenchanging rapidly similar to the switchover in making maleic anhydride. In1983 28% of phthalic anhydride came from naphthalene, 72% from o-

xylene. No naphthalene-based plants were open in 1989. In 1993naphthalene rebounded and was used to make 20% of the phthalic anhydrideagain because of a price increase for o-xylene, but as of 1998 no phthalicanhydride is made from naphthalene. Despite the better yield in thenaphthalene process, energetic factors make this less favorable economicallycompared to the oxylene route.

The uses of phthalic anhydride include plasticizers (53%), unsaturatedpolyester resins (22%), and alkyd resins (15%).

Phthalic anhydride reacts with alcohols such as 2-ethylhexanol to formliquids that impart great flexibility when added to many plastics withouthurting their strength. Most of these plasticizers, about 80%, are forpoly(vinyl chloride) flexibility. Dioctyl phthalate (DOP), also called di-(2-ethylhexyl)phthalate (DEHP), is a common plasticizer.

2-ethylhexanol

DOP or DEHP

High doses of DEHP have been found to cause liver cancer in rats andmice and it is on the "Reasonably Anticipated to Be Human Carcinogens"list. In 2000 a report by the National Toxicology Program found seriousconcern that DEHP in vinyl medical devices may harm the reproductiveorgans of critically ill and premature male infants exposed during medicaltreatment. They also expressed concern that development of male unbornbabies would be harmed by the pregnant mothers' exposure to DEHP or that

the child would be harmed by other DEHP exposure during the first fewyears of life. Certain plasticizer applications, such as those in infants'pacifiers and squeeze toys, as well as blood bags, respiratory masks, oxygentubing, and intravenous bags softened with DEHP, may be affected in theyears ahead. Other diesters of phthalic anhydride do not seem to have thetoxic effects of DEHP so substitutes should be easy to find.

Suggested Readings

Chemical Profiles in Chemical Marketing Reporter, 3-2-98, 4-13-98, 6-8-98,6-15-98, 7-6-98, 2-8-99, 2-15-99, and 3-29-99.

Kent, RiegeVs Handbook of Industrial Chemistry, pp. 849-862.Szmant, Organic Building Blocks of the Chemical Industry, pp. 407-574.Wiseman, Petrochemicals, pp. 101-140.Wittcoff and Reuben, Industrial Organic Chemicals, pp. 234-293.