Chapter 11
Derivatives of the Basic Aromatics
1.
BENZENE DERIVATIVES
There are nine chemicals in the top 50 that are manufactured from
benzene. These are listed in Table 11.1. Two of these, ethy!benzene 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 11.1 Benzene Derivatives in the Top 50
Ethylbenzene
Styrene
Cumene
Phenol
Acetone
Bisphenol A
Cyclohexane
Adipic acid
Nitrobenzene
acetone
phenol
cumene
benzene
bisphenol A
ethylbenzene
styrene
adipic acid
cyclohexane
caprolactam
nitrobenzene
aniline
Figure 11.1 Synthesis of benzene derivatives.
2.
PHENOL (CARBOLIC ACID)
The major manufacturing process for making phenol was discussed in
Chapter 10, Section 4, since it is the co-product with acetone from the acidcatalyzed rearrangement of cumene hydroperoxide. The student should
review this process. It accounts for 95% of the total phenol production and
has dominated phenol chemistry since the early 1950s. But a few other
syntheses deserve some mention.
A historically important method, first used about 1900, is sulfonation of
benzene followed by desulfonation with caustic. This is classic aromatic
chemistry. In 1924 a chlorination route was discovered. Both the
sulfonation and chlorination reactions are good examples of electrophilic
aromatic substitution on an aromatic ring. Know the mechanism of these
reactions. These routes are no longer used commercially.
high pressure
A minor route, which now accounts for 2% of phenol, takes advantage of
the usual surplus of toluene from petroleum refining. Oxidation with a
number of reagents gives benzoic acid.
Further oxidation to phydroxybenzoic acid and decarboxylation yields phenol. Here phenol
competes with benzene manufacture, also made from toluene when the
surplus is large. The last 2% of phenol comes from distillation of petroleum
and 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 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 alky !phenols will be discussed
later. Caprolactam and aniline are mentioned in the following sections in
this chapter.
Although phenol ranked thirty-fourth in 1995, it is still the highest ranked
derivative of benzene other than those using ethylene or propylene along
with benzene. Its 2000 price was 38C/lb. That gives a total commercial
value of $1.6 billion for the 4.2 billion Ib produced.
3.
CYCLOHEXANE (HEXAHYDROBENZENE,
HEXAMETHYLENE)
Benzene can be quantitatively transformed into cyclohexane by
hydrogenation over either a nickel or platinum catalyst. This reaction is
carried out at 21O 0 C and 350-500 psi, sometimes in several reactors placed
in series. The yield is over 99%.
Although many catalytic reactions are not well understood, a large
amount of work has been done on hydrogenations of double bonds. The
metal surface acts as a source of electrons. The TT bonds as well as hydrogen
atoms are bound to this surface. Then the hydrogen atoms react with the
complexed carbons one at a time to form new C—H bonds. No reaction
occurs without the metal surface. The metal in effect avoids what would
otherwise have to be a free radical mechanism that would require
considerably more energy. The mechanism is outlined as follows.
Table 11.3 shows the main uses of cyclohexane. Adipic acid is used to
manufacture nylon 6,6, the major nylon used currently in the U.S.
Caprolactam is the monomer for nylon 6, for which there is a growing
market.
4.
ADIPIC ACID (1,6-HEXANDIOIC ACID)
Nearly all the adipic acid manufactured, 98%, is made from cyclohexane
by oxidation. Air oxidation of cyclohexane with a cobalt or manganese (II)
naphthenate or acetate catalyst at 125-16O0C and 50-250 psi pressures gives
a mixture of cyclohexanone and cyclohexanol. Benzoyl peroxide is another
Table 11.3 Uses of Cyclohexane
Adipic acid
55%
Caprolactam
26
Miscellaneous
19
Source: Chemical Profiles
possible catalyst. The yield is 75-80% because of some ring opening and
other further oxidation that takes place. The cyclohexanone/cyclohexanol
mixture (sometimes referred to as ketone-alcohol, KA mixture, or "mixed
oil") is further oxidized with 50% nitric acid with ammonium vanadate and
copper present as catalysts at 50-9O0C and 15-60 psi for 10-30 min.
1:3 mixed oil
The mechanism of cyclohexane oxidation involves cyclohexane
hydroperoxide as a key intermediate.
then (2), (3), (2), (3), etc.
The cyclohexane hydroperoxide then undergoes a one-electron transfer
with cobalt or manganese (II). Chain transfer of the cyclohexyloxyl radical
gives cyclohexanol or p-scission gives cyclohexanone.
to step (6) or (7)
to step (3)
to step (2)
Figure 11.2 shows a cyclohexane oxidation reactor. The further
oxidation of the ketone and alcohol to adipic acid is very complex but occurs
in good yield, 94%, despite some succinic and glutaric acid by-products
being formed because the adipic acid can be preferentially crystallized and
centrifuged.
A small amount of adipic acid, 2%, is made by hydrogenation of phenol
with a palladium or nickel catalyst (15O0C, 50 psi) to the mixed oil, then
nitric acid oxidation to adipic acid.
If palladium is used, more
cyclohexanone is formed. Although the phenol route for making adipic acid
is not economically advantageous because phenol is more expensive than
benzene, the phenol conversion to greater cyclohexanone percentages can be
used successfully for caprolactam manufacture (see next section), where
cyclohexanone is necessary.
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 6,6
has large markets in textiles, carpets, and tire cords. It is made by reaction
of HMDA and adipic acid.
Table 11.4 Uses of Adipic Acid
Nylon 6,6 fibers
72%
Nylon 6,6 resins
18
Polyurethanes
5
Plasticizer
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 Ce
carboxylic acid, caproic acid. Caprolactam is the cyclic amide (lactam) of 6aminocaproic 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 40O0C. 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 100-12O0C is common but phosphoric acid is
also used, since after treatment with ammonia the by-product becomes
ammonium phosphate, which can be sold as a fertilizer. The caprolactam
can be extracted and vacuum distilled, bp 1390C at 12 mm. The overall
yield is 90%.
cyclohexane
or
phenol
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.
In the rearrangement of cumene hydroperoxide we saw an industrial
example of a rearrangement of electron-deficient oxygen. The Beckmann
rearrangement of caprolactam is a successful large-scale example of a
rearrangement to electron-deficient nitrogen. Protonation of the hydroxyl
followed by loss of a water molecule forms the positive nitrogen, but the R
group can migrate while the water leaves, so the nitrenium ion may not be a
discreet intermediate. Attack of water on the rearranged ion and a proton
shift to form the amide completes the process.
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.
6.
NITROBENZENE
Aniline is an important derivative of benzene that can be made in two
steps by nitration to nitrobenzene and either catalytic hydrogenation or
acidic metal reduction to aniline. Both steps occur in excellent yield.
Almost all nitrobenzene manufactured (97%) is directly converted into
aniline. The nitration of benzene with mixed acids is an example of an
electrophilic aromatic substitution involving the nitronium ion as the
attacking species. The hydrogenation of nitrobenzene has replaced the ironReaction:
catalyst
aniline
Mechanism:
acid reduction process. At one time the special crystalline structure of the
Fe3O4 formed as a by-product in the latter process made it unique for use in
pigments. But the demand for this pigment was not great enough to justify
continued 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 various
vulcanization accelerators and age resistors. By far the most important and
growing use for aniline is in the manufacture of/7,^-methylene diphenyl
diisocyanate (MDI), which is polymerized with a diol to give a polyurethane.
MDA
MDI
Table 11.5 Uses of Aniline
MDI
80%
Rubber-processing chemicals
11
Herbicides
3
Dyes and pigments
3
Specialty fibers
2
Miscellaneous
1
Source: Chemical Profiles
Two moles of aniline react with formaldehyde to give p,pmethylenedianiline (MDA). MDA reacts with phosgene to give MDI. The
student should develop the mechanism of this electrophilic aromatic
substitution.
We have already been introduced to polyurethane chemistry in Chapter
10, Section 2, where we used toluene diisocyanate (TDI) reacting with a diol
to give a polyurethane. Polyurethanes derived from MDI are more rigid than
those from TDI. New applications for these rigid foams are in home
insulation and exterior autobody parts. The intermediate MDA is now on the
"Reasonably Anticipated to Be Human Carcinogens" list and the effect of
this action on the market for MDI remains to be seen. The TLV-TWA
values for MDA and MDI are some of the lowest of the chemicals we have
discussed, being 0.1 and 0.005 ppm respectively.
7.
TOLUENE DERIVATIVES
Other than benzene, 30% of which is made from toluene by the
hydrodealkylation process, there are no other top 50 chemicals derived from
catalyst
para only
zeolites
para only
Figure 11.3 Conversion of toluene to other aromatic compounds.
toluene in large amounts. However, a few important chemicals are made
from toluene. As we learned earlier in this chapter, Section 2, a very small
amount of phenol is made from toluene. Toluene also provides an alternate
source that is becoming more popular for the xylenes, especially /?-xylene.
These routes are indicated in Fig. 11.3.
The first example, the
disproportionation of toluene to benzene and the xylenes, is being used in the
U.S. to the extent of 3-4 billion Ib of benzene and xylenes. The last two
examples provide routes respectively to terephthalic acid and /?-xylene
without the need for an isomer separation, a very appealing use for toluene
that is often in excess supply as compared to the xylenes.
Two other derivatives of toluene are the important explosive
trinitrotoluene (TNT) and the polyurethane monomer toluene diisocyanate
(TDI). TNT requires complete nitration of toluene. TDI is derived from a
mixture of dinitrotoluenes (usually 80% o,p and 20% 0,0) by reduction to the
diamine and reaction with phosgene to the diisocyanate. TDI is made into
flexible foam polyurethanes for cushioning in furniture (35%), transportation
(25%), carpet underlay (20%), and bedding (10%). A small amount is used
in polyurethane coatings, rigid foams, and elastomers.
TNT
TDI
Finally, benzaldehyde, an ingredient in flavors and perfumes, is made by
dichlorination of toluene (free radically via the easily formed benzyl radical)
followed by hydrolysis.
benzaldehyde
8.
TEREPHTHALIC ACID AND DIMETHYL
TEREPHTHALATE
TA, TPA5 or PTA
DMT
There are only two top 50 chemicals, terephthalic acid and dimethyl
terephthalate, derived from /?-xylene and none from o- or w-xylene. But
phthalic anhydride is made in large amounts from o-xylene.
Terephthalic acid is commonly abbreviated TA or TPA.
The
abbreviation PTA (P = pure) is reserved for the product of 99% purity for
polyester manufacture. For many years polyesters had to be made from
dimethyl terephthalate (DMT) because the acid could not be made pure
enough economically. Now either can be used. TA is made by air oxidation
of/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 is
formed 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 cyclohexanonexyclohexanol
process discussed in this chapter, Section 4.
The crude TA is cooled and crystallized. The acetic acid and xylene are
evaporated and the TA is washed with hot water to remove traces of the
catalyst and acetic acid. Some /7-formylbenzoic acid is present as an
impurity from incomplete oxidation. This is most easily removed by
hydrogenation to /7-methylbenzoic acid and recrystallization of the TA to
give 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 and
manganese salt catalysis.
Table 11.6 Uses of TA/DMT
Polyester fiber
Polyester resin
Polyester film
Miscellaneous
Source: Chemical Profiles
50%
33
8
9
DMT must be carefully purified via a five-column distillation system, bp
2880C, mp 1410C. The present distribution of the TA/DMT market in the
U.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 reacted
with ethylene glycol to give poly(ethylene terephthalate) (90%) but
sometimes it is combined with 1,4-butanediol to yield poly(butylene
terephthalate). Polyester fibers are used in the textile industry. Films find
applications as magnetic tapes, electrical insulation, photographic film, and
packaging. Polyester bottles, especially in the soft drink market, are
growing rapidly in demand.
9.
PHTHALIC ANHYDRIDE
The manufacturing method of making phthalic anhydride has been
changing rapidly similar to the switchover in making maleic anhydride. In
1983 28% of phthalic anhydride came from naphthalene, 72% from o-
xylene. No naphthalene-based plants were open in 1989. In 1993
naphthalene rebounded and was used to make 20% of the phthalic anhydride
again because of a price increase for o-xylene, but as of 1998 no phthalic
anhydride is made from naphthalene. Despite the better yield in the
naphthalene process, energetic factors make this less favorable economically
compared to the oxylene route.
The uses of phthalic anhydride include plasticizers (53%), unsaturated
polyester resins (22%), and alkyd resins (15%).
Phthalic anhydride reacts with alcohols such as 2-ethylhexanol to form
liquids that impart great flexibility when added to many plastics without
hurting their strength. Most of these plasticizers, about 80%, are for
poly(vinyl chloride) flexibility. Dioctyl phthalate (DOP), also called di-(2ethylhexyl)phthalate (DEHP), is a common plasticizer.
2-ethylhexanol
DOP or DEHP
High doses of DEHP have been found to cause liver cancer in rats and
mice and it is on the "Reasonably Anticipated to Be Human Carcinogens"
list. In 2000 a report by the National Toxicology Program found serious
concern that DEHP in vinyl medical devices may harm the reproductive
organs of critically ill and premature male infants exposed during medical
treatment. They also expressed concern that development of male unborn
babies would be harmed by the pregnant mothers' exposure to DEHP or that
the child would be harmed by other DEHP exposure during the first few
years of life. Certain plasticizer applications, such as those in infants'
pacifiers and squeeze toys, as well as blood bags, respiratory masks, oxygen
tubing, and intravenous bags softened with DEHP, may be affected in the
years ahead. Other diesters of phthalic anhydride do not seem to have the
toxic 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.