Alkylation of benzene

Benzene can be alkylated in the presence of a Lewis or a Bronsted acid catalyst. Olefins such as ethylene, propylene, and C12-C14 alpha olefins are used to produce benzene alkylates, which have great commercial value. Alkyl halides such as monochloroparaffms in the C12-C14 range also serve this purpose. 

The first step in alkylation is the generation of a carbocation (carbo-nium ion). When an olefin is the alkylating agent, a carbocation intermediate forms. 

Carboncations also form from an alkyl halide when a Lewis acid catalyst is used. Aluminum chloride is the commonly used Friedel-Crafts alkylation catalyst. Friedel-Crafts alkylation reactions have been reviewed by Roberts and Khalaf  

The next step is an attack by the carbocation on the benzene ring, followed by the elimination of a proton and the formation of a benzene alkylate  

Ethylbenzene (EB) is a colorless aromatic liquid with a boiling point of 136.2°C, very close to that of p-xylene. This complicates separating it from the Cg aromatic equilibrium mixture obtained from catalytic reforming processes. (See Chapter 2 for separation of Cg aromatics). Ethylbenzene obtained from this source, however, is small compared to the synthetic route. 

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For the refiner, the reduction in benzene concentration to 3% is not a major problem it is achieved by adjusting the initial point of the feed to the catalytic reformers and thereby limiting the amount of benzene precursors such as cyclohexane and Cg paraffins. Further than 3% benzene, the constraints become very severe and can even imply using specific processes alkylation of benzene to substituted aromatics, separation, etc. 

The second source of sulfonic acid uses the following reaction scheme alkylation of benzene by a propylene oligomer then sulfonation of the alkylbenzene. 

Alkylation of benzene with alkyl halides m the presence of aluminum chloride was discovered by Charles Friedel and James M Crafts m 1877 Crafts who later became president of the Massachusetts Institute of Technology collaborated with Friedel at the Sorbonne m Pans and together they developed what we now call the Friedel-Crafts reaction into one of the most useful synthetic methods m organic chemistry... 

One drawback to Fnedel-Crafts alkylation is that rearrangements can occur espe cially when primary alkyl halides are used For example Friedel-Crafts alkylation of benzene with isobutyl chloride (a primary alkyl halide) yields only tert butylbenzene... 

Direct alkylation of benzene using 1 chlorobutane and aluminum chloride would yield sec butylbenzene by rearrangement and so could not be used... 

Isopropylbenzene is prepared by the Friedel-Crafts alkylation of benzene y using isopropyl chloride and aluminum chloride (Section 12 6) j... 

The EB present in recovered mixed xylenes is largely converted to xylenes or benzene. The EB used to make styrene is predominately manufactured by the alkylation of benzene with ethylene. 

Mechanism. The mechanism of alkylation and of other related Friedel-Crafts reactions is best explained by the carbocation concept. The alkylation of benzene with isopropyl chloride may be used as a general example ... 

For example, ia the iadustriaHy important alkylation of benzene with ethylene to ethylbenzene, polyethylbenzenes are also produced. The overall formation of polysubstituted products is minimized by recycling the higher ethylation products for the ethylation of fresh benzene (14). By adding the calculated equiUbrium amount of polyethylbenzene to the benzene feed, a high conversion of ethylene to monoethylbenzene can be achieved (15) (see also... 

Because of isomerization, alkylation of benzene with tertiary alkyl haUdes can also yield secondary alkylbenzenes rather than only tertiary alkylbenzenes (20). For example, the / fAhexylbenzene, which is first formed by the reaction of benzene with 2-chloro-2,3-dimethylbutane and AlCl isomerizes largely to 2,2-dimethyl-3-phenylbutane by a 1,2-CH2 shift. With ferric chloride as the catalyst, / fAhexylbenzene does not undergo isomerization and is isolated as such. 

Catalysts used in the polymerization of C-5 diolefins and olefins, and monovinyl aromatic monomers, foUow closely with the systems used in the synthesis of aHphatic resins. Typical catalyst systems are AlCl, AIBr., AlCl —HCl—o-xylene complexes and sludges obtained from the Friedel-Crafts alkylation of benzene. Boron trifluoride and its complexes, as weU as TiCl and SnCl, have been found to result in lower yields and higher oligomer content in C-5 and aromatic modified C-5 polymerizations. 

Diisopropjibenzenes (DIPB) are readily obtained via Eriedel-Crafts alkylation of benzene or cumene by propylene. This reaction inhquid phase has not evolved drastically since 1980 with the exception of the large variety of heterogeneous acid catalysts that are now being used, mainly zeoHtes, type HZSM-12, giving a para/meta ratio = 0.7 (4). In fact, propylene can also be replaced by isopropyl alcohol coming from the hydrogenation of acetone that... 

A typical phenol plant based on the cumene hydroperoxide process can be divided into two principal areas. In the reaction area, cumene, formed by alkylation of benzene and propylene, is oxidized to form cumene hydroperoxide (CHP). The cumene hydroperoxide is concentrated and cleaved to produce phenol and acetone. By-products of the oxidation reaction are acetophenone and dimethyl benzyl alcohol (DMBA). DMBA is dehydrated in the cleavage reaction to produce alpha-methylstyrene (AMS). 

The following HF alkylation reactions are based on straight-chain olefins. A similar chemistry can be written for the branched-chain process. The main reaction is the alkylation of benzene with the straight-chain olefins to yield a linear alkylbenzene ... 

Polynuclear Aromatics. The alkylation of polynuclear aromatics with olefins and olefin-producing reagents is effected by acid catalysts. The alkylated products are more compHcated than are those produced by the alkylation of benzene because polynuclear aromatics have more than one position for substitution. For instance, the alkylation of naphthalene [91-20-3] with methanol over mordenite and Y-type zeoHtes at 400—450°C produces 1-methylnaphthalene [90-12-0] and 2-methylnaphthalene at a 2-/1- ratio of about 1.8. The selectivity to 2-methylnaphthalene [91-57-6] is increased by applying a ZSM-5 catalyst to give a 2-/1- ratio of about 8 (102). 

Cumene. Cumene (qv) is produced by Friedel-Crafts alkylation of benzene by propylene (103,104). The main appHcation of cumene is the production of phenol (qv) and by-product acetone (qv). Minor amounts are used in gasoline blending (105). 

Styrene is manufactured from ethylbenzene. Ethylbenzene [100-41-4] is produced by alkylation of benzene with ethylene, except for a very small fraction that is recovered from mixed Cg aromatics by superfractionation. Ethylbenzene and styrene units are almost always installed together with matching capacities because nearly all of the ethylbenzene produced commercially is converted to styrene. Alkylation is exothermic and dehydrogenation is endothermic. In a typical ethylbenzene—styrene complex, energy economy is realized by advantageously integrating the energy flows of the two units. A plant intended to produce ethylbenzene exclusively or mostly for the merchant market is also not considered viable because the merchant market is small and sporadic. 

Production of a-methylstyrene (AMS) from cumene by dehydrogenation was practiced commercially by Dow until 1977. It is now produced as a by-product in the production of phenol and acetone from cumene. Cumene is manufactured by alkylation of benzene with propylene. In the phenol—acetone process, cumene is oxidized in the Hquid phase thermally to cumene hydroperoxide. The hydroperoxide is spHt into phenol and acetone by a cleavage reaction catalyzed by sulfur dioxide. Up to 2% of the cumene is converted to a-methylstyrene. Phenol and acetone are large-volume chemicals and the supply of the by-product a-methylstyrene is weU in excess of its demand. Producers are forced to hydrogenate it back to cumene for recycle to the phenol—acetone plant. Estimated plant capacities of the U.S. producers of a-methylstyrene are Hsted in Table 13 (80). 

Styrene. Commercial manufacture of this commodity monomer depends on ethylbenzene, which is converted by several means to a low purity styrene, subsequendy distilled to the pure form. A small percentage of styrene is made from the oxidative process, whereby ethylbenzene is oxidized to a hydroperoxide or alcohol and then dehydrated to styrene. A popular commercial route has been the alkylation of benzene to ethylbenzene, with ethylene, after which the cmde ethylbenzene is distilled to give high purity ethylbenzene. The ethylbenzene is direcdy dehydrogenated to styrene monomer in the vapor phase with steam and appropriate catalysts. Most styrene is manufactured by variations of this process. A variety of catalyst systems are used, based on ferric oxide with other components, including potassium salts, which improve the catalytic activity (10). 

AH commercial processes for the manufacture of caprolactam ate based on either toluene or benzene, each of which occurs in refinery BTX-extract streams (see BTX processing). Alkylation of benzene with propylene yields cumene (qv), which is a source of phenol and acetone ca 10% of U.S. phenol is converted to caprolactam. Purified benzene can be hydrogenated over platinum catalyst to cyclohexane nearly aH of the latter is used in the manufacture of nylon-6 and nylon-6,6 chemical intermediates. A block diagram of the five main process routes to caprolactam from basic taw materials, eg, hydrogen (which is usuaHy prepared from natural gas) and sulfur, is given in Eigute 2. 

All lation. An exceUent example of alkylation is the Mobil-Badger process, which uses ZSM-5-type zeoHte to produce ethylbenzene by alkylation of benzene with ethylene (12,40) ... 

Catalytic dehydrogenation of cumene, obtained by alkylation of benzene with propylene, will give a-methylstyrene (Figure 16.15). 

There are relatively few kinetic data on the Friedel-Crafts reaction. Alkylation of benzene or toluene with methyl bromide or ethyl bromide with gallium bromide as catalyst is first-order in each reactant and in catalyst. With aluminum bromide as catalyst, the rate of reaction changes with time, apparently because of heterogeneity of the reaction mixture. The initial rate data fit the kinetic expression ... 

The principal use of the alkylation process is the production of high octane aviation and motor gasoline blending stocks by the chemical addition of C2, C3, C4, or C5 olefins or mixtures of these olefins to an iso-paraffin, usually isobutane. Alkylation of benzene with olefins to produce styrene, cumene, and detergent alkylate are petrochemical processes. The alkylation reaction can be promoted by concentrated sulfuric acid, hydrofluoric acid, aluminum chloride, or boron fluoride at low temperatures. Thermal alkylation is possible at high temperatures and very high pressures. 

Ethylbenzene can also be produced by catalytic alkylation of benzene with ethylene. Benzene is alkylated with ethylene in a fixed bed alkylator. An excess of benzene is used to suppress the formation of di- and triethyl- benzenes. The excess benzene is removed from the alkylate by fractionation and recycled to the alkylator. The ethylbenzene is separated from the polyalkylated benzenes which are in turn fed to a separate reactor. Here benzene is added to convert the polyalkylated benzenes to monoethylbenzene by transalkylation. 

Because acylation of an aromatic ring can be accomplished without reanangement, it is frequently used as the first step in a procedure for the alkylation of aromatic compounds by acylation-reduction. As we saw in Section 12.6, Friedel-Crafts alkylation of benzene with primary alkyl halides nonrrally yields products having rearranged alkyl groups as substituents. When a compound of the type ArCFl2R is desued, a two-step sequence is used in which the first step is a Friedel-Crafts acylation. 

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