Toluene side-chain chlorination

Rapid side-chain chlorination of toluene proceeds in the dark with sulphuryl chloride in the presence of dibenzoyl peroxide (0-001-0 005 mol per mol of SOjCl,) as catalyst ... 

The only industrially important processes for the manufacturing of synthetic benzaldehyde involve the hydrolysis of benzal chloride [98-87-3] and the air oxidation of toluene. The hydrolysis of benzal chloride, which is produced by the side-chain chlorination of toluene, is the older of the two processes. It is no longer utilized ia the United States. Other processes, including the oxidation of benzyl alcohol, the reduction of benzoyl chloride, and the reaction of carbon monoxide and benzene, have been utilized ia the past, but they no longer have any iadustrial appHcation. 

Only trace amounts of side-chain chlorinated products are formed with suitably active catalysts. It is usually desirable to remove reactive chlorides prior to fractionation in order to niinimi2e the risk of equipment corrosion. The separation of o- and -chlorotoluenes by fractionation requires a high efficiency, isomer-separation column. The small amount of y -chlorotoluene formed in the chlorination cannot be separated by fractionation and remains in the -isomer fraction. The toluene feed should be essentially free of paraffinic impurities that may produce high boiling residues that foul heat-transfer surfaces. Trace water contamination has no effect on product composition. Steel can be used as constmction material for catalyst systems containing iron. However, glass-lined equipment is usually preferred and must be used with other catalyst systems. 

Benzyl chloride is manufactured by the thermal or photochemical chlorination of toluene at 65—100°C (37). At lower temperatures the amount of ring-chlorinated by-products is increased. The chlorination is usually carried to no more than about 50% toluene conversion in order to minimize the amount of benzal chloride formed. Overall yield based on toluene is more than 90%. Various materials, including phosphoms pentachloride, have been reported to catalyze the side-chain chlorination. These compounds and others such as amides also reduce ring chlorination by complexing metallic impurities (38). 

Benzotrichloride is produced from total side-chain chlorination of toluene or of residual products from benzyl chloride production. In Western Europe, Bayer has the largest capacity (14,000 t/yr), and there are only two significant producers in the United States Occidental Chemical in Niagara EaUs, New York (20,000 t/yr), and Velsicol Chemical (11,000 t/yr). Total capacity in the western world is 68,000 t/yr and production of benzotrichloride in 1988 was estimated at 31,500 t. 

Table 3. Toxicology of Side-Chain Chlorinated Toluenes...

Ring-Substituted Derivatives The ring-chlorinated derivatives of benzyl chloride, benzal chloride, and benzotrichloride are produced by the direct side-chain chlorination of the corresponding chlorinated toluenes or by one of several indirect routes if the required chlorotoluene is not readily available. Physical constants of the main ring-chlorinated derivatives of benzyl chloride, benzal chloride, and benzotrichloride are given in Table 4. 

New interesting applications have been in the epoxidation of difficult olefin compounds (including hexafluoropropene) with NaOCl, side-chain chlorination of substituted toluenes, diazotization of pentafluoroaniline, polymerization with free radicals, etc. 

Lewis acid catalyst is normally required when ammonium polyhalides are used, although recourse does not have to be made to strong acids, such as aluminium trichloride. Bromination and iodination reactions are normally conducted in acetic acid in the presence of zinc chloride [32], but chlorination using the ammonium tetrachloroiodate in acetic acid does not require the additional presence of a Lewis acid [33]. Radical chlorination of toluenes by benzyltrimethylammonium tetrachloroiodate in the presence of AIBN gives mixtures of the mono-and dichloromethylbenzenes [34], Photo-catalysed side-chain chlorination is less successful [35], Radical bromination using the tribromide with AIBN or benzoyl peroxide has also been reported [36, 37],... 

Side chain chlorination of toluene gives benzal chloride, which on hydrolysis gives benzaldehyde. This is a commercial method of manufacture of benzaldehyde. 

Side-Chain Chlorination of Toluene. Benzyl chloride, used mainly in the manufacture of plasticizers, may be prepared by the thermal or photochemical chlorination of toluene.191,192 In the thermal process chlorine is passed through toluene at 65-100°C. To minimize the formation of benzal chloride and benzotrichloride, the conversion is limited to about 50%. Since the density of the reaction mixture increases linearly with the formation of benzyl chloride,145 measurement of density is used to monitor the progress of the reaction. The overall yield based on toluene is about 90%, and the maximum conversion to benzyl chloride is above 70%. Higher yields in photochemical chlorination may be achieved. 

Toluene, xylene Cl2 Side-chain chlorinated products... 

Side-chain chlorinated alkyl aromatics based on toluene and xylene play an important role as chemical intermediates. They are used in the manufacture of a variety of chemical products including plastics, pharmaceuticals, flavors, pesticides, catalysts, and more. Monochlorination of the sidechain (Table 4, entry 37) is rather difficult. 

At even higher pH values, metal catalysis is required for chlorination to proceed. Thus, at pH =11, nickel(salen) catalyzes the chlorination of adamantane, cyclohexane and toluene785 and manganese porphyrin promotes the chlorination of cyclohexane786. Selective side-chain chlorination by sodium hypochlorite under PTCs at pH = 8.5 has been used for benzylic chlorinations787,788 and for the functionalization of poly(4-methyl-styrene)789,790. Similarly, with calcium hypochlorite in acetic acid, ring chlorination is reported for toluene, xylenes, anisole and other activated aromatics791. 

The chlorination of alkyl aromatics by sulfuryl chloride promoted by free-radical initiators, which was originally discovered by Kharasch and Brown990, can be modified by incorporation of transition metal complexes. Matsumoto and coworkers have observed that, upon addition of Pd(PPh3)4, in place of a radical initiator, the side-chain monochlorination of toluene is substantially more selective991. Davis and his colleagues992 have extended this study and report that Pt(0) and Pd(0) are effective initiators for side-chain chlorination of toluene by sulfuryl chloride and dichlorine. Mn, Re, Mo and Fe complexes, on the other hand, behave more like Friedel-Crafts catalysts. Gas-phase chlorination of olefins to allyl chlorides is catalyzed by PdCl2 or by PtCl2993. 

Side-chain chlorination of toluene can yield successively the mono-, di-, and trichloro compounds. These are known as benzyl chloride benzol chloride, and... 

Benzylidene chloride and its nuclear-substituted derivatives are important mainly as intermediates on the way to aldehydes, so that the difficulty of separating them from benzotri-chlorides must be overcome. On hydrolysis they give benzoic acids, which are readily separable from the aldehydes. Phosphorus trichloride (ca. 2%) has been recommended as addition for side-chain chlorination of toluenes. 

Like radical side-chain bromination, side-chain chlorination by S02C12 and a peroxide occurs mainly on the a-carbon atom ethylbenzene gives mainly (l-chloroethyl)benzene cumol gives 90% of the a- and 10% of the /9-chloro product. Chlorine enters the jS-position of terf-butylbenzene. o- and p-Nitro-toluene cannot be converted into the corresponding benzyl chlorides by S02C12 and a peroxide.419... 

Liquid-phase chlorination of benzene, side-chain chlorination of toluene, etc. 

Several other companies oxidize toluene to benzoic acid, as above. Except for Dow and Snia Viscosa (nylon-6), outlets are mainly into speciality uses. Benzaldehyde is often recovered as a by-product, while Rhone-Poulenc and others oxidize toluene to give mainly benzaldehyde and benzyl alcohol (phenylmethanol). However, many derivatives, including the major benzyl esters, are produced via the side-chain chlorination of toluene. 

One of the most important reactions in the production of industrial aromatics is electrophilic aromatic substitution another prominent type of reaction is nucleophilic substitution, which is favored for aromatics with electron-withdrawing groups. Free radical reactions, which occur especially in thermal pyrolysis processes and in side-chain oxidation and chlorination reactions, are even more important, in quantitative terms, than electrophilic and nucleophilic substitution reactions. Typical examples are thermal cracking of naphtha and gas-oil fractions, the oxidation of naphthalene to phthalic anhydride, and the side-chain chlorination of toluene. Rearrangement reactions are less significant. 

Radical reactions also play a dominant role in the side-chain chlorination of alkylated aromatics, such as toluene. 

Whereas the nitro derivatives of benzene are produced by electrophilic aromatic substitution, further important derivatives of toluene are predominantly obtained through reactions of the methyl group they include the production of oxidation products such as benzoic acid and the side-chain chlorinated toluene compounds. 

The chlorination of toluene can be carried out in the side chain and in the aromatic nucleus both product groups are of commercial significance, although side-chain-chlorinated toluenes are predominant in terms of quantity. 

The degree of substitution in side-chain chlorination is dependent on the chlo-rine/toluene molar ratio, as shown in Figure 6.4. For the manufacture of benzyl chloride and benzal chloride, the reaction has to be restricted to a low conversion to suppress the production of benzotrichloride. 

Figure 6.4 Product distribution in the side-chain chlorination of toluene versus the molar chlo-rine/toluene ratio...

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