Side-Chain Chlorination of Toluene

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. 

A higher than twofold chlorine excess in this reaction results in the formation of benzal chloride in about 70% yield. When the chlorination is carried out at higher temperature (100-140°C) with illumination, the synthesis of benzotrichloride in 95% yield may be accomplished. 

The purity of the toluene is critical and traces of water are generally removed by distillation before chlorination. 

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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. 

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. 

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

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. 

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. 

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

Benzotrichloride is produced by the side-chain chlorination of toluene with excess chlorine at 100 to 140 °C. West European production in 1985 was 15,0001. The most important application for benzotrichloride is the production of benzoyl chloride (see Chapter 6.2) by hydrolysis with equimolar quantities of water at 120 to 140 °C, with the addition of ZnCl2 the benzoyl chloride is purified by distillation. 

Side-chain chlorination of toluene takes place in the gas phase at 400-600°C or in the presenee of UV light. When an excess of chlorine is used, multiple chlorinations of the side chain occur ... 

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