Toluene benzylic chlorination

Chlorinated paraffins. Chlorinated ben2enes. Ring-chlorinated toluenes. Benzyl chloride. 

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. 

In Leonard s Method (81) toluene is chlorinated to benzyl chloride which is, in turn, converted to phenylacetic ester, and then to PEA. 

The impurities present after chlorination are unchanged toluene, benzyl chloride and benzotrichloride. 

In the absence of catalysts, treatment of toluene with chlorine (or bromine) at the boiling point, preferably with exposure to sunlight or other bright light source, results in halogenation in the side chain. The introduction of the first chlorine atom, for example, proceeds at a much faster rate than the introduction of the second chlorine atom so that in practice the major portion of the toluene is converted into benzyl chloride (1) before appreciable chlorination of benzyl chloride occurs to give benzylidene chloride (2) and benzotrichloride (3). 

Isotope effect and relative rate studies also suggest an early TS for benzylic chlorination and bromination. The benzylic position is only moderately activated toward uncomplexed chlorine atoms. Relative to ethane, toluene reactivity is increased only by a factor of 3.3. The kinetic isotope effect observed for bromination and chlorination of toluene suggest little rehybridization at the TS. 

We have already seen in this chapter that we can substitute bromine and chlorine for hydrogen atoms on the benzene ring of toluene and other alkylaromatic compounds using electrophilic aromatic substitution reactions. We can also substitute bromine and chlorine for hydrogen atoms on the benzylic carbons of alkyl side chains by radical reactions in the presence of heat, light, or a radical initiator like a peroxide, as we first saw in Chapter 10, (Section 10.9). This is made possible by the special stability of the benzylic radical intermediate (Section 15.12A). For example, benzylic chlorination of toluene takes place in the gas phase at 400-600 °C or in the presence of UV light, as shown here. Multiple substitutions occur with an excess of chlorine. 

Versatile aromatic intermediates, especially in the manufacture of plant protection agents and dyestuffs, are the chlorinated toluene derivatives o- and p-chloro-toluene, benzyl chloride, and benzotrichloride, together with toluene nitro-deriva-tives. Sulfonic acids of toluene have extensive applications as surfactants. 

The question of ami nation of aromatics by protonated amino radicals has been examined extensively by Minisci and co-workers (16). Benzene is aminated in good yields by a variety of N-chloroamines in acidic solutions, catalyzed by Iron(II) sulfates. With activated aromatics competing chlorination and sulfonation complicates the reactions, but In many cases good yields of the substituted anilines are obtained. In the case of a1kylbenzenes, benzylic chlorination competes with nuclear amlnatlon. The latter is favored by high acid concentrations. Thus, the reaction of toluene with N-chlorodimethyl amine (17) gives 95 percent amination and 6 percent benzylic chlorination in neat H SO., but 100 percent benzylic chlorination In neat acetic acid. The amount of nuclear amination increases with the concentration of H2SO4 in acetic acid. 

It is prepared by the direct chlorination of toluene in the presence of PClj. It is purified by fractionation from the unchanged toluene and the higher chlorinated products. It is used for benzylating amines and for preparing benzyl alcohol. 

The comparative ease with which a benzylic hydrogen is abstracted leads to high selectivity m free radical halogenations of alkylbenzenes Thus chlorination of toluene... 

Benzyl Chloride. Benzyl chloride is manufactured by high temperature free-radical chlorination of toluene. The yield of benzyl chloride is maximized by use of excess toluene in the feed. More than half of the benzyl chloride produced is converted by butyl benzyl phthalate by reaction with monosodium butyl phthalate. The remainder is hydrolyzed to benzyl alcohol, which is converted to ahphatic esters for use in soaps, perfume, and davors. Benzyl salicylate is used as a sunscreen in lotions and creams. By-product benzal chloride can be converted to benzaldehyde, which is also produced directiy by oxidation of toluene and as a by-product during formation of benzoic acid. By-product ben zotrichl oride is not hydrolyzed to make benzoic acid but is allowed to react with benzoic acid to yield benzoyl chloride. 

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. 

In the past benzal and benzyl chlorides were co-produced for the manufacture of benzaldehyde and benzyl alcohol, but today the vast majority of the benzaldehyde produced from benzal chloride is that which is made from recovered (by-product) material. For an historical article regarding the chlorination of toluene and the subsequent production of benzaldehyde, benzyl alcohol, and benzoic acid, see reference 4. 

Manufacture. Today benzyl alcohol is almost universally manufactured from toluene which is first chlorinated to give benzyl chloride [100-44-7]. This is then hydrolyzed to benzyl alcohol by treatment with aqueous sodium carbonate. 

Continuous chlorination of benzene at 30—50°C in the presence of a Lewis acid typically yields 85% monochlorobenzene. Temperatures in the range of 150—190°C favor production of the dichlorobenzene products. The para isomer is produced in a ratio of 2—3 to 1 of the ortho isomer. Other methods of aromatic ring chlorination include use of a mixture of hydrogen chloride and air in the presence of a copper—salt catalyst, or sulfuryl chloride in the presence of aluminum chloride at ambient temperatures. Free-radical chlorination of toluene successively yields benzyl chloride, benzal chloride, and benzotrichloride. Related chlorination agents include sulfuryl chloride, tert-huty hypochlorite, and /V-ch1orosuccinimide which yield benzyl chloride under the influence of light, heat, or radical initiators. 

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). 

Under typical Hquid-phase chlorination conditions the maximum conversion to benzyl chloride of about 70% is reached after reaction of about 1.1 moles of chlorine per mole of toluene (39). Higher yields of benzyl chloride have been claimed 80% for low temperature chlorination (40) 80—85% for light-catalyzed chlorination in the vapor phase (41) and 93.6% for continuous chlorination above 125°C in a column packed with glass rings (42). 

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. 

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. 

The chlorination of toluene by substituting the methyl hydrogens is a free radical reaction. A mixture of three chlorides (benzyl chloride, ben-zal chloride and benzotrichloride) results. 

The ratio of the chloride mixture mainly derives from the toluene/chlo-rine ratio and the contact time. Benzyl chloride is produced hy passing dry chlorine into hoiling toluene (110°C) until reaching a density of 1.283. At this density, the concentration of henzyl chloride reaches the maximum. Light can initiate the reaction. 

The same principle can be applied to chlorinations and iodinations (ref. 2) as well as to the benzylic bromination of toluenes and related substrates as intermediates to benzaldehydes and benzoic acids (ref. 6). 

The results of chain transfer studies with different polymer radicals are compared in Table XIV. Chain transfer constants with hydrocarbon solvents are consistently a little greater for methyl methacrylate radicals than for styrene radicals. The methyl methacrylate chain radical is far less effective in the removal of chlorine from chlorinated solvents, however. Vinyl acetate chains are much more susceptible to chain transfer than are either of the other two polymer radicals. As will appear later, the propagation constants kp for styrene, methyl methacrylate, and vinyl acetate are in the approximate ratio 1 2 20. It follows from the transfer constants with toluene, that the rate constants ktr,s for the removal of benzylic hydrogen by the respective chain radicals are in the ratio 1 3.5 6000. Chain transfer studies offer a convenient means for comparing radical reactivities, provided the absolute propagation constants also are known. 

The side-chain substitution of toluene, p-chlorotoluene, etc. is industrially practised. This reaction is carried out in a photochemical reactor. It is an exothermic reaction in which HCl is produced. The reaction is consecutive, and hence CL first reacts with toluene reacts to form the desired benzyl chloride, which is then converted to benzal chloride, and finally benzotrichloride. We may, however, well be interested in the selectivity to benzyl chloride. An additional complication arises due to nuclear chlorination, which is most undesirable. A distillation-column reactor can offer advantages (Xu and Dudukovic, 1999). 

No catalyst) Benzyl chloride Benzal chloride Benzotrichloride Rapid side-chain chlorination of toluene proceeds in the dark with sulphuryl chloride in the presence of dibenzoyl peroxide (O 001-0 005 mol per mol of SOjClj) as catalyst ... 

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