Side-chain-chlorinated aromatic hydrocarbons

Depending on the reaction conditions, the action of elemental chlorine can lead either to addition or substitution on the aromatic ring or to substitution in the aliphatic side-chain. The side-chain chlorinated all l aromatics are exceptional important intermediates for the production of numerous chemicals, including dyes, plastics, pharmaceuticals, flavors, perfumes, pesticides, catalysts, inhibitors, etc. The toluene derivatives benzyl chloride, benzal chloride and benzotrichloride have the greatest commercial significance. 

The substitution of the hydrogen in the aliphatic side-chain by chlorine occurs in as a radical chain mechanism. In industrial chlorinations the formation of chlorine radicals is achieved either by irradiation (ultraviolet light, beta-radiation), or by the use of elevated temperature (100 - 200 °C). The reactants must be free of dissolved iron salts (build-up of Friedel-Crafts-catalysts), oxygen (radical scavenger), and water (build-up of hydrochloric acid). 

The stage of chlorination is a function of the ratio chlorine toluene, see Fig. 103. 

Benzyl chloride is used mainly to produce plasticizers (eg. benzyl butyl phthalate), benzyl alcohol, and phenyl acetic acid via benzyl cyanide (used for the production of penicillin). Small-scale uses are for quaternary ammonium salts, benzyl esters, dyes, dibenzyl disulfide, benzyl phenols, and benzylamines. 

Benzotrichloride is used mainly to produce benzoyl chloride by partial hydrolysis. Photochemical chlorination is widely used for the production. In order to avoid excessive chlorination and the formation of ring-chlorinated substances, cascades of six to ten reactors are applied in continuous processes. Like benzal chloride, benzotrichloride is toxic and suspected to have a carcinogenic potential. 

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K. Lipper Side-Chain-Chlorinated Aromatic Hydrocarbons in UUmann, 5th ed., 6, 355-367, and 6th ed.. Electronic Release 1999. 

If chlorine and bromine are allowed to act upon an aromatic hydrocarbon like toluene, which has a side-chain, substitution may occur in the nucleus or the side-chain, according to the conditions. Generally speaking, in the cold and in presence of a halogen carrier, nuclear substitution occurs, Irut at a high temperatuie the halogen passes into the side-chain (see Piep. 

Chlorine atoms react with aromatic hydrocarbons, but only at a significant rate with those having saturated side chains from which the chlorine atom can abstract a hydrogen or unsaturated side chains to which it can add. For example, the rate constant for the Cl atom reaction with benzene is 1.3 X 10"15 enr3 molecule-1 s-1 (Shi and Bernhard, 1997). On the other hand, the rate constants for the reactions with toluene and p-xylene are 0.59 X 10-10 and 1.5 X 10-l() enr3 molecule"1 s"1, respectively (Shi and Bernhard, 1997), and that for reaction with p-cymene is 2.1 X 10"10 cm3 molecule"1 s-1 (Finlayson-Pitts et al., 1999). Hence... 

Under the preparation of brombenzene it has already been mentioned that by the action of chlorine or bromine on aromatic hydrocarbons containing aliphatic side-chains, different products are formed,... 

Aromatic hydrocarbons can be chlorinated in the side chain in either the liquid or vapor phase. In the liquid phase, the chlorine is generally bubbled into the boiling hydrocarbon in the presence of light 65, 69). With temperatures as high as 300 to 400° C. no light is necessary. Nuclear substitution occurs in the presence of halogen carriers such as iron, aluminum, or iodine 28 at low temperatures. In the vapor phase, nuclear chlo-rinations occur at high temperatures and in the absence of a catalyst. 

There are in principle three possibilities for reaction of halogens with aromatic hydrocarbons, namely, addition, substitution in the nucleus, and substitution in a side chain. The last of these is discussed on pages 152 and 157. Substitution of benzene by chlorine or bromine is an ionic reaction,114 whereas photochemical or peroxide-catalyzed addition of these halogens involves a radical-chain mechanism.115 Substitution in the side chain also proceeds by a radical mechanism,116 addition rather than side-chain substitution being favored by higher chlorine concentrations.115... 

Perchlorination of an alkylaromatic hydrocarbon could a priori be achieved via two routes either exhaustive chlorination of its aromatic nucleus, followed by that of its side-chain, or vice versa, as shown for the synthesis of perchlorotoluene (1). 

The chlorophenoxyalkanoic acids constitute yet another economically important group of halogenated aromatic hydrocarbons. They are widely used as herbicides to control dicotyledonous weeds. The most important of them are 2,4-D and its propionic and butyric acid homologs, 4-chloro> 2-methylphenoxyacetic acid (MCPA), and 2,4,5>trichlorophenoxyacetic add (2,4,5-T). In all the cases studied, aerobic biodegradation proceeds by removal of the aliphatic side chain with the formation of the corresponding chlorinated phenoL Thus, formation of 2,4 chlorophenol from both 2,4-D and 4-(2,4-dichlorophenoxy)-butyric acid, 4-chloro-o-cresol from MCPA, and 2,4,5-trichlorophenol from 2,4,5-T has been... 

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