Chlorination of -chlorotoluene

The ring-chlorinated derivatives of toluene form a group of stable, industrially important compounds. Many chlorotoluene isomers can be prepared by direct chlorination. Other chlorotoluenes are prepared by indirect routes involving the replacement of amino, hydroxyl, chlorosulfonyl, and nitro groups by chlorine and the use of substituents, such as nitro, amino, and sulfonic acid, to orient substitution followed by their removal from the ring. 

The first systematic study of the reaction of chlorine with toluene was carried out in 1866 by Bedstein and Geitner. During the next 40 years, many studies were performed to isolate and identify the various chlorination products (1). During the early 1930s, Hooker Electrochemical Co. (Hooker Chemicals Plastics Corp.) and the Heyden Chemical Corp. (Tenneco) began the manufacture of chlorotoluenes. Hooker Electrochemical Co. was later acquired by Occidental Petroleum Corp. and became the Occidental Chemical Corp. In the mid-1970s, Heyden exited chlorotoluenes production Occidental thus is the sole U.S. producer of chlorotoluenes. 

Reactions of the Aromatic Ring. Ring chlorination of o-chlorotoluene yields a mixture of all four possible dichlorotoluenes, the 2,3-, 2,4-, 2,5-, and 2,6-isomers as shown in equation 1 (14). 

The rate of chlorination of toluene relative to that of ben2ene is about 345 (61). Usually, chlorination is carried out at temperatures below 70°C with the reaction proceeding at a profitable rate even at 0°C. The reaction is exothermic with ca 139 kj (33 kcal) of heat produced per mole of monochlorotoluene formed. Chlorine efficiency is high, and toluene conversion to monochlorotoluene can be carried to about 90% with the formation of only a few percent of dichlorotoluenes. In most catalyst systems, decreasing temperatures favor formation of increasing amounts of -chlorotoluene. Concentrations of requited catalysts are low, generally on the order of several tenths of a percent or less. 

A number of analytical methods have been developed for the determination of chlorotoluene mixtures by gas chromatography. These are used for determinations in environments such as air near industry (62) and soil (63). Liquid crystal stationary columns are more effective in separating m- and chlorotoluene than conventional columns (64). Prepacked columns are commercially available. ZeoHtes have been examined extensively as a means to separate chlorotoluene mixtures (see Molecularsieves). For example, a Y-type 2eohte containing sodium and copper has been used to separate y -chlorotoluene from its isomers by selective absorption (65). The presence of ben2ylic impurities in chlorotoluenes is determined by standard methods for hydroly2able chlorine. Proton (66) and carbon-13 chemical shifts, characteristic in absorption bands, and principal mass spectral peaks are available along with sources of reference spectra (67). 

Chlorination of OCT with chlorine at 90°C in the presence of L-type 2eohtes as catalyst reportedly gives a 56% yield of 2,5-dichlorotoluene (79). Pure 2,5-dichlorotoluene is also available from the Sandmeyer reaction on 2-amino-5-chlorotoluene. 3,4-Dichlorotoluene (l,2-dichloro-4-methylben2ene) is formed in up to 40% yield in the chlorination of PCT cataly2ed by metal sulfides or metal halide—sulfur compound cocatalyst systems (80). 

Dichlorotoluene (l,2-dichloro-3-methylben2ene) is present in about 10% concentration in reaction mixtures resulting from chlorination of OCT. It is best prepared by the Sandmeyer reaction on 3-arnino-2-chlorotoluene. 

Dichlorotoluene (l,3-dichloro-2-methylben2ene) is prepared from the Sandmeyer reaction on 2-arnino-6-chlorotoluene. Other methods include ring chlorination of -toluenesulfonyl chloride followed by desulfonylation (81), and chlorination and dealkylation of 4-/ f2 -butyltoluene (82) or... 

The side-chain chlorine contents of benzyl chloride, benzal chloride, and benzotrichlorides are determined by hydrolysis with methanolic sodium hydroxide followed by titration with silver nitrate. Total chlorine determination, including ring chlorine, is made by standard combustion methods (55). Several procedures for the gas chromatographic analysis of chlorotoluene mixtures have been described (56,57). Proton and nuclear magnetic resonance shifts, characteristic iafrared absorption bands, and principal mass spectral peaks have been summarized including sources of reference spectra (58). Procedures for measuring trace benzyl chloride ia air (59) and ia water (60) have been described. 

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 2- and 4-monochloro and 2,4- and 3,4-dichlorobenzyl chloride, benzal chloride, and benzotrichlorides are manufactured by side-chain chlorination of the appropriate chlorotoluene. -Chlorobenzotrichloride (1 -chioro-4-trichioromethylhenzene) can be prepared by peroxide-catalyzed chlorination of -toluenesulfonyl chloride or di-/)-toluylsulfone (71). 2,4-Dichlorobenzotrichloride (1,3-dich1 oro-4-trichi oromethylhen zene) is obtained by the chlorination of 2-chloro-4-chlorosulfonyltoluene (72). 

Stock and Baker2 5 9 measured the relative rates of chlorination of a number of halogenated aromatics in acetic acid containing 20.8 M H20 and 1.2 M HC1 at 25 °C and the values of the second-order rate coefficients (103Ar2) are as follows p-xylene (11,450), benzene (4.98), fluorobenzene (3.68), chlorobenzene (0.489), bromobenzene (0.362), 2-chlorotoluene (3.43), 3-chlorotoluene (191), 4-chloro-toluene (2.47), 4-fluorotoluene (9.70), 4-bromotoluene (2.47). Increasing the concentration of the aromatic, however, caused, in some cases, a decrease in the rate coefficients thus an increase in the concentration of chlorobenzene from 0.1 M to 0.2 M caused a 20 % decrease in rate coefficient, whereas with 4-chloro-and 4-bromo-toluene, no such change was observed. 

Our own earlier work on the chlorination of toluene had been subject to similar constraints. In this case, chlorination with ferf-butyl hypochlorite had proved to be advantageous. In the presence of silica gel as catalyst the yield of chlorotoluenes was quantitative but the regioselectivity was more or less statistical (ref. 8). However, the use of proton-exchanged zeolite X allowed the production of chlorotoluenes with a para-selectivity of more than 90 % (Fig. 4) (ref. 9). No HCl is generated in this process since the by-product is tert-butanol, and there is no inhibition of the catalyst. Indeed, the catalyst can be reused if necessary. 

Ester (9) can easily be made from acid (H)- You might consider two approaches to this a one-carbon electrophile addition via chloromethylation (Table T 2.2) and oxidation or FGl (Table 2,3) back to p-chlorotoluene (12). The latter is easier on a large scale. The p-chlorotoluene (12) can be made either by direct chlorination of toluene or by the diazotisation route (p T 12) again from toluene. 

The anodic chlorination in some cases allows one to achieve better regioselec-tivities than chemical alternatives (p/o ratio of chlorotoluene in chlorination of toluene anodic 2.2, chemical alternative 0.5-1.0) [215]. Anodic oxidation of iodine in trimethyl orthoformate afforded a positive iodine species, which led to a more selective aromatic iodination than known methods ]216]. Aryliodination is achieved in good yield, when an aryhodide is oxidized in HOAc, 25% AC2O, 5% H2SO4 in the presence of an arene ]217, 218]. Alkyl nitroaromatic compounds, nitroaromatic ketones, and nitroanihnes are prepared in good yields and regioselectivity by addition of the corresponding nucleophile to a nitroarene and subsequent anodic oxidation of the a-complex (Table 13, number 11) ]219, 220]. 

The starting snbstance 2-chlorotriphenylmethylchloride is made in varions ways. In par-ticnlar, chlorinating 2-chlorotoluene under light makes 2-chlorotrichloromethylbenzene (35.2.22), which is reacted with benzene in the presence of aluminum chloride to give 2-chlorotriphenylmethylchloride (35.2.20). 

Although the disagreement between the mt values is not large, the discrepancy is almost certainly greater than the experimental error. The deviations are not ordered. To examine the origin of the discrepancy in more detail, the relative rates predicted from ortho and para partial rate factors were compared with experimental rates of chlorination of o-and TO-chlorotoluene. 

The electrochemical nuclear chlorination of substituted aromatics in some cases allows to achieve better regioselectivities than the chemical alternatives. DOW 93 94) has shown that, in the anodic chlorination of toluene in aprotic electrolytes, the p/o ratio of the chlorotoluenes can be increased to about 2.2 (chemical alternatives 0.5-1, depending on substances added to the reaction mixture) ... 

Noncatalytic desulfonation takes place in the simple direct chlorination of sulfonyl chlorides at 100 °C. />-Toluenesulfonyl chloride is converted to / <2ra-chlorotoluene and, at a higher temperature, to />-chlorobenzotrichloride (equation 168)1088. 

Sunlight or ultraviolet light generally facilitates smooth chlorination of the side chain, especially in the case of chlorotoluenes (see page 162). A 600-1000 watt lamp can be used satisfactorily in the laboratory as the light source. 

PCTAs can be prepared by chlorination of thianthrene. The method of Buckholtz uses sulfuryl chloride as a chlorination agent in a mixture of o- and p-chlorotoluene [34]. For environmental studies small amounts of 2378-TeCTA and some other PCTAs have been prepared by a modified method of Buckholtz... 

Chlorination of toluene with tert-butyl hypochlorite in the presence of silica gel gave a mixture of chlorotoluenes in which the ortho isomer predominated. Rutming this reaction using a large pore zeolite, H-X, gave p-chlorotoluene in... 

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