Gem-Dichlorocyclohexadienones

The role of polychlorinated gem-dichlorocyclohexadienones as reaction intermediates which can then produce desired me/a-chlorinated products or imdesired coupled products has been described.17 Chlorination of phenol and sometimes the ether complicates the hydrolysis of some primary alkyl phenyl ethers in concentrated aqueous perchloric acid solution.18 Unexpected chlorination has also been established in the reaction of 2-amino-5-chlorobenzophenone with HC1 in aqueous methanol (1 1 v/v), 2-amino-3,5-dichlorobenzophenone being amongst the products.19... 

When the carbon has already been substituted by an atom, ipso attack leads to some polychlorinated gem-dichlorocyclohexadienones (Fig. 2). 

Polychlorinated gem-dichlorocyclohexadienones can be obtained through the action of t-butyl hypochlorite (ref. 7), hypochlorous acid (ref. 8) or chlorine in acetic acid (refs. 9, 10) with a chloro phenol. 

The only items of interest to be found in the literature are those provided by P. SVEC and coll. (refs. 6, 11) using gas phase chromatography in c.c.m.. P. SVEC and V. KUBELKA (ref. 11) observed that the polychlorinated gem-dichlorocyclohexadienones were broken down in gas phase chromatography. This result was confirmed in the laboratory by work performed on different gas phase chromatography columns which showed almost quantitative reversion of the polychlorinated ge/n-dichlorocyclohexadienones back to the initial state of chlorophenol (eqn. 8). For this reason GPC cannot be used. 

Some polychlorinated gem-dichlorocyclohexadienones were separated by P. SVEC (6) using thin-layer chromatography on a silica plate with eluants of hexane, cyclohexane, and benzene type. Using hexane as an eluant it was possible to transpose this separation of elements to HPLC with a silica column (Fig. 3). 

Fig. 3. Chromatogram of the separation of polychlorinated gem-dichlorocyclohexadienones and gem-dichlorocyclohexenones with silica column...

These circumstances obliged us to research into other conditions using HPLC. We observed that the polychlorinated gem-dichlorocyclohexadienones were eluted (Fig. 4) when placed under the conditions that we used to quantitatively analyse the chlorophenols, but unfortunately the retention periods were interfering with those of the chlorophenols (Fig. 5). 

Because similar retention periods are used for some chlorophenols and polychlorinated gem-dichlorocyclohexadienones a problem of peak times was created and it was necessary to use a double detection method enabling differentiation of both products. In order to obtain the best possible sensitivity and specificity for the polychlorinated gem-dichlorocyclohexadienones, we opted for electrochemical detection based on reduction of gem-dichlorocyclohexadienones at an imposed potential of -0 volt (Fig. 6). 

The limit of detection of polychlorinated gem-dichlorocyclohexadienones by HPLC using equipment fitted with a double detection system, UV and electrochemical, is approximately 0.01 % in a synthetic chlorophenol mixture. 

Since the analytical method we used enabled us to detect down to 0.01 % of polychlorinated gem-dichlorocyclohexadienone in a chlorophenol mixture, we were able to detect this compound in a chlorination reaction mass. This confirmed some of the assumptions we had made when we started work on the subject. 

Detection of the polychlorinated gem-dichlorocyclohexadienones was performed by means of numerous tests and only one example of the chlorination of 2,4,6-trichlorophenol in the presence of A1C13 (ref. 12) is described below ... 

Chlorophenoxyphenols and other polychlorinated impurities were released when the polychlorinated gem-dichlorocyclohexadienones were generated in the reaction mixture. This fact confirms our hypothesis that polychlorinated gem-dichlorocyclo-hexadienones are probably the cause of the parasite chemistry. 

Table 4. Percentage of polychlorinated gem-dichlorocyclohexadienones transformed after heating for 8 hours at different temperatures. 

When heated throughout, polychlorinated gem-dichlorocyclohexadienones remain stable up to a temperature of 150°C, and do not produce the impurities observed during the chlorination process. 

Obtained results indicate the strong influence of the type of phenol and polychlorinated gem-dichlorocyclohexadienones on the changes in reaction mixture. This is the reason why we will distinguish a) the process related to phenols containing chlorine atoms with a 2.4.6 position from that b) of phenols containing at least one hydrogen with a 2.4.6 position. 

This mechanism shows the catalytic role of polychlorinated gem-dichlorocyclohexadienone 2 explained in a different way. (Fig. 10)... 

Apart from their greater reactivity, the behaviour of these chlorophenols with polychlorinated gem-dichlorocyclohexadienones is close to that of trihalogenated chlorophenols with a 2.4.6 position. 

The reactivity of polychlorinated gem-dichlorocyclohexadienones decreases as the number of chlorine atoms increases. 

As soon as they reach 70°C, phenols with low chlorine content (phenol, monochlorophenols, dichlorophenols) produce polychloro phenoxyphenols in the presence of polychlorinated gem-dichlorocyclohexadienones. In this case there is some consumption of polychlorinated gem-dichlorocyclohexadienones. 

Chlorination of 2,4,6-trichlorophenol to tetrachlorophenol or pentaclorophenol is usually performed with an acid (refs. 12, 13). For this reason it was important to observe the reactivity of polychlorinated gem-dichlorocyclohexadienones in chlorophenols in the presence of acids. To do this we studied the behaviour of a mixture containing the following components at 70°C ... 

When performed under similar conditions, gaseous hydrochloric acid formed during the chlorination process hardly creates any isomerization of polychlorinated gem-dichlorocyclohexadienones to chlorophenols. This difference in reactivity explains why it is necessary to use either a strong acid or a Lewis acid to reach the tetra or penta stage. 

According to the polychlorinated gem-dichlorocyclohexadienone and acid used, the reaction mechanism is either intermolecular, or intramolecular. 

To conclude, strong acids and Lewis acids transform polychlorinated gem-dichlorocyclohexadienones in chlorophenols. Polychlorinated gem-dichlorocyclo-hexadienones are true intermediates in the formation of chlorophenols that have one chlorine atom in meta position of the OH. 

According to the reaction system used (nature of the acid, of the polychlorinated gem-dichlorocyclohexadienone, and of the polychlorophenol) either an intramolecular migration of the chlorine (isomerization) or a intermolecular transfer occurs. 

When these chlorophenols are used to run the process, hardly any transformation of polychlorinated gem-dichlorocyclohexadienones in chlorophenols takes place. 

Polychlorinated gem-dichlorocyclohexadienones in the presence of a strong acid do not change into chlorophenols when processed in chlorophenols containing at least one hydrogen atom in ortho or para position. The main products formed are polychlorophenoxyphenols and polychlorodihydroxybiphenyls. 

Chlorine reacts with polychlorinated gem-dichlorocyclohexadienones to produce polychlorinated cyclohexenones (ref.6) (eqns. 34, 35). 

This action again confirms the essential part played by the polychlorinated gem-dichlorocyclohexadienones in the parasite chemistry. 

The development of an efficient analytical method enabled us to detect the presence of polychlorinated gem-dichlorocyclohexadienones in chlorination reaction masses. 

These results confirm our hypothesis, and prove the essential role of polychlorinated gem-dichlorocyclohexadienones as reaction intermediates which can react to give either noble products (chlorinated phenols in meta), or unwanted condensation products (polychloro phenoxy phenols, polychloro dihydroxy biphenyls, etc.). 

Improvement of the quality of chlorophenols implies that the chemical evolution of the polychlorinated gem-dichlorocyclohexadienones is controlled during the chlorination reaction. 

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