BALLESTER 2 Perchlorination

Some of the most outstanding questions encountered in the perchlorination of aromatic compounds arise in the synthesis and behaviour of simple perchloroalkylbenzenes. In fact, the synthesis and properties of perchloro-toluene are excellent paradigms for perchloro-organic chemistry (Ballester, 1948, 1954 Ballester and Molinet, 1954 Ballester et al., 1960a), and indeed form its core, to which any review must frequently refer. 

It was inferred that a most powerful yet selective perchlorinating agent had somehow been formed from the starting components, the so-called reagent BMC (Ballester et ai, 1960a Fieser and Fieser, 1967). 

In some cases, the steric situation is reversed, and the adduct is less strained than the substrate. This situation is found in the perchlorination of fused polycyclic aromatic chlorocarbons with reagent BMC (Ballester et al., 1980c). The products, instead of being the corresponding quasiplanar aromatic chlorocarbons, are the less strained chlorine adducts. From naphthalene, anthracene, phenanthrene and acenaphthylene, perchloro-1,2-... 

Because of steric shielding, the hydrolysis of aromatic chlorinated esters and amides cannot be carried out under normal conditions, not even with the Hammett-Newman method, i.e. treatment with concentrated sulphuric acid and then with water. For comparison, the hydrolysis of highly hindered esters, such as ethyl mesitoate, can readily be performed in concentrated sulphuric acid (Treffers and Hammett, 1937 Newman et al., 1945). Hydrolysis of the perchlorinated esters can be effected with oleum in excellent yields. (To perform that of perchloroamides, hot (160°C) oleum is required (Ballester et al., 1978b).) It is assumed that the mechanism of hydrolysis for perchloroesters with oleum is analogous to that proposed by Newman, i.e. 

In view of the results described so far, it was questioned whether the Darzens condensation between two perchlorinated components such as pentachloro-benzaldehyde and a//,a/f-a-bromopentachloroacetophenone aH,aH-pentachlorophenacyl bromide) would occur. The condensation takes place with sodium hydroxide in aqueous dioxane at low temperature (0-5°C), the main product being the expected oxirane [90], However, a substantial proportion of bromohydrin [89] is formed, which has been found to be a precursor of the oxirane (M. Ballester and J. Martinez-Manzanares, unpublished). 

In the same manner, other triphenylmethyl chlorocarbon radicals have been synthesized, including perchloro-p-phenyltriphenylmethyl (PPTM ), perchloro-/ ,/ -diphenyltriphenylmethyl (PDTM-) and perchloro-/>,/>, />"-tri-phenyltriphenylmethyl (PTTM ). They all display the same extremely high chemical inertness and thermal stability. Nearly one hundred perchlorinated radicals have been synthesized by Ballester and coworkers. Before the discovery of these radicals, the most stable carbon free radical was 2-(4-biphenylyl)bis(biphenylenyl)allyl, described by Kuhn and Neugebauer (1964) its half-life in air, in solution and at room temperature, is about 10 h. 

Such a puzzling SET has been accounted for by assuming the formation of a transient p Tt eharge-transfer complex between the perchlorinated radical and the hydroxide ion (Ballester and Pascual, 1985). This is a reasonable assumption, since some perchloro aromatie compounds are known to act as acceptors, giving charge-transfer molecular complexes (p. 351). 

It has been indicated before that its perchlorinated counterpart, per-chloro-p-xylylene (p. 304) withstands, surprisingly enough, even aggressive reagents and moderately high temperatures (Ballester and Castaner, 1960a Ballester et al, 1966). 

It was reasoned that, as with diphenylmethyl radicals, perchlorination might stabilize dephenylaminyl radicals. The synthesis of perchlorodiphenylaminyl [PDA ] has been effected (i) by oxidation of A -H-decachlorodiphenylamine (PDA—H) with either Fe(CN)6 "/HO" (in H O/CgHg) or Ag"0 (in CgHg) (ii) by dechlorination of the quinonoid perchloro compound [134] with molecular silver (in CCI4) (192) (Ballester et al., 1974a). 

In some perchlorinated compounds, the two types of distortion concur. Thus the secondary bands for perchlorobenzocycloalkenes, such as per-chloroindane (Ballester and Riera, 1960 Ballester and Castaner, 1970) and perchlorobenzocyclobutene (Roedig et al., 1971), show significant concurrent hyperchromic and bathochromic shifts. This is especially so in ring-strained perchloroindene (secondary band 342 nm, e = 2850) (Ballester and Castaner, 1970). 

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