And halogenation of aromatic

Such long ago known in organic chemistry reactions as nitration, sulfonation, and halogenation of aromatic compounds are reactions of electrophilic substitution. 

Peroxidases have been used very frequently during the last ten years as biocatalysts in asymmetric synthesis. The transformation of a broad spectrum of substrates by these enzymes leads to valuable compounds for the asymmetric synthesis of natural products and biologically active molecules. Peroxidases catalyze regioselective hydroxylation of phenols and halogenation of olefins. Furthermore, they catalyze the epoxidation of olefins and the sulfoxidation of alkyl aryl sulfides in high enantioselectivities, as well as the asymmetric reduction of racemic hydroperoxides. The less selective oxidative coupHng of various phenols and aromatic amines by peroxidases provides a convenient access to dimeric, oligomeric and polymeric products for industrial applications. 

Edwards EA, Liang LN, Grbic-Galic D. 1993. Anaerobic microbial transformation of aromatic hydrocarbons and mixtures of aromatic hydrocarbons and halogenated solvents. Stanford, CA Environmental Engineering and Science program. Department of Civil Engineering. 

SCHEME 185. Chlorination and bromination of aromatic amines, hydrocarbons and naphthols with in situ generated active halogen... 

Apart from the usual electrophilic aromatic suhstitution reactions, other reactions can he carried out involving the methyl group in toluene, e.g. oxidation and halogenation of the alkyl group. 

Halogenation of Aromatic and Heterocyclic Compounds by Means of N-Halogenated Amides N. P. Buu-Hoi, Rec. Chem. Prog., 1952,13, 30-36. 

III. Structure and properties of aromatic polyethers 1. Halogen substituted polyphenylene ethers... 

Strong differences in the reactivity of the aromatic C=C double bond compared to the reactivity of the C=C double bond of olefins are observed olefinic electrophilic additions are faster than aromatic electrophilic substitutions. For instance, the addition of molecular bromine to cyclohexene (in acetic acid) is about 1014 times faster than the formation of bromobenzene from benzene and bromine in acetic acid113,114. Nevertheless, the addition of halogens to olefins parallels the Wheland intermediate formation in the halogenation of aromatic substrates. 

Dolfing J (1998) Halogenation of Aromatic Compounds Thermodynamic, Mechanistic and Ecological Aspects. FEMS Microbiol Lett 167 271... 

There are some known unsuccessful attempts to carry out alkylation (Mel, Me2S04), halogenation (tert-butyl hypochloride) and nitration of aromatic dihydrobenzodiazepines [7, 105]. Such attempts only resulted in the destruction of the seven-membered heterocycle. As a rule, these destructive processes are typical of dihydrodiazepine systems and often manifest themselves during the synthesis and study of these compounds. Therefore, the results of the destruction of a seven-membered heterocycle are most widespread and include its decomposition into ortho-diamine and carbonyl compounds (Scheme 4.43, reactions A and B) [105, 106] and benzimidazole rearrangement accompanied by splitting out of a methyl aryl ketone molecule (Scheme 4.43, reaction C) [117]. 

This chapter is the second of a three-part series reviewing halogenation of aromatic heterocycles. Part 1 [93AHC(57)291] described halogenation methods and their application to five-membered systems. Part 3 will cover the benz and other fused heterocyclic compounds. Material published since 1978 is emphasized, although earlier references are included where... 

The halogenation of phenols and aromatic amines in aqueous solution also provides evidence for diffusion control, but the interpretation is complicated by the fact that either the formation of the o-complex or the proton loss from the (7-complex can be rate-determining. The reaction path for the halogenation of aromatic amines in aqueous acids is believed to be that shown for N,N-dialkyl anilines in Scheme 9. Where the formation of the o-complex is rate-determining, the kinetic form for attack by the molecular halogen is given by (39). In this equation, the observed rate coefficient (k ) is related to the rate coefficient for the reaction of the amine molecule (k) by (40), where KSH+ is the... 

Most of the rate comparisons in the halogenation of aromatic amines refer to bromination rate coefficients for para-substitution are collected in Table 10. Further results for o/7/io-substitution are provided in the cited references. Some of the early calculation based on (39) and (40) may be in error, because it was not then realized that the appropriate acidity function in (40) depends on the structure of the substrate (cf. Bell and Ramsden., 1958 Bell and Ninkov, 1966). The appropriate acidity function was used for the results listed in Table 10 but it is still advisable for rate comparisons to be based on experiments carried out under the same conditions. 

Beaven, G.H., Hassan de la Mare, P.B.D.M., Johnson, E.A., Klassen, N.V. (1961) The kinetics and mechanisms of aromatic halogen substitution. Part X. Products in chlorination of biphenyl in acetic acid. J. Chem. Soc. 2749. 

R.J. Alberts, E.C. Kooymann, Halogenation of aromatics. IX. Vapor-phase chlorination and bromination of benzotrifluoride, Reel. Trav. Chim. Pays-Bas. 83 (8) (1964) 930-936. 

The nuclear-substituted halogens of aromatic /V-hetereocycles may also be susceptible to hydrogenolysis. In particular, those at the 2 and 6 positions of pyridines and at the 2 and 4 positions of quinolines are readily hydrogenolyzed, as shown in eqs. 13.129— 13.131. In the example shown in eq. 13.131, it was noticed that the rate of hydrogenolysis of the 4-chlorine was considerably greater than that of the 7-chlorine in the presence of an excess of alkali, and the selective dechlorination of the 4-chlorine was successful in an alcoholic solution containing 1.25 equiv of potassium hydroxide at room temperature and atmospheric pressure.240... 

Alkyl and halogen substituted aromatics have 3 values within 0.05 unit of the parent compound. 

In the halogenation of aromatic molecules the role of the zeolite is to polarize the Cl, or Br, molecule in order to enable it to attack the aromatic nucleus. The polarization is aided by an alkali or an alkali earth cation [117]. In most cases addition of Cl, to benzene dominates over MFI and FAU type molecular sieves leading to chlorocyclohexane intermediates. A minor portion of the aromatic molecules, however, also reacts directly to chlorobenzenes via electrophilic substitution. Larger pore zeolites usually lead to a higher degree of chlorination which can be explained by the availability of the space in the zeolite pores[l 18]. 

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