Halogenation of aromatics

Halogenation of aromatic compounds Rearrangement of N-haloamines (Orton)... 

Scheme 39 Halogenation of aromatic compounds catalyzed by FeCl3...

The rate of the second reaction is quite low as long as there is enough free sulfonyl chloride to react with additional amounts of aluminum chloride. Once this is no longer true, further additions of catalyst enormously increase the reaction rate (27). The recently discovered swamping catalyst effect in the halogenation of aromatic donor species (35) probably exhibits analogous kinetic behavior. The basic rate expression found by Olivier consisted of only one term for reaction when a relatively small amount of aluminum chloride was present ... 

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

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... 

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. 

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... 

Figure 3.106 Halogenation of aromatics employing HX/hydrogen peroxide.

The aryl group participation process is mechanistically a Friedel-Crafts alkylation. The r value of the Friedel-Crafts alkylation by alkyl carbocations was found to be significantly lower than that of protonation or of halogenation of aromatic substrates (Yukawa et al., 1966). Olah interpreted the low r value in terms of an earlier transition state, i.e. less advanced aryl C bond formation with r = 0.6 at the transition state prior to formation of the... 

Electrophilic aromatic substitution (Sections 16.1 -16.4) ta> Halogenation of aromatic compoundB (Section 16.1)... 

Halogenation of aromatic nuclei may also be achieved by hdogen or positively charged species of halogen formed in solution by anodic oxidation of halide anions (equation 70). ... 

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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