Halogenation, electrophilic

The benzenesulphonylation of benzene did not give a single kinetic form. Satisfactory second-order rate coefficients could be obtained provided that the initial benzene concentration was not small, when a small increase in the calculated rate coefficients as the reaction proceeds is observed. However, the values of the second-order rate coefficients are not independent of the initial concentration of reactants (Table 51). It was concluded that benzene reacts by both mechanisms  

RATE COEFFICIENTS FOR THE AICI3-CATALYSED REACTION OF PhSOjCl WITH PhH IN 

The very large effect of adding an amount of aluminium chloride greater than stoichiometric as noted by Olivier has been attributed187 to an as yet unspecified property of the dimer which differs from that of the monomer. 

Finally, it is interesting to note that the similarity between benzenesulphonylation and benzoylation also shows up in the log f0 log fp ratios for toluene which are 0.56184 and 0.54188, respectively, indicating a similar size of electrophile in each case. 

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You have just seen that cyclic halonmm ion intermediates are formed when sources of electrophilic halogen attack a double bond Likewise three membered oxygen containing rings are formed by the reaction of alkenes with sources of electrophilic oxygen... 

Beyer synthesis, 2, 474 electrolytic oxidation, 2, 325 7r-electron density calculations, 2, 316 1-electron reduction, 2, 282, 283 electrophilic halogenation, 2, 49 electrophilic substitution, 2, 49 Emmert reaction, 2, 276 food preservative, 1,411 free radical acylation, 2, 298 free radical alkylation, 2, 45, 295 free radical amidation, 2, 299 free radical arylation, 2, 295 Friedel-Crafts reactions, 2, 208 Friedlander synthesis, 2, 70, 443 fluorination, 2, 199 halogenation, 2, 40 hydrogenation, 2, 45, 284-285, 327 hydrogen-deuterium exchange, 2, 196, 286 hydroxylation, 2, 325 iodination, 2, 202, 320 ionization constants, 2, 172 IR spectra, 2, 18 lithiation, 2, 267... 

As with addition of other electrophiles, halogenation of conjugated dienes can give 1,2- or 1,4-addition products. When molecular bromine is used as the brominating agent in chlorinated hydrocarbon solvent, the 1,4-addition product dominates by 7 1 in the case of butadiene. ... 

With more electrophilic halogenated olefins, additions of tnfluoromethyl hypofluonte appear to go by radical processes [139] and give oligomeric products [140] or one-to-one adducts [141,142] (equation 4) depending on reaction conditions... 

The hydroxyl group is a strongly activating, ortho- and para-directing substituent in electrophilic aromatic substitution reactions (Section 16.4). As a result, phenols are highly reactive substrates for electrophilic halogenation, nitration, sulfonation, and lTiedel-Crafts reactions. 

The two isomeric possibilities are 1,2- and 2,1-benzisoxazole. Both are preferentially halogenated by electrophilic halogen in the homocyclic ring, initially in the 5-position, although substituents can modify this behavior [67AHC(8)277]. Nucleophiles attack the heteroring (84MI26). 

With phenazine (97) there is considerable resistance to any electrophilic halogenation, but in aqueous solution chlorine converted it into a mixture of 1-chloro, 1,4-dichloro-, 1,4,6-trichloro-, and 1,4,6,9-tetrachloro-phenazines (53G327). 

Both electrophilic and nucleophilic reactions can generate halogenopur-ines with differences in regioselectivity dependent on substituents and on the nature of the substrate (anion, neutral molecule, or cation). In the neutral molecule nucleophilic displacements occur in the order 2 > 4 > 6 in the anion the imidazole ring may be sufficiently 7r-excessive for attack to occur at C-2, and the nucleophilic substitution order becomes 4 > 6 > 2. Strong electron donors are usually necessary to promote 2-halogenation by electrophilic halogen sources. 

Isoxazolo[4,5-6]pyrazine (176) is resistant to electrophilic halogenation because the potentially reactive site is at the ring junction. The 7-oxide, though, was converted into the 6-chloro derivative of 176 on heating with phosphoryl chloride (73JHC181). 

Although the simple mono-(2-,4-,6-, and 7-) and some di-(6,7-, 2,4-) tri-(2,4,6-, 2,4,7-), and tetra-(2,4,6,7-) -chloro derivatives are known, they are often unstable and highly susceptible to nucleophilic attack, and little is known about some of the compounds (64JCS1666 84MI10). No electrophilic halogenations have yet been reported, but there have been a few instances of chlorine introduction by Meisenheimer reaction of pteridine N-oxides (78JOC680 82LA2135). 

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