Electrophilic substitution halogenation

However, the transannular effect of the substituents was shown to operate essentially through an inductive mechanism. The effects were studied most extensively with the redox potentials of the X-substituted ferrocene versus the X-substituted ferricinium system. The potentials of the ferrocene derivatives offer a parallel to the activity of the ferrocene nucleus as a function of the substituents. Alkyl ferrocenes are oxidized more readily than ferrocene and they are more active in their electrophilic substitutions. Halogens, COOH, COOCHj, NHCOCH3, HgCl, CH OH, C Hj, to say nothing about RC(0) and CN groups, are electron acceptors... 

Chlorination of 2-methylpyrazine occurs under such mild conditions that it is almost certain that an addition/elimination sequence is involved, rather than a classical aromatic electrophilic substitution. Halogenation of pyrimidines may well also involve such processes. ... 

One amino group is sufficient in most cases to allow easy electrophilic substitution, halogenation for example, and two amino groups activate the ring to attack even by weaker electrophilic reagents - for example by thiocyanogen. Diamino-pyrimidines will couple with diazonium salts, ° which provides a means for the introduction of a third nitrogen substituent. 

From the results of their studies the authors conclude that the methylene group in indoxazene-3-acetic acid is particularly sensitive to electrophilic substitution, halogenation and sulfonation taking place there in preference to the carbocyclic ring. The reasons for this enhanced reactivity and for the anomalous nitration results are as yet unexplained 47... 

Both phenanthrene and anthracene have a tendency to undergo addition reactions under the conditions involved in certain electrophilic substitutions. Halogenation and nitration may proceed in part via addition intermediates ... 

The most widely used reactions are those of electrophilic substitution, and under controlled conditions a maximum of three substituting groups, e.g. -NO2 (in the 1,3,5 positions) can be introduced by a nitric acid/sul-phuric acid mixture. Hot cone, sulphuric acid gives sulphonalion whilst halogens and a Lewis acid catalyst allow, e.g., chlorination or brom-ination. Other methods are required for introducing fluorine and iodine atoms. Benzene undergoes the Friedel-Crafts reaction. ... 

There are a wide variety of methods for introduction of substituents at C3. Since this is the preferred site for electrophilic substitution, direct alkylation and acylation procedures are often effective. Even mild electrophiles such as alkenes with EW substituents can react at the 3-position of the indole ring. Techniques for preparation of 3-lithioindoles, usually by halogen-metal exchange, have been developed and this provides access not only to the lithium reagents but also to other organometallic reagents derived from them. The 3-position is also reactive toward electrophilic mercuration. 

SUBSTITUENT EFFECTS IN ELECTROPHILIC AROMATIC SUBSTITUTION HALOGENS... 

Substituent Effects in Electrophilic Aromatic Substitution Halogens... 

Reactions. The CF O— group exerts predominant para orientation in electrophilic substitution reactions such as nitration, halogenation, acylation, and alkylation (350). 

Beside being acidic, a significant industrial chemical property of phenol is the extremely high reactivity of its ring toward electrophilic substitution. If steric conditions permit, the substitution leads first to the formation of the 2- or 4-mono derivative, then to the 2,4- or 2,6-diderivative, and finally to the 2,4,6-triderivative. The halogenation of phenol produces mono-, di-, and tribal ophenols. 

No simple electrophilic substitution, for example nitrosation, nitration, sulfonation or halogenation of a C—H bond, has so far been recorded in the pteridine series. The strong 7T-electron deficiency of this nitrogen heterocycle opposes such electrophilic attack, which would require a high-energy transition state of low stability. 

In still other cases, the product of reaction of an electrophile with an aminoazole is from electrophilic attack at a ring carbon. This is electrophilic substitution and is the general result of nitration and halogenation (see Section 4.02.1.4). In such cases, reactions at both cyclic nitrogen and at an amino group are reversible. 

Electrophilic substitution occurs readily in Af-phenyl groups, e.g. 1-phenyI-pyrazoIes, -imidazoles and -pyrazolinones are all nitrated and halogenated at the para position. The aryl group is attacked preferentially when the reactions are carried out in strongly acidic media, where the azole ring is protonated. 

Halogenation is one of the most studied electrophilic substitutions in the pyrazole series (67HC(22)1, B-76MI40402). The results concern chlorination, bromination and iodination since there is no report on direct fiuorination of pyrazoles (fiuoropyrazoles are prepared by other... 

Azaindolizine, 5-chloro-nucleophilic substitution, 4, 458 8-Azaindolizine, 7-chloro-nucleophilic substitution, 4, 458 Azaindolizines basicity, 4, 454 electronic spectra, 4, 445 electrophilic substitution, 4, 453 halogenation, 4, 457 hydrogen/deuterium exchange, 4, 458 NMR, 4, 447, 449 nucleophilic attack, 4, 458 protonation, 4, 453 reaction with isothiocyanates, 4, 513 reactions, 5, 267 reviews, 4, 444 UV spectra, 4, 446, 449 Azaindolizines, amino-tautomerism, 4, 452... 

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... 

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