Halobenzenes, alkylation

Aromatic ring. Must be at least as reactive as a halobenzene. Alkyl halide. Primary alkyl halides undergo carbocation rearrangement. 

Neither Friedel-Crafts acylation nor alkylation reactions can be earned out on mtroben zene The presence of a strongly deactivating substituent such as a nitro group on an aromatic ring so depresses its reactivity that Friedel-Crafts reactions do not take place Nitrobenzene is so unreactive that it is sometimes used as a solvent m Friedel-Crafts reactions The practical limit for Friedel-Crafts alkylation and acylation reactions is effectively a monohalobenzene An aromatic ring more deactivated than a mono halobenzene cannot be alkylated or acylated under Friedel-Crafts conditions... 

Carbocations, usually generated from an alkyl halide and aluminum chloride, attack the aromatic ring to yield alkylbenzenes. The arene must be at least as reactive as a halobenzene. Carbocation rearrangements can occur, especially with primary alkyl halides. 

Many variations of the reaction can be carried out, including halogenation, nitration, and sulfonation. Friedel-Crafts alkylation and acylation reactions, which involve reaction of an aromatic ling with carbocation electrophiles, are particularly useful. They are limited, however, by the fact that the aromatic ring must be at least as reactive as a halobenzene. In addition, polyalkylation and carbocation rearrangements often occur in Friedel-Crafts alkylation. 

Marko and coworkers [12] dehalogenated several alkyl and benzyl halides as well as halobenzenes (X=C1, Br, I) with [(l,5-hexadiene)RhCl]2/PPh3 in the presence of Et2NH using a medium of p-xylene/water at 50°C and atmospheric H2 pressure. Grushin and Alper [13] elaborated a similar (but more effective)... 

Hall-Petch relationship, 13 497 Haloacetic acids (HAAs), removal from drinking water, 17 806—807 Haloacetones, 1 163 Haloalkylating agents, 12 167 Haloalkylation, 12 166-168 Haloalkylbenzenes, intramolecular alkylation of, 12 169 IV-Halo-a-amino acids, 13 107 Haloaromatics, 13 573 Halobenzenes, as Friedel-Crafts arylating agents, 12 171 Halobismuthines, 4 28-29 Haloboration, 14 270 Halobrom (BCDMH,... 

Changes in intramolecular selectivity in the bromination and nitration of alkyl-benzenes in acidic media have been attributed to changes in medium polarity or changes in electrophile solvation. Mass spectrometric studies of the first stage in the gas-phase reactions of halobenzenes, furan, thiophene and pyrrole with alkyl cations have been rationalized in terms of co-existing a- and tt-complexes. The extent of... 

A limited number of reports of indoles arising from o-bromoaniline and enolates have appeared (equation 107) (80JOC1546,8lT(S9)393). The final cyclization step is of the condensation type already recognized in several other procedures. The inertness of unactivated halobenzenes, such as o-bromoaniline, requires an alternative to direct aromatic nucleophilic substitution and those cases where success has been reported depend upon photoinitiated substitution by an electron transfer process. The scope of this method remains to be explored but it appears that alkyl, alkoxy and carboxy groups can be tolerated on the aromatic ring. When the enolates are derived from an unsymmetrical ketone in which one group is methyl, there appears to be a preference for exclusive involvement of the less substituted enolate, leading to 2-alkylindoles. 

The reactivities of aryl halides, such as the halobenzenes, are exceedingly low toward nucleophilic reagents that normally effect displacements with alkyl halides and activated aryl halides. Substitutions do occur under forcing conditions of either high temperatures or very strong bases. For example, chlorobenzene reacts with sodium hydroxide solution at temperatures around 340° and this reaction was once an important commercial process for the production of benzenol (phenol) ... 

Neumann, R. and Sasson, Y., Poly(ethylene glycols) as phase-transfer catalysts in the alkoxylation of halobenzenes of alkyl aryl ethers, Tetrahedron, 1983, 39, 3437. 

Triflic acid is also efficient in the alkylation of electron-rich aromatics (anisole, 1,3-dimethoxybenzene, 2-methylfurane, pyrrole, benzofurane, indole) with secondary benzylic alcohols and 3-phenylallyl alcohols (50°C, 1-9 h, 66-95% yield).201 Benzene, toluene, and halobenzenes are also alkylated with hydroxy-biindantetraone 53 in triflic acid within 1-2h202 [Eq. (5.78)]. Suprisingly, however, the primary products (with the exception of the 4-methylphenyl-substituted compound) undergo rearrangement upon prolonged treatment to yield alkenes... 

Much as benzene and phenyl are the simplest aromatic hydrocarbon or arene and aryl group respectively, methane and methyl are the simplest aliphatic hydrocarbon or alkane and alkyl group respectively. The resonance energy of the various halobenzenes may be identified as the negative of the exothermicity of reaction 34. 

At elevated temperatures and in a sealed tube pentaphenylstiborane reacts with alkyl halides such as chloroform, tetrachloromethane142) or iodomethane143> to give tetraptenylstibonium halides, benzene, halobenzene and other products. Homo-lytic cleavage of Sb—C bonds is assumed to play a leading role here. In a more detailed study, a radical chain reaction was proposed for such decompositions,... 

The alkylation cannot be used with strongly deactivated derivatives. Also true Only benzene, halobenzenes, and activated derivatives are suitable. 

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