Halogen-substituted Toluenes

Halogen substituted toluenes are readily converted into nitriles because electron-withdrawing substituents enhance the reactivity of such compounds in the ammoxidation reaction. The fluoro-, chloro-, bromo-, and iodo-substituted toluenes [e. g. 41,74-76] can, therefore, be converted to the corresponding nitriles. Whereas the conversion rate of /7-halotoluenes (over vanadium phosphate catalysts [41,75]) is nearly independent of the nature of the halogen substituent, the selectivity decreases in the sequence p-Cl /7-Br p-l. Ammoxidation of isomeric chloro-toluenes results in different conversion p o m) and selectivity p o m) sequences [41,75,76]. 

The reaction of dichloro-substituted toluenes is much more influenced by the position of the substituents-closer proximity of the substituents to the methyl group results in lower conversion and nitrile selectivity (2,6-di-Cl 2,5-di-C1 2,3-di-Cl 2,4-di-Cl 3,4-di-Cl) [39,77], Because, however, 2,6-dichloro-benzonitrile has special importance as an intermediate for pesticides, catalysts and conditions have been described which afford acceptable yields [29,78]. Thus 2,6-dichlorobenzonitrile is formed in 85 % yield by use of a fluid-bed reactor with separate introduction of the educt gases over a vanadium-molybdenum-containing multicomponent catalyst [79]. 

Halogen-substituted xylenes can also be ammoxidized 3,4,5,6,-tetrachloro-phthalodinitrile is formed from the corresponding o-xylene derivative (45 % yield) [80]. 

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Some radicals (e.g., tert-butyl, benzyl, and cyclopropyl), are nucleophilic (they tend to abstract electron-poor hydrogen atoms). The phenyl radical appears to have a very small degree of nucleophilic character. " For longer chains, the field effect continues, and the P position is also deactivated to attack by halogen, though much less so than the a position. We have already mentioned (p. 896) that abstraction of an a hydrogen atom from ring-substituted toluenes can be correlated by the Hammett equation. 

Apart from complex formation involving metal ions (as discussed in Chapter 4), crown ethers have been shown to associate with a variety of both charged and uncharged guest molecules. Typical guests include ammonium salts, the guanidinium ion, diazonium salts, water, alcohols, amines, molecular halogens, substituted hydrazines, p-toluene sulfonic acid, phenols, thiols and nitriles. 

Analogous investigations of deuteriated methanol were carried out on the OD-valency vibration roD = 2689 cm in CCI4 as the standard solvent (Tamres, 1952, Searless and Tamres, 1951). Altogether 12 aromatic compounds, including halogen-substituted ones, were used as solvents. In the series of solvents, carbon tetrachloride, benzene, toluene, 0-, m-, j-xylene and mesitylene, i/qd shifted from 2689 to 2670 in benzene and to 2655 cm in mesitylene. 

The reduction of basicity of benzene by halogen substitution similarly applies to the case of toluene, as deduced by Tamres (1952) from I.R. measurements and by Ogimachi et al. (1955) from complex formation with ICl. The basicity decreases even more for multiple halogen substitution (Tamres, 1952). 

When a halogen substitution product of benzene, e.g. brom benzene, is treated with a methyl halide and metallic sodium, toluene is obtained. This proves that toluene is methyl benzene. 

Isomerism.— As all of the second group, in which substitution occurs in the side chain, considered as benzene derivatives, are mono-substituted benzenes, they do not exist in isomeric forms, and only one compound of each formula is known. The first group, however, in which halogen substitution occurs in the ring, are all poly-substitution products of benzene, since toluene itself is a mono-substituted benzene. Mono-chlor toluene is, therefore, a di-substituted benzene, and occurs in the three forms, as follows ... 

When a radical attacks a C—H or C-halogen bond, the interactions are with a and a orbitals. The latter orbitals are usually high in energy, and we can expect that the major interaction is therefore with the HOMO (Fig. 5-4a), namely the a orbital. Radicals abstracting hydrogen atoms are generally regarded as electrophilic. Reactions of various radicals with p-substituted toluenes have been studied and Hammett plots made (Table 5-1). The p-values are small,... 

The reactions may also be accelerated by the addition of organic compounds to the hydrocarbon. The oxidation of toluene under the action of light is accelerated by the addition of phenanthraquinone, which is itself attacked in the reaction, however, to form diphenic acid.110 Anthraquinone and its halogen-substitution products are of greater effect.120... 

If chlorine and bromine are allowed to act upon an aromatic hydrocarbon like toluene, which has a side-chain, substitution may occur in the nucleus or the side-chain, according to the conditions. Generally speaking, in the cold and in presence of a halogen carrier, nuclear substitution occurs, Irut at a high temperatuie the halogen passes into the side-chain (see Piep. 

The above mechanism for chloromethylation seems to be general for halo-methylation since bromomethylation gives the same ortho para ratio for toluene, ethylbenzene, and i-propylbenzene, which is entirely in accord with the halogen being substituted in a non rate-determining step of the reaction386. 

By introducing reasonable values (about 2 for nitrogen, 4 for oxygen) for the electron affinity parameter relative to carbon, 8, and for the induced electron affinity for adjacent atoms (32/8i = Vio), we have shown that the calculated permanent charge distributions for pyridine, toluene, phenyltrimethylammonium ion, nitrobenzene, benzoic acid, benzaldehyde, acetophenone, benzo-nitrile, furan, thiophene, pyrrole, aniline, and phenol can be satisfactorily correlated qualitatively with the observed positions and rates of substitution. For naphthalene and the halogen benzenes this calculation does not lead to results... 

Detailed investigations of the chemical reactivity of the diketiminato-stabilized phosphenium cations like 28 (Scheme 17) are to date rare and include only two reports dealing with the substitution and reduction of P-halogen-derivatives. Thus, reaction of 28 (X=Br) with sodium hydroxide in toluene was reported to proceed with displacement of the halide substituent at phosphorus and conservation of the heterocyclic ring to give a mixture of bromide and triflate salts containing a P-hydroxy-substituted cation, both of which were isolated in small yields [89], The products are remarkable as they represent one of very few examples of a stable phosphinous acid which does not rearrange to the tautomeric secondary phosphine oxide. Potassium reduction of the P-chloro-substituted derivative 34 produced the... 

Similar reactions occur with all aliphatic halides and the rates of substitution are related to the degree of ionic character of the carbon-halogen bond. For preparation purposes, trityl bromide or propargyl bromide are more convenient than allyl bromide. The compounds obtained are listed in Table XI. They were obtained pure and characterized fully. Zr (allyl) 3Br and Zr(allyl)2Br2 are sufficiently soluble in toluene for polymerizations to be initially homogeneous. Their relative reactivities are listed in Table XI. In all cases hydrogen was used to reduce the molecular weight of the polymer formed. In this respect the polymer derived from Zr (allyl )3Br was more readily modified than that from Zr (allyl) 4, but in order to avoid... 

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