Halogenated benzenes monosubstituted

Table 27. Estimated partial rate factors for fiara halogenation of monosubstituted benzenes .

A point in case is provided by the bromination of various monosubstituted benzene derivatives it was realized that substituents with atoms carrying free electron pairs bonded directly to the benzene ring (OH, NH2, etc) gave 0- and p-substituted benzene derivatives. Furthermore, in all cases except of the halogen atoms the reaction rates were higher than with unsubstituted benzene. On the other hand, substituents with double bonds in conjugation with the benzene ring (NO2, CHO, etc.) decreased reaction rates and provided m-substituted benzene derivatives. 

The additivity treatment also allows one to evaluate the influence of substituents which are otherwise obtainable only with difficulty. The study of the non-catalytic bromination of the halo-substituted poly-methylbenzenes by Illuminati and Marino (1956) allowed the evaluation of the partial rate factors for the highly deactivating m- and p-halogens. These data for the slow, highly selective bromination are inaccessible by other techniques. Analysis of the relative rates is made by application of the additivity equations (5) and (6) as described in Section I. An important aspect of the chemistry of the substituted polymethyl-benzenes, in contrast to the monosubstituted benzenes, is the large difference in p for bromination. The partial rate factors derived for each reaction are correlated with good precision by the tr4 -constants (Figs. 11 and 19). Yet the susceptibility of the reactions to the influence of substituents is altered by more than 25%. As already noted, this aspect of the problem is not well defined and is worthy of additional attention. 

Considering what was said about stabilization energies in our previous discussion of thermochemical mimics, we now turn to aryl halides. There are several conceptual approaches to their thermochemistry one can take. The first is to consider halogenated derivatives of benzene, then of naphthalene, then of the isomeric anthracene and phenan-threne, etc. This approach, perhaps more appropriate for a study of generally substituted aromatic hydrocarbons, is immediately thwarted. Although there are many appropriate derivatives of benzene worthy of discussion, thermochemical data on halogenated naphthalenes are limited to the isomeric 1- and 2-monosubstituted derivatives, and halogenated derivatives of other aromatics remain thermochemically unstudied. 

Metal-halogen exchange of the readily THF-soluble polymer 3 with alkyllithiums gave lithiate 3A, which is insoluble in THF, but stable for hours at -78 °C. In a small scale reaction, the lithiation yield was shown to be over 90%, based on successful trapping experiments with chloro-trimethylsilane. When the anion is quenched with methanol (MeOH) or acetonitrile, IR peaks appear at 700 and 779 cm-1 due to monosubstituted benzene, as well as peaks corresponding to trisubstituted benzene. The lithiate can also be treated with various electrophiles, such as C02, yielding the water-soluble carboxylate (3B). Hydrolysis of this polymer provides a water-insoluble, but THF-soluble, polyacid (3J). 

With monosubstituted benzenes, the tendency of OH to undergo ipso addition seems to be small (<10%). However, if the ipso position is activated by a second substituent like CH3O, HO or 0 , ipso addition may contribute up to 25% to the overall reactivity . Due to the small size of the OH radical, steric effects (i.e. the size of X) do not seem to be of mnch importance in determining the probability of OH attack at the ipso position. Oxidative replacement of halogen has been observed also with pentafluoro-, pentachloro-, pentabromo- and 2,4,6-triiodo-phenol, where it occurs in parallel to electron transfer". ... 

To get an idea of such a prediction process, we will look at a result for the prediction of a monosubstituted benzene derivative. The data set for this experiment contains 50 benzene derivatives and their spectra-descriptor pairs. Figure 6.5 shows the infrared spectrum of the query compound reduced to 128 components by the FHT method described already. The spectrum contains some bands that may indicate the existence of an aromatic system and of halogen atoms. However, the spectrum is not particularly characteristic. 

All monosubstituted benzenes react with the same regio- and enantio-selectivity whether the substituent is halogen, alkyl, alkenyl, C02H, CN, COMe, OMe or CF3. Thus toluene gives the diol 76 that can be produced at 3g/litre in the broth described above. It is also a commercial product. 

Constancy of A/ Values for Halogen SubstituentsxSrtvB net Adsorption of Monosubstituted and />ara-Disubstitut --Benzenes on 0.7% H2O-AI2O3 from Pentane... 

Halogen atoms activated by carbonyl groups adjacent to the benzene ring can be, in some cases, displaced with nucleophiles. For example, compound (250) treated with iV,A(-dimethylaminopropyl amine gives the product of monosubstitution (251) together with the rearrangement 2 1 product (252). No product of normal disubstitution is formed (Equation (34)) <92CPB2905>. 

Aromatic compounds like benzene undergo a highly characteristic reaction called electrophilic substitution. For example, halogens, such as chlorine and bromine, instead of simply adding to the formal double bonds as if it were an olefin (i.e. electrophilic addition in which both halogen atoms add to the double bond), displace one of the hydrogen atoms to give a monosubstituted aryl halide... 

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