Monosubstituted benzenes electrophilic aromatic

Polycyclic aromatic hydrocarbons undergo electrophilic aromatic substitution when treated with the same reagents that react with benzene In general polycyclic aromatic hydrocarbons are more reactive than benzene Most lack the symmetry of benzene how ever and mixtures of products may be formed even on monosubstitution Among poly cyclic aromatic hydrocarbons we will discuss only naphthalene and that only briefly Two sites are available for substitution m naphthalene C 1 and C 2 C 1 being normally the preferred site of electrophilic attack... 

A hydroxyl group is a very powerful activating substituent, and electrophilic aromatic substitution in phenols occurs far- faster, and under milder conditions, than in benzene. The first entry in Table 24.4, for exfflnple, shows the monobromination of phenol in high yield at low temperature and in the absence of any catalyst. In this case, the reaction was carried out in the nonpolar- solvent 1,2-dichloroethane. In polar- solvents such as water it is difficult to limit the bromination of phenols to monosubstitution. In the following exfflnple, all three positions that are ortho or para to the hydroxyl undergo rapid substitution ... 

The most familiar set of organic reactions is perhaps the electrophilic aromatic substitutions. For monosubstituted benzenes the major products from the process are either o- or p-disubstituted benzenes or m-disubstituted analogs. 

We will restrict our consideration to reactions of substituted benzenes and to nitrogen heteroaromatic systems in which the reaction takes place first with the n system. The simplest example of reaction of a monosubstituted benzene with an electrophile (Lewis acid) is shown in Scheme 11.1. The electrophile may attach itself to the n system (step A) in four distinct modes, ipso, ortho, meta, and para. The reactivity of the aromatic ring and the mode of attachment of the electrophile will be influenced by the specific nature of the substituent group, which may be X , Z, or C type. Detachment of the electro-... 

A quantitative description of the reactivity of monosubstituted benzenes to electrophilic substitution based on considerations of inductive effect parameters and con-jugative effect parameters from the 13 C chemical shifts of the aromatic compounds has been proposed.3 MO calculations on the proton migration in the ipso adducts formed in the reaction of CH3+ and SiH3+ with benzene have been described.4 With SiH3+ the ipso adduct is the most stable of possible isomers, whereas for CH3+ the >ara-protonated isomer is the most stable. 

Deactivated monosubstituted benzenes Traditionally, qualitative discussions of electrophilic substitution emphasize the deactivating influence of positive or partial positive charges on substituent atoms (Ingold, 1953). Only in recent years, however, have valid quantitative measurements been performed to allow the elucidation of the extent of the deactivation of the aromatic nucleus by such groups. The available information, still very limited in scope, is summarized in Table 18. 

MO studies of aromatic nitration cast doubt on the existence of jt-complexes and electron-transfer complexes in liquid-phase nitrations.14 The enthalpy of protonation of aromatic substrates provides a very good index of substrate reactivity to nitration. Coulomb interaction between electrophile and substituent can be a special factor influencing regioselectivity. A detailed DFT study of the reaction of toluene with the nitronium ion has been reported.15 Calculated IR spectra for the Wheland intermediates suggest a classical SE2 mechanism. MO calculations of cationic localization energies for the interaction of monosubstituted benzenes with the nitronium ion correlate with observed product yields.16... 

Regioselectivity in the formation of regioisomers is also observed in electrophilic aromatic substitution reactions. In the case of monosubstituted benzene derivatives, there are three possible regiosomeric products that form at different rates, based on the mechanism of the reaction (see Figure 13). see also Berzelius, Jons Jakob Chirality Dalton, John Davy, Humphry Molecular Structure Scheele, Carl Wohler, Friedrich. 

The vast majority of reactivities and regioselectivities observed in the reaction with electrophiles on monosubstituted benzenes (Table 5.2) are in agreement with the preceding generalizations (columns 2 and 4). The very few substituents that are not in agreement (column 3) deactivate the aromatic compound as do electron acceptors, but they are para- > ortho-directing as are electron donors. 

With monosubstituted benzenes a mixture of isomeric biphenyls is obtained. The substitution pattern corresponds to that observed in electrophilic aromatic substitution, consistent with a mechanism involving electrophilic attack by PdCl2 shown in Eq. (388). 

Aromatic compounds are the only type of i-nucleophiles involved in reactions with organoxenonium salts. While pentafluorobenzenes, CeFsZ (Z = H, F, CN, SiMej), are not reactive towards [C6F5Xe][AsF6] in MeCN at 20 °C, a series of 2,3,4,5,6-pentafIuorobiphenyls was obtained from monosubstituted benzene derivatives, CeHsZ, under these conditions. The reaction rates diminish in the sequence Z = CH3 > F > CF3 CN > NO2, which is consistent with the electrophilic nature of the process. Nevertheless, the isomer distribution in the C6F5C6H4Z products shows unambiguously the radical character of the pentafluorophenylation reaction (eq 17) 46). 

Aromatic bromides (3, 286). The definitive paper on electrophilic aromatic bromination with bromine and thallium(lll) acetate has been published. The two most oul.standingfeaturc.s are I) monobromination is observed in almost all cases, and 2) exclusive para substitution is observed with almost all monosubstituted benzenes. Electron-withdrawing groups inhibit bromination of monosubstituted benzenes. It... 

The isomer distribution for anodic acetoxylation of a number of monosubstituted benzenes has been determined [122]. The reaction closely resembles ordinary electrophilic aromatic substitution processes, perhaps on the side of low-selectivity reactions. The isotope effect, A h//cd, for nuclear acetoxylation in anisole was found to be 1.0, whereas for a-substitution in ethylbenzene a value of 2.6 was observed. The interpretation of these values is not straightforward [126]. 

Since the positions of the Is,min are the locations, on the average, of the most easily removed electrons, these should also be the sites that are most reactive toward electrophilic attack. This has been fully confirmed for a group of monosubstituted benzene derivatives [10,21]. The Is,min correctly predict the ortho/para- or meta- directing tendencies of the substituents, even the rather unusual NH3+, which is a metal para director [22]. Furthermore, the magnitudes of the Is,min relative to that of unsubstituted benzene correctly indicate whether each substituent activates or deactivates the aromatic ring toward electrophiles. These analyses have been extended to other aromatic systems, including azines and azine N-oxides [18,23,24]. 

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