Monosubstituted, 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. 

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. 

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 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]. 

The major products of further substitution in a monosubstituted naphthalene can usually be predicted by the following rules. As we shall see, these rules are reasonable ones in light of structural theory and our understanding of electrophilic aromatic substitution. 

Protonation and alkylation of arenes afford cyclohexadienyl cations (arenium ions) which are also of importance in electrophilic aromatic substitution. The hepta-methylbenzenium ion (16) is a very stable species30-, but even the parent benzenium ion (17a) has been observed as have most of its alkyl, halo, and alkoxy derivatives51. The benzenium ion (17a) undergoes a rapid degenerate rearrangement which equilibrates the seven protons over six carbons. Data for the monosubstituted benzenium ions show that (17) is the most stable of the possible isomeric forms. Positive charge... 

The synthesis of 2-monosubstituted or 2,2 -disubstituted precursor compounds can be achieved by two different routes direct electrophilic aromatic substitution or the use of substituted derivatives of 9-fluorenone. hi many cases, the first route is complicated due to the fact that the two fluorene moieties in spirobifluorene react chemically independently of each other. The formation of products monosubstituted in the 2-position is therefore often accompanied by the formation of byproducts (Fig. 6). A good example is the nitration of spirobifluorene, which was investigated by Weisburger and coworkers [23]. They discovered that a careful control of reaction times and conditions allowed for the formation of 2-nitro-9,9 -spirobifluorene or 2,2 -dinitro-9,9 -spirobifluorene as the main product. Especially in the case of 2-nitro-9,9 -spirobifluorene, however, the conversions are low because the reaction must be quenched before a suitable amoimt of the dinitro compound is formed. 

In the electrophilic aromatic substitution of a monosubstituted benzene, three isomeric products are possible The new group may be oriented ortho, meta, or para to the existing group. On the basis of a wealth of experimental observations, chemists have made the... 

To explain electrophilic aromatic substitution of substituted benzene derivatives, a generic benzene derivative, 61, is used in Figure 21.1, with a substituent X. This is used rather than a specific example in order to show the similarities and differences in reactivity for electron-releasing versus electron-withdrawing groups. The carbon of the benzene ring attached to the substituent is defined as the ipso carbon ( ) in Figure 21.1. There are only three possible arenium ion intermediates for the reaction of any monosubstituted benzene derivative 61 with an electrophile such as Br+ 62, 63, and 64. 

Many monosubstituted benzene derivatives can be prepared by electrophilic aromatic substitution. If the substituent is -Cl, -Br, -NO2, -SO3H, or -COR (acyl), then a possible retrosynthesis involves a disconnection of the bond connecting that substituent to the benzene ring. The aromatic carbon was the nucleophile (simply add a hydrogen here) and the substituent was the electrophile (usually a positively charged atom that is generated by the appropriate set of reagents). 

You will be measuring the relative rates of bromination of six monosubstituted arenes 27-32. This particular electrophilic aromatic substitution is selected because the relative rates may be measured both qualitatively and quantitatively. The results should allow you to determine the order of ring activation associated with the various substituents that you will investigate. 

---