Aromatic rings, origin

The best-known equation of the type mentioned is, of course, Hammett s equation. It correlates, with considerable precision, rate and equilibrium constants for a large number of reactions occurring in the side chains of m- and p-substituted aromatic compounds, but fails badly for electrophilic substitution into the aromatic ring (except at wi-positions) and for certain reactions in side chains in which there is considerable mesomeric interaction between the side chain and the ring during the course of reaction. This failure arises because Hammett s original model reaction (the ionization of substituted benzoic acids) does not take account of the direct resonance interactions between a substituent and the site of reaction. This sort of interaction in the electrophilic substitutions of anisole is depicted in the following resonance structures, which show the transition state to be stabilized by direct resonance with the substituent ... 

Birch and others originally suggested that two electrons add to the aromatic ring affording a dicarbanion (34) which is subsequently protonated [Eq. (1)]. The preference for maximum separation of charge in dicarbanion... 

The hydrogenation of ring A aromatic steroids over ruthenium occurs, almost invariably, from the a side and all substituents on the original aromatic ring are cis in the resulting cyclohexane. Estrone (62) is hydrogenated over ruthenium to 5a,10a-estrane-3/3,17j6-diol (63) in 85-90% yield. 

Figure 15.14 The origin of aromatic ring-current. Aromatic protons are deshielded by the induced magnetic field caused by delocalized tt electrons circulating in the molecular orbitals of the aromatic ring.

In the production of TNT from the reaction between toluene and mixed acids (nitric/sulfuric), TeNMe forms in amounts between 0.2—0.4% of the total wt of TNT. This TeNMe has been held responsible for several expins which have occurred in TNT plants, causing fatal injuries to personnel and severe damage to facilities. These expins were attributed to the presence of TeNMe in the acid fume lines and the acid storage tanks. Mixts of TeNMe and readily oxidizable materials are known to form very powerful and sensitive expl mixts. Since TeNMe is also isolated from the nitration of Nitrobenzene (NB), the TeNMe formed in the nitration of toluene may arise from the oxidation of the aromatic ring and/or methyl group. In an effort to gain more informa-. tion on the origin of TeNMe from TNT production, radioactive carbon-14 (14C) was used as a tracer to determine the extent to which each of the carbon atoms in the toluene skeleton of the various nitro-substituted isomers contributes to... 

Aromatic hydrocarbons, which originally got their name from the distinctive odors many of them have, are called arenes. They all contain an aromatic ring, usually the six-membered ring of benzene, which was introduced in Sections 2.7, 3.7, and 3.12. An abundant source of arenes is coal, which is a very complex mixture of compounds, many of which consist of extensive networks containing aromatic rings (Section 18.10). 

Scheme 2.11 shows some examples of Robinson annulation reactions. Entries 1 and 2 show annulation reactions of relatively acidic dicarbonyl compounds. Entry 3 is an example of use of 4-(trimethylammonio)-2-butanone as a precursor of methyl vinyl ketone. This compound generates methyl vinyl ketone in situ by (3-eliminalion. The original conditions developed for the Robinson annulation reaction are such that the ketone enolate composition is under thermodynamic control. This usually results in the formation of product from the more stable enolate, as in Entry 3. The C(l) enolate is preferred because of the conjugation with the aromatic ring. For monosubstituted cyclohexanones, the cyclization usually occurs at the more-substituted position in hydroxylic solvents. The alternative regiochemistry can be achieved by using an enamine. Entry 4 is an example. As discussed in Section 1.9, the less-substituted enamine is favored, so addition occurs at the less-substituted position. 

Closely related to the 1,3,2-dithiazolyl radicals are the isomeric 1,2,3-dithiazolyl radicals. The benzo-fused derivatives were originally prepared by Herz in 192276 from the reaction of aniline and its derivatives with an excess of S2C12 (Scheme 10). Almost invariably the aromatic ring becomes substituted by chlorine para to the amine N atom. 

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