Aromatic ring system

The immediate site of the adsorbent-adsorbate interaction is presumably that between adjacent atoms of the respective species. This is certainly true in chemisorption, where actual chemical bond formation is the rule, and is largely true in the case of physical adsorption, with the possible exception of multilayer formation, which can be viewed as a consequence of weak, long-range force helds. Another possible exception would be the case of molecules where some electron delocalization is present, as with aromatic ring systems. 

Rings can either stabilize or destabilize molecules beyond what is to be expected from a simple additivity scheme. Stabilization comes from aromatic ring systems... 

The organic chemical structural types believed to be characteristic of coals include complex polycyclic aromatic ring systems with connecting bridges and varied oxygen-, sulfur-, and nitrogen-containing functionalities. 

Amine oxides, known as A[-oxides of tertiary amines, are classified as aromatic or aliphatic, depending on whether the nitrogen is part of an aromatic ring system or not. This stmctural difference accounts for the difference in chemical and physical properties between the two types. 

This type of addition process is particularly likely to be observed when the electrophile attacks a position that is already substituted, since facile rearomatization by deprotonation is then blocked. Reaction at a substituted position is called ipso attack. Addition products have also been isolated, however, when initial electrophilic attack has occurred at an unsubstituted position. The extent of addition in competition with substitution tends to increase on going to naphthalene and the larger polycyclic aromatic ring systems. ... 

Table 23.2 lists the oxidation states of the elements along with representative examples of their compounds. Coordination numbers as high as 12 can be attained, but those over 7 in the case of Cr and 9 in the cases of Mo and W involve the presence of the peroxo ligand or rr-bonded aromatic rings systems such as rj -CsHs or rj -C(,H(,. 

At elevated temperatures and possibly under the catalytic influence of the sorbent surface there is probably elimination of functional groups to yield aromatic ring systems that are excited to fluorescence under long-wavelength UV light (X = 365 nm). 

Radke et al. [28] described an automated medium-pressure liquid chromatograph, now commonly called the Kohnen-Willsch instrument. At present, the method is widely used to isolate different fractions of soluble organic matter (for instance, as described in Reference 29 to Reference 31). A combination of normal phase and reversed-phase liquid chromatography has been used by Garrigues et al. [32] to discriminate between different aromatic ring systems and degrees of methylamine in order to characterize thermal maturity of organic matter. 

Extension of aromaticity. The extension of the aromatic ring system through fusion (as in naphthalene) or direct substitution (as in biphenyl) appears to increase log P, especially if the heteroaromatic atom is next to the juncture. If the ring-joining carbons are attached only to other aromatic carbons, electron delocalization is minimal the correction is -rO.lO for each 1C. If the ICs are also attached... 

In a similar fashion therefore, quaternary substituents can also be introduced to aromatic ring systems by the aromatic counterpart SRN1-procedure as investigated mainly by Bunnett115. In an extreme situation of steric shielding, however, a response... 

Thiemann and coworkers [68] sought novel types of steroids with different biological activity, and in doing so prepared areno-annulated compounds such as 6/1-133 (Scheme 6/1.35). This is achieved with a Heck reaction of 6/1-132 with an acrylate, followed by an electrocydic ring closure of the formed hexatriene. The reaction is then terminated by removal of the nitro group, with formation of the aromatic ring system. 

Phenyl methyl ketone 1 was brominated to give l-phenyl-2-bromoethanone 2. Compound 2 was treated with methylsulfonic acid to yield the corresponding methylsulfonate 3. Etherification of 3 gave the a-benzyloxy derivative 4 and compound 4 was then chlorinated to give the 2,4-dichlorinated derivatives in both aromatic ring systems 5. Compound 5 reacted with imidazole in dimethylformamide to give miconazole 6 [7], which is converted to miconazole nitrate. 

Although the number of potential molecules to be made is quite large, the number of small aromatic ring systems is much smaller. A recent study identified 24,847 such ring systems and found that only 1701 have been synthesized. The remaining were subjected to synthetic feasibility calculations and more than 3000 additional ring systems were identified (Figure 3) that have never been made [26]. 

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