Electron aromatic rings

Conjugated ring systems offer an alternative mode for the stabilization of a carbanion center. The most common situation is where deprotonation completes a cyclic n system leading to a highly stabilized, aromatic anion. The best known example is cyclopentadiene, which leads to a six-electron, aromatic ring after... 

It should also be noted here that recent work in Japan has shown that a free germanium cation can be prepared and structurally characterized. Thus, the reaction of tetrakis (tri-z-butylsilyl)cyclotrigermene with trityl tetraphenylborate affords (t-Bu3SiGe)3+ BPI14 as a yellow crystalline solid. X-ray crystallography shows that there is no close interaction between the anion and the cation. The stability of the compound is presumably due not only to the steric protection afforded by the bulky t-BtcSi substituents, but also to the fact that it contains a 2jr-electron aromatic ring.198... 

The germanium-ring cyclopropene analogs can be oxidized to yield 2 r-electron aromatic rings. These compounds,... 

Correlations have been found between certain absorption patterns in the infrared and the concentrations of aromatic and paraffinic carbons given by the ndA/method (see article 3.1.3.). The absorptions at 1600 cm due to vibrations of valence electrons in carbon-carbon bonds in aromatic rings and at 720 cm (see the spectrum in Figure 3.8) due to paraffinic chain deformations are directly related to the aromatic and paraffinic carbon concentrations, respectively. )... 

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. 

An alternative drivirg force could involve a donor - acceptor interaction. The electron-poor pyridine ring that is coordinated to the copper cation can act as electron acceptor with respect to the aromatic ring of the -amino acid. The fact that donating substituents on the amino acid increase the efficiency... 

In TT-complexes formed from aromatic compounds and halogens, the halogen is not bound to any single carbon atom but to the 7r-electron structure of the aromatic, though the precise geometry of the complexes is uncertain. The complexes with silver ions also do not have the silver associated with a particular carbon atom of the aromatic ring, as is shown by the structure of the complex from benzene and silver perchlorate. ... 

Success of the reactions depends considerably on the substrates and reaction Conditions. Rate enhancement in the coupling reaction was observed under high pressure (10 kbar)[l 1[. The oxidative addition of aryl halides to Pd(0) is a highly disfavored step when powerful electron donors such as OH and NHt reside on aromatic rings. Iodides react smoothly even in the absence of a... 

As expected, the formation of a carbonyl group is not possible with tert-allylic alcohols. Although the aromatic ring bears electron-donating groups, the 2,2-disubstituted chromene 119 was formed smoothly with the tert-allylic alcohol 118[100]. 

The alkylpalladium intermediate 198 cyclizes on to an aromatic ring, rather than forming a three-membered ring by alkene insertion[161], Spirocyclic compounds are easily prepared[l62]. Various spiroindolines such as 200 were prepared. In this synthesis, the second ring formation involves attack of an alkylpalladium species 199 on an aromatic ring, including electron-rich or -poor heteroaromatic rings[l6.5]. 

Step 1 An electron is transferred from sodium (the reducing agent) to the n system of the aromatic ring The product is an anion radical... 

Arylamines contain two functional groups the amine group and the aromatic ring they are difunctional compounds The reactivity of the amine group is affected by its aryl substituent and the reactivity of the ring is affected by its amine substituent The same electron delocalization that reduces the basicity and the nucleophilicity of an arylamme nitrogen increases the electron density in the aromatic ring and makes arylamines extremely reactive toward electrophilic aromatic substitution... 

A nitro group behaves the same way m both reactions it attracts electrons Reaction is retarded when electrons flow from the aromatic ring to the attacking species (electrophilic aromatic substitution) Reaction is facilitated when electrons flow from the attacking species to the aromatic ring (nucleophilic aromatic substitution) By being aware of the connection between reactivity and substituent effects you will sharpen your appreciation of how chemical reactions occur... 

In resonance terms the shorter carbon-oxygen bond distance in phenol is attrib uted to the partial double bond character that results from conjugation of the unshared electron pair of oxygen with the aromatic ring... 

Many of the properties of phenols reflect the polarization implied by the resonance description The hydroxyl oxygen is less basic and the hydroxyl proton more acidic in phenols than m alcohols Electrophiles attack the aromatic ring of phenols much faster than they attack benzene indicating that the ring especially at the positions ortho and para to the hydroxyl group is relatively electron rich... 

The aromatic ring of a phenol like that of an arylamine is seen as an electron rich functional unit and is capable of a variety of reactions In some cases however it IS the hydroxyl oxygen that reacts instead An example of this kind of chemical reac tivity IS described m the following section... 

Although a hydroxyl group strongly activates an aromatic ring toward electrophilic attack an oxyanion substituent is an even more powerful activator Electron delocaliza tion m phenoxide anion leads to increased electron density at the positions ortho and para to oxygen... 

Notice too that the most shielded carbons of the aromatic ring are the ones that are ortho and para to the hydroxyl group m keeping with our experience that the OH group donates electrons preferentially to these positions... 

Phenols are polar compounds but less polar than alcohols They resem ble arylammes m having an electron rich aromatic ring... 

Phenylalanine and tryptophan have side chains that incorporate aromatic rings which are large and hydrophobic The aromatic portion of tryptophan is bicyclic which makes it larger than phenylalanine Tryptophan also has a more electron rich aromatic ring and is more polarizable than phenylalanine Its role is more specialized and it is less abundant m proteins than most of the other ammo acids... 

Aromatic rings ca 1600 (m) ca 1580 (m) ca 1470 (m) ca 1510 (m) Stronger when ring is further conjugated When substituent on ring is electron acceptor When substituent on ring is electron donor... 

An additional curious feature of alkylaromatic oxidation is that, under conditions where the initial attack involves electron transfer, the relative rate of attack on different alkyl groups attached to the same aromatic ring is quite different from that observed in alkane oxidation. For example, the oxidation of -cymene can lead to high yields of -isopropylbenzoic acid (2,205,297,298). 

The instabihty of tert-huty areneperoxysulfonates is increased by the presence of electron-withdrawing substituents on the aromatic ring and decreased by electron-donating substituents. However, even the most stable members decompose violently on warming, as indicated in Table 14. These peroxyesters appear to decompose heterolyticaHy without the formation of free radicals (44). 

This is due to a resonance effect. Aniline is stabilized by sharing its nitrogen lone-pair electrons with the aromatic ring. In the anilinium ion, the resonance stabilization is dismpted by the proton bound to the lone pair. 

---