Aromatics positive ions

In a 7r electron system the orbitals of an aromatic positive ion are similar to the corresponding orbitals of the neutral molecule. In contrast, in small molecules electronic rearrangement following excitation is often sufficiently important that changes in nuclear geometry, correlation energy, etc., are all essential to the correct interpretation of the excitation phenomenon. Because of the similarity in the orbital systems of neutral and positive ion aromatic compounds, we shall assume that it is possible to describe the photoionization of an aromatic molecule within the framework of a one-electron model. Given that the n and a electrons are describ-able by a set of separable equations of motion, we need consider only the initial and final orbitals of the most weakly bound electron to determine the ionization cross section near to the threshold of ionization. 

In Chapter 3, it was mentioned that positive ions can form addition complexes with 7T systems. Since the initial step of electrophilic substitution involves attack by a positive ion on an aromatic ring, it has been suggested that such a complex, called a % complex (represented as 10), is formed first and then is converted to the arenium ion 11. Stable solutions of arenium ions or 7t complexes (e.g., with Br2, l2> picric... 

Direct Electron Transfer. We have already met some reactions in which the reduction is a direct gain of electrons or the oxidation a direct loss of them. An example is the Birch reduction (15-14), where sodium directly transfers an electron to an aromatic ring. An example from this chapter is found in the bimolecular reduction of ketones (19-55), where again it is a metal that supplies the electrons. This kind of mechanism is found largely in three types of reaction, (a) the oxidation or reduction of a free radical (oxidation to a positive or reduction to a negative ion), (b) the oxidation of a negative ion or the reduction of a positive ion to a comparatively stable free radical, and (c) electrolytic oxidations or reductions (an example is the Kolbe reaction, 14-36). An important example of (b) is oxidation of amines and phenolate ions ... 

Dienes and polyenes show a pronounced molecular ion in the mass spectra and hence the molecular weight of polyenes can be determined by positive ion mass spectra. The easy removal of a 7r-electron from a diene is usually the reason for the distinct M +. The mass spectral investigation of conjugated polyenes is somewhat similar to that of aromatic structures, due to the high stability of the rearranged ions formed after the... 

Finally, the five-feature a pharmacophore was generated from those ald antagonists, which were 100-fold selective over ala and 40-fold selective over alb receptors. The pharmacophore consisted of the following features (a) one positive ion, (b) one aromatic ring, (c) one H-bonding acceptor and (d) two hydrophobic groups [102]. 

As portrayed above, no aromatic carbonium ion is formed as such. Rather, one positive group is expelled as the other one enters. However, other schemes have been suggested, such as that shown below (12,17), as well as more complicated ones which involve several resonance structures. 

Aromatic carbenium ions such as cyclopropenium and cycloheptatrienium ion (Fig. 3.2) [76, 495] are efficiently stabilized as the positive charge is delocalized within the (An + 2) n electron system. Carbon-13 shifts are smaller than 180 ppm. 

A considerable difference between the spectra of positive and negative ions is also typical for highly aromatic compounds. Fig. 7 shows the spectra of benzopyrene as an example. The positive ion spectrum is amazingly simple. Detection of the radical, parent ion is... 

Azo coupling is the most widely used industrial reaction in the production of dyes, lakes and pigments. Aromatic diazonium ions acts as electrophiles in coupling reactions with activated aromatics such as anilines or phenols. The substitution normally occurs at the para position, except when this position is already occupied, in which case ortho position is favoured. The pH of solution is quite important it must be mildly acidic or neutral, since no reaction takes place if the pH is too low. 

Fossil organic particles in situ usually release positive ions at masses 23,24, 39, 41, 54, 56 thus indicating Na, Mg, K and Fe. But other signals also occur (Fig. 19). The negative ion mass spectra are dominated by carbon clusters with zero, one or two hydrogen atoms attached. They resemble spectra obtained from polymer foils (Gardella et al., 1980)42> rather than those from coals and carbon films (Fiirstenau et al., 1979)43). Consequently, the material contains more long hydrocarbon chains rather than aromatic constituents. Peaks at m/e = 79 (benzylium) and 90 (tropylium) indicate aromatic constituents. Unspecific ions like CN, CNO, and Cl are commonly present (Fig. 11). 

The reactivity of these positive ions toward aromatic amines is considered in Section IV, D. They also react in acetic acid containing a small quantity of pyridine with active methylene compounds such... 

Analogous experimental conditions (i.e. Cl, 0.1-1 Torr) allowed for the detection by tandem mass spectrometry of the collision complexes formed in the ion-molecule reactions of several aromatic radical cations M+ (M = C6H5X, X= Me, N02, Cl pyridine, furan, thiophene) and neutral iodoalkanes RI (R= n-Pr, 2-Pr, n-Bu, 2-Bu, etc.) en route to areni-um ions34,35. The collision complexes are covalently bonded species, namely nonisomeriz-ing iodonium radical positive ions 4 which dissociate to arenium ions 5 via reductive elimination of I (Scheme 7)34. 

Disproportionation of radicals by electron transfer to yield the negative and positive ions, and the subsequent reactions of the cation to either add OH- or lose H+, has been discussed in the previous section. Several aromatic and heterocyclic ions have been suggested to be intermediates in such processes, and the lifetime of these intermediates appears to be long enough to allow an effect of pH on their subsequent reaction (e.g. Haysom et al., 1972). 

Because of the presence of nitrogen in the aromatic ring, electrons in pyridine are distributed in such a way that their density is higher in positions 3 and 5 (the P-positions). In these positions, electrophilic substitutions such as halogenation, nitration, and sulfonation take place. On the contrary, positions 2, 4, and 6 (a- and y-positions, respectively) have lower electron density and are therefore centers for nucleophilic displacements such as hydrolysis or Chichibabin reaction. In the case of 3,5-dichlorotrifluoropyridine, hydroxide anion of potassium hydroxide attacks the a- and y-positions because, in addition to the effect of the pyridine nitrogen, fluorine atoms in these position facilitate nucleophilic reaction by decreasing the electron density at the carbon atoms to which they are bonded. In a rate-determining step, hydroxyl becomes attached to the carbon atoms linked to fluorine and converts the aromatic compound into a nonaromatic Meisenheimer complex (see Surprise 67). To restore the aromaticity, fluoride ion is ejected in a fast step, and hydroxy pyridines I and J are obtained as the products [58],... 

Another aromatic molecule containing six n electrons is C7H7+, the tropylium ion, derived from cycloheptatriene. This positive ion forms fewer complexes than does benzene, and they are less thoroughly studied. A molecule that has 10 electrons and has an aromatic structure is the cyclooctatetraenyl ion, C8H82. Some sandwich compounds containing this ligand are known as well as complexes of the type... 

Most reactions such as halogenation, nitration, sulphonation etc. are reactions with a positive ion, with an electrophilic reagent therefore, in which the aromatic molecule reacts nu-cleophilically. In hydrolysis, alcoholysis and aminolysis of aryl halides the reagents are nucleophilic. Radical reactions are also possible, especially in the gas phase at higher temperatures. 

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