Of aromatic nuclei

Hydrogen bromide adds to acetylene to form vinyl bromide or ethyHdene bromide, depending on stoichiometry. The acid cleaves acycHc and cycHc ethers. It adds to the cyclopropane group by ring-opening. Additions to quinones afford bromohydroquinones. Hydrobromic acid and aldehydes can be used to introduce bromoalkyl groups into various molecules. For example, reaction with formaldehyde and an alcohol produces a bromomethyl ether. Bromomethylation of aromatic nuclei can be carried out with formaldehyde and hydrobromic acid (6). 

Tnfluoromethylation of double bonds or of aromatic nuclei is performed by bis(trifluorotnethyl) tellurium [145] (equation 125)... 

The yield of hydroxylated products is always very low, and there are usually a number of by-products. For instance, side chains of aromatic nuclei are easily attacked, as shown by the formation of 5-hydroxymethyluracil from thymine. Breslow and Lukens measured both the amount of 3-hydroxyquinoline formed and the quinoline consumed during hydroxylation with Fenton s reagent and EDTA in the presence of several adducts (Table XII). 

Coke formed on the catalyst surface is thought to he due to polycondensation of aromatic nuclei. The reaction can also occur through a car-honium ion intermediate of the benzene ring. The polynuclear aromatic structure has a high C/H ratio. 

In graphite each carbon atom is bound to three others in the same plane and here the assumption of inversion of a puckered layer is improbable, because of the number of atoms involved. A probable structure is one in which each carbon atom forms two single bonds and one double bond with other atoms. These three bonds should lie in a plane, with angles 109°28 and 125°16,l which are not far from 120°. Two single bonds and a double bond should be nearly as stable as four single bonds (in diamond), and the stability would be increased by the resonance terms arising from the shift of the double bond from one atom to another. But this problem and the closely related problem of the structure of aromatic nuclei demand a detailed discussion, perhaps along the lines indicated, before they can be considered to be solved. 

The behaviour of the frontier electrons was also attributed to a certain type of electron delocalization between the reactant and the reagent 40). A concept of pseudo-n-orbital was introduced by setting up a simplified model, and the electron delocalization between the 71-electron system of aromatic nuclei and the pseudo-orbital was considered to be essential to aromatic substitutions. The pseudo-orbital was assumed to be built up out of the hydrogen atom AO attached to the carbon atom at the reaction center and the AO of the reagent species, and to be occupied by zero, one, and two electrons in electrophilic, radical, and nucleophilic reactions. A theoretical quantity called "superdelocalizability was derived from this model. This quantity will be discussed in detail later in Chap. 6. 

The hetero radicals that have already been referred to—(9, p. 301), (10, p. 302), (14, p. 302) and (15, p. 302)—owe their relative stability [with respect to their dimers—apart from l,l-diphenyl-2-picrylhydrazyl (10)] to a variety of factors (a) the relative weakness of N—N, S—S and 0—0 bonds, (b) the delocalisation through the agency of aromatic nuclei, and (c) steric inhibition of access to the atom with the unpaired electron, or to an aryl p-position, cf. (50). The latter factor bulks large (in addition to the weakness of O—O bonds) in the great stability of (15, cf. p. 302) and all three factors operate to stabilise (51), which is wholly dissociated in solution ... 

Rhodium-on-alumina, catalyzed reduction of aromatic nuclei, 51, 105... 

Tab. 13 Indirect electrooxidation of aromatic nuclei with cerium mediators...

Naphthol has been reduced to 1-decalol using platinum,5 Raney nickel,6 and Raney copper.7 The reactions catalyzed by nickel and copper required elevated temperatures and pressure. The present procedure allows the preparation of substantial quantities of 1-decalol under much more convenient conditions and shorter reaction times. Previous methods5-7 require costly catalysts or high-pressure equipment and frequently result in a high degree of hydrogenolysis. The submitters have found that the present method is applicable to a wide variety of aromatic nuclei, some of which are listed in Table I. 

Whereas, photodecomposition of diazoketones in the presence of aromatic nuclei induces an enlargement of the ring system, irradiation of cyelic-diazoketones results in ring contraction with loss of nitrogen,... 

It has been found that oxygen-containing substitutents (particularly phenols) and alkyl substitutents on an aromatic nucleus increase dramatically the susceptibility to oxidation of the original aromatic compound (18, 24, 25). Oxidation of the aromatic nuclei in coal undoubtedly takes place at periphery sites hence, an increase in size of aromatic nuclei would be expected to lower the oxidation rate owing to a decrease in the number of oxidation sites per unit area of coal surface. 

Irradiation of powdered titanium dioxide suspended in solutions containing aromatic compounds and water under oxygen has recently been shown to induce hydroxylation of aromatic nuclei giving phenolic compounds and oxidation of side chains of the aromatic compounds (50-55). These reactions have been assumed to proceed through hydroxyl and other radical intermediates, but the mechanism for their generation, whether reactive free radicals result from oxidation of water, from reduction of oxygen, or from oxidation of the substrates on the surfaces of the excited titanium dioxide, has not been clear. 

The chain stiffness expressed by the flex parameter F which, for a given chain of molar mass Me, is given by F = Me/Ne, where Ne is the number of elementary (undeformable) segments. F is essentially an increasing function of the content of aromatic nuclei, Ar. 

On treatment with oxidants such as chlorine, hypochlorite anion, chlorine dioxide, oxygen, hydrogen peroxide, and peroxy acids, the aromatic nuclei in lignin typically are converted to o- and p-quinonoid structures and oxirane derivatives of quinols (E, D, C, resp., Fig. 1.4). It should be noted that the conversion of aromatic nuclei to o-quinonoid rings is accompanied by loss of the methoxyl group as methanol and that conversion to p-quinonoid groups in many cases leads to displacement of the side chain. 

Alkaline hydrogen peroxide has not found wide use in the oxidation of aromatic nuclei, although hydroxyquinones can be formed from hydroquinones under strongly alkaline conditions (Figure 3.111).470... 

Sometimes the polarographic behavior of heteroaromatic compounds is complicated by adsorption of depolarizer and/or products the adsorbability generally increases with the number of aromatic nuclei in the molecule. In the reduction of benzoyl pyiidines270 it was found that the adsorption of both depolarizer and products was important in acid solution, whereas only the former was adsorbed at high pH. 

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