Electrophilic aromatic substitution anthracene

The polycyclic aromatic hydrocarbons such as naphthalene, anthracene, and phenan-threne undergo electrophilic aromatic substitution and are generally more reactive than benzene. One reason is that the activation energy for formation of the c-complex is lower than for benzene because more of the initial resonance stabilization is retained in intermediates that have a fused benzene ring. 

Anodic oxidation of n-alkanes in acetonitrile results in mixtures of A -s-alkylacetamides but skeletal rearrangement of the intermediate i-carbenium ions is not observed. Aromatic compounds can undergo direct acetamidation in the ring. Thus, acetophenone, which normally undergoes electrophilic aromatic substitution at the meta position, affords the o- and p-acetamides (Scheme 44). Anthracene is cleanly converted into the acetamide (84) when the reaction is performed in the presence of TFAA as water scavenger (equation 41). ... 

Polycyclic aromatic compounds such as naphthalene, anthracene, and phenanthrene give electrophilic aromatic substitution reactions. The major product is determined by the number of resonance-stabilized intermediates for attack at a given carbon and the number of fully aromatic rings (intact rings) in the resonance structures. 

Polynuclear aromatic hydrocarbons such as naphthalene, anthracene, and phenanthrene undergo electrophilic aromatic substitution reactions in the same manner as benzene. A significant difference is that there are more carbon atoms, more potential sites for substitution, and more resonance structures to consider. In naphthalene, it is important to recognize that there are only two different positions Cl and C2 (see 122). This means that Cl, C4, C5, and C8 are chemically identical and that C2, C3, C6, and C7 are chemically identical. In other words, if substitution occurs at Cl, C4, C5, and C8 as labeled in 122, only one product is formed 1-chloronaphthalene (121), which is the actual product isolated from the chlorination reaction. Chlorination of naphthalene at Cl leads to the five resonance structures shown for arenium ion intermediate 127. 

In anthracene (123), there are three different positions (Cl, C2, and C9) and there are five different positions (Cl, C2, C4, C5, and C9) in phenanthrene (124). Electrophilic aromatic substitution of anthracene leads to substitution primarily at C9 because that gives an intermediate with the most resonance forms and the most intact benzene rings. A comparison of attack at Cl and at C2 in anthracene will show that there are more resonance forms for attack at Cl and more fully aromatic rings. Attack at C9 leads to an intermediate with even more resonance, and electrophilic substitution of anthracene leads to C9 and Cl products, with little reaction at C2. 

Substituents such as alkene units, alkyne units, and carbonyls can be reduced by catalytic hydrogenation. Lithium aluminum hydride reduces many heteroatom substituents, including nitrile and acid derivatives 56, 57, 104, 105, 106, 107, 108, 109. Polycyclic aromatic compounds such as naphthalene, anthracene, and phenanthrene give electrophilic aromatic substitution reactions. The major product is determined by the number of resonance-stabilized intermediates for attack at a given carbon and the number of fully aromatic rings (intact rings) in the resonance structures 59, 60, 61, 62, 63, 64, 65, 85, 104, 106, 107, 108,109,110,113,114,118. 

Further support for the notion that arylnitrenes need to be much more electrophilic than phenylnitrene to accomplish aromatic substitution comes from the work of Huisgen. 4,5-Dimethyl-2-pyrimidinylnitrene (P3T-N) readily gives substitution with activated aromatics (e.g., naphthalene, anthracene, anisole, and others) ... 

The nitration of polycyclic aromatic compounds by NO is analogous to aromatic substitution by other electrophilic radicals. The mechanism involves a rate-determining O-complex formation. This is illustrated in the following scheme using anthracene as the representative compound [40] ... 

For radical cations this situation is typically observed when deprotonation of the dimer dication is slow and for radical anions under conditions that are free from electrophiles, for example, acids, that otherwise would react with the dimer dianion. Most often, this type of process has been observed for radical anions derived from aromatic hydrocarbons carrying a substituent that is strongly electron withdrawing, most notably and well documented for 9-substituted anthracenes [112,113] (see also Chapter 21). Examples from the radical cation chemistry include the dimerization of the 1,5-dithiacyclooctane radical cations [114] and of the radical cations derived from a number of conjugated polyenes [115,116]. 

Synthesis of nitriles.2 The reagent reacts with aromatic hydrocarbons that undergo ready electrophilic substitution (e.g., anthracene) to give carboxamine-N-sulfo-chlorides. These are converted into nitriles by loss of S03 and HC1 when treated... 

The increasing importance of electron-transfer reactions with increasing aromatic hydrocarbon size is illustrated in the reaction of bromine with various aromatic compounds. With benzene (with a Lewis acid) and with naphthalene, electrophilic substitution occurs, and with anthracene, oxidative addition occurs (6) however, with graphite, only oxidation to the exclusion of carbon-bromine bond formation occurs, even at a stoichiometry of C8Br (II, 12). 

In the early 1960 s it was described 20,24,55-5 ) salt-like compounds of aromatic hydrocarbons are o-complexes, i.e. their cations AH possess the structure of arenium ions. This conclusion was first based on indirect arguments ensuing from the analysis of the AH -cation electronic absorption spectra (in particular, from the similarity of the spectra of anthracene and 1,1-diphenylethylene solutions in cone. HjSO j. It also results from the linear dependence of the logarithms of the. relative stability constants of A HF BF3 complexes on tho% of the rate constants of electrophilic substitution reaction of the hydrocarbons A Direct proof of this point of view was obtained from studies into the A HY mMY complexes and the solutions of aromatic hydrocarbons or their derivatives in various acids (HF, HF + BFj, HSO3F and others) by the nuclear magnetic resonance nKasurements of Dutch investigators... 

The competition between thermodynamic and kinetic stability may lead to different answers with respect to the question of aromaticity. There are exceptions to the rule that aromatic compounds undergo electrophilic substitution rather than addition reactions. ° There is an increasing tendency to addition within the series naphthalene, anthracene, tetracene, and pentacene. For anthracene and tetracene, 1.4-addition becomes more important and it is dominant in pentacene. ... 

Although the attack on (or by) the aromatic ring that leads to substitution can be brought about by free radical, electron-rich (nucleophilic) or electron-poor (electrophilic) reagents, the surfeit of electrons associated with aromaticity provides a richness that dictates that the latter should be most common. Nonetheless, aU three substitution pathways will be considered. Indeed, the three that will be examined are benzene (CeHe) and the polycyclic arenes (naphthalene [CioH ], anthracene [C14H10], and phenanthrene [CmHjo]). 

Substituted benzene compounds belong to a class of conjugated compounds called arenes. Examples include benzene, naphthalene, anthracene, and phenanthrene. The common structural feature of arenes is a monocyclic or polycyclic system of k electrons that results in a special stability called aromaticity. As a result, aromatic compounds are much less reactive in electrophilic addition reactions than we would expect based on the reactivity of polyenes. 

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