Naphthalene electrophilic attack

Polycyclic aromatic hydrocarbons undergo electrophilic aromatic substitution when treated with the same reagents that react with benzene In general polycyclic aromatic hydrocarbons are more reactive than benzene Most lack the symmetry of benzene how ever and mixtures of products may be formed even on monosubstitution Among poly cyclic aromatic hydrocarbons we will discuss only naphthalene and that only briefly Two sites are available for substitution m naphthalene C 1 and C 2 C 1 being normally the preferred site of electrophilic attack... 

Having its pyrazolic 4-position substituted, electrophilic attack on indazoles takes place in the 3-position and in the homocycle (the 5- and 7-positions). The condensation of a benzene ring results in a decrease of the aromaticity of the pyrazole moiety, as in naphthalene compared to benzene, and therefore basic ring cleavage is easier in indazoles than in pyrazoles (Section 4.04.2.1.7(v)). 

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. ... 

The relative stability of the intermediates determines the position of substitution under kinetically controlled conditions. For naphthalene, the preferred site for electrophilic attack is the 1-position. Two factors can result in substitution at the 2-position. If the electrophile is very bulky, the hydrogen on the adjacent ring may cause a steric preference for attack at C-2. Under conditions of reversible substitution, where relative thermodynamic stability is the controlling factor, 2-substitution is frequently preferred. An example of this behavior is in sulfonation, where low-temperature reaction gives the 1-isomer but at elevated temperatures the 2-isomer is formed. ... 

Two sites are available for substitution in naphthalene, C-1 and C-2, C-1 being normally the prefened site of electrophilic attack. 

One way to anticipate the favored product is to consider the shape of naphthalene s best electron-donor orbital, the highest-occupied molecular orbital (HOMO). Display the HOMO in naphthalene and identify the sites most suitable for electrophilic attack. Which substitution product is predicted by an orbital-control mechanism Ts this the experimental result ... 

The HOMO of naphthalene reveals the likely site of electrophilic attack. 

In alternant hydrocarbons (p. 55), the reactivity at a given position is similar for electrophilic, nucleophilic, and free-radical substitution, because the same kind of resonance can be shown in all three types of intermediate (cf. 20,22, and 23). Attack at the position that will best delocalize a positive charge will also best delocalize a negative charge or an unpaired electron. Most results are in accord with these predictions. For example, naphthalene is attacked primarily at the 1 position by NOj, NHJ, and Ph, and always more readily than benzene. 

Naphthalene undergoes electrophihc substitutions at the a rather than p position. The Hueckel molecular orbital calculations show that all the carbons have the same jt electron density 1.0. This is not in agreement with the theory of organic reactions based on the Coulombic interaction that electrophilic attack occurs on the most negatively charged atom. Fukui [7] proposed the frontier orbital theory for the discrepancy between the theory and the experimental observation. The importance of... 

With naphthalene, electrophilic substitution (e.g. nitration) is found to take place preferentially at the 1- (a-), rather than the alternative 2- (/ -), position. This can be accounted for by the more effective delocalisation, and hence stabilisation, that can take place in the Wheland intermediate for 1 - attack (60a - 606) compared with that for 2-attack (61) ... 

This may be rationalized by considering the stability of intermediate addition cations. When the electrophile attacks at C-5 or C-8, the intermediate cation is stabilized by resonance, each having two favourable forms that do not perturb the aromaticity of the pyridinium system. In contrast, for attack at C-6 or C-7 there is only one such resonance form. We used similar reasoning to explain why naphthalene... 

Condensed aromatic hydrocarbons are more reactive towards electrophilic reagents, and naphthalene, for example, may be brominated quite readily in solution in carbon tetrachloride without the need for a catalyst electrophilic attack takes place at the more reactive a-position to yield 1-bromonaphthalene (Expt 6.25). 

The sulphonation of toluene (Expt 6.37) with concentrated sulphuric acid at 100-120°C results in the formation of toluene-p-sulphonic acid as the chief product, accompanied by small amounts of the ortho and meta isomers these are easily removed by crystallisation of the sodium salt of the para isomer in the presence of sodium chloride. Sulphonation of naphthalene at about 160°C yields largely the 2-sulphonic acid (the product of thermodynamic control) (Expt 6.38) at lower temperatures (0-60 °C) the 1-sulphonic acid (the product of kinetic control) is produced almost exclusively. In both cases the product is isolated as its sodium salt. In anthraquinone the carbonyl groups deactivate the aromatic nucleus towards electrophilic attack and vigorous conditions of sulphonation are required, i.e. oleum at about 160 °C. The product is largely sodium anthraquinone-2-sulphonate (Expt 6.39). 

Although no detailed mechanism for the conversion was proposed, the initial step likely involves the formation of the superelectrophilic, dipro-tonated species (213), which reacts with benzene by electrophilic attack. There is some 1,3-dicationic character in 213, however it is understood that the positive charge is delocalized throughout the naphthalene ring-system. Evidence for the dicationic species comes from cryoscopic... 

Deserves your attention. MO s are given on p. 273 Predict the site of electrophilic attack at naphthalene, azulene, indole and benzofuran. 

If a deactivating group is present on a monosubstituted naphthalene, the incoming electrophile attacks the a position of the other ring (sulfonation may go to the 0 position). 

There is neither a partial positive nor a partial negative charge on the two nonequivalent positions 1 and 2 of naphthalene, which are poised for electrophilic substitution. Based on the above-mentioned incorrect mechanistic model, one would consequently predict that electrophiles attack naphthalene without regiocontrol. Furthermore, this... 

The Buchner synt)tesls is the result of an electrophilic attack of an aromatic ring. Cyclohcptatrienes are formed (eg., from benzene and carbalkoxycarbenes). In other cases, a norcaradiene is obtained (e.g., from naphthalene) and, in fad, an equilibrium may occur between both forms, according to the nature of the substituents present on the aromates and the carbene [30],... 

Electrophilic attack on naphthalene, anthracene and phenan-threne... 

In contrast to benzene, the bond lengths in naphthalene are not all equal, as illustrated in 4. The resonance energy of naphthalene is 255 kJ mol", which is higher than, though not twice that of, benzene (151 kJ mol" ). In the canonical forms 5 and 7 that contribute to the valence bond structure for naphthalene, only one of the two rings is fully benzenoid. Naphthalene is less aromatic than benzene, which accounts for its higher reactivity towards electrophilic attack compared with benzene. 

Electrophilic attack on naphthalene is easier than that on benzene and occurs at the -position. 

In spite of the elegant solution to the problem as represented in the structure of 206, it is rather far from being optimal. In fact, the presence of the naphthalene system in this base made the latter rather vulnerable to electrophilic attack directed at aromatic rings. Owing to this complication, 206 is used in syntheses less often than other sterically hindered bases like 4-methyl-2,6-di-tcrt-butylpyridine 207, ethyldiisopropylamine, 208 (Hunig s base) or ethyldi-cyclohexylamine 209. All of these reagents are now manufactured commercially and widely used in cases where it is essential to carry out a reaction with strong electrophiles under strictly non-acidic conditions with the removal of proton acids as they are formed. In fact, the preparation of 205 was succesfully carried out in the presence of 208. °... 

---