Electrophilic substitution in naphthalene

In Chapter 2, electrophilic substitution in naphthalene was discussed, when consideration of the stability of the cationic intermediates arising... 

FIGURE 5.33. Distributions of formal charge in transition states for electrophilic substitution in naphthalene and anthracene and corresponding reactivity numbers. 

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

Electrophilic substitution in 12.2 indicates that the 7-position (corresponding to the p-position of naphthalene) is the more reactive, and this follows since this position is para conjugated with sulfur (12.8), whereas the 6-position is ortho conjugated (12.9). Yields are lower than in substitution of 12.1, but this does not necessarily indicate that 12.2 is less reactive, since substitution is accompanied by ring-contraction reactions. No quantitative data are yet available. 

In our study of electrophilic substitution in the benzene ring (Chap. II), we found that we could account for the observed orientation on the following basis (a) the controlling step is the attachment of an electrophilic reagent to the aromatic ring to form an intermediate carbonium ion and (b) this attachment takes plaqe in such a way as to yield the most stable intermediate carbonium ion. Let us see if this approach can be applied to the nitration of naphthalene. 

Rate and Regioselectivity in the Nitration of (Trifluoromethyl)benzene 474 Substituent Effects in Electrophilic Aromatic Substitution Activating Substituents 476 Substituent Effects in Electrophilic Aromatic Substitution Strongly Deactivating Substituents 480 Substituent Effects in Electrophilic Aromatic Substitution Halogens 482 Multiple Substituent Effects 484 Retrosynthetic Analysis and the Synthesis of Substituted Benzenes 486 Substitution in Naphthalene 488 Substitution in Heterocyclic Aromatic Compounds 489... 

The resonance energy of naphthalene is 255 kj moH. We can compare this with benzene, which has a resonance energy of 157 kJ mohh Effectively, naphthalene is less aromatic than benzene—although the MO theory is beyond the scope of this course, we can see that only one ring at a time can be fully Hiickel compliant with six i-electrons. The consequence for the chemistry of naphthalene is that electrophilic substitution is easier than for benzene—less resonance energy is lost in the initial step to form the Wheland intermediate, as one benzene ring will effectively remain intact and fully aromatic. Electrophilic substitution of naphthalene could, in principle, occur at either the 1- or 2-position. In most cases, we see only the kinetic product, which derives from substitution at the 1-position (Figure 12.43). 

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... 

Fluoronaphthalene [321-38-0] is prepared from 1-naphthylamine by the Balz-Schiemaim reaction in 52% yield or by diazotization in anhydrous hydrogen fluoride in 82% yield. Electrophilic substitution occurs at the 4-position, eg, nitration with fuming nitric acid in acetic acid gave 88% yield of l-fluoro-4-nitro-naphthalene [341 -92-4]. 

In compounds with a fused benzene ring, electrophilic substitution on carbon usually occurs in the benzenoid ring in preference to the heterocyclic ring. Frequently the orientation of substitution in these compounds parallels that in naphthalene. Conditions are often similar to those used for benzene itself. The actual position attacked varies compare formulae (341)-(346) where the orientation is shown for nitration sulfonation is usually similar for reasons which are not well understood. 

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)). 

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 ... 

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