Substitution, electrophilic arenium ions, isolation

Note that both the bromination and the acylation of naphthalene result in the substitution of the electrophile at the 1 position. None of the isomeric product with the electrophile bonded to the 2 position is isolated in either case. The higher reactivity of the 1 position can be understood by examination of the resonance structures for the arenium ion. When the electrophile adds to the 1 position, the arenium ion has a total of seven resonance structures, whereas only six exist for the arenium ion resulting from addition of the electrophile to the 2 position. 

The remaining two steps proceed along very similar lines in most electrophilic aromatic substitution reactions. The attack by the electrophile is usually the rate limiting step. The cationic intermediate is called a Wheland intermediate, or o-complex or an arenium ion, and can sometimes be isolated. 

A simple example illustrates the reaction. When benzene reacts with benzyl chloride in the presence of 0.4 equivalents of AICI3, diphenylmethane (45) is isolated in 59% yield. If this reaction proceeds by electrophilic aromatic substitution, then the sp carbon of benzyl chloride is a precursor to a carbocation. To form a carbocation from benzyl chloride, the chlorine atom must react as a Lewis base with AICI3 to form PhCH2 AlCL. Benzene reacts with this carbocation via electrophilic aromatic substitution in the same manner as the reaction with Br in the previous section to form an arenium ion intermediate (see 40) to give 45. 

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. 

---