Organic Chemistry: Concepts and Applications - Headley Allan D. 2020

Aromaticity and Aromatic Substitution Reactions
17.9 Electrophilic Substitution Reactions of Polycyclic Aromatic Compounds

Close examination of naphthalene reveals that there are two different types of hydrogens and they are shown in Reaction (17-60) as H_α and H_β. Hydrogens labeled H_α are close to the point of ring fusion and the other types of hydrogens, H_β, are removed from the point of ring fusion by a carbon. Thus, there are two products possible for the electrophilic substitution reaction as shown in Reaction (17-60).

(17-60)

By varying the reaction conditions for this substitution reaction, it is possible to produce one of these products as the major product compared to the other. If E⁺ is bulky, such as the sulfonyl group, there will be steric crowing around the electrophile in the product if the substitution is in the alpha (α) position. On the other hand, if the substitution is in the beta (β) position, there would be less steric crowding. This concept is illustrated in Reaction (17-61).

(17-61)αβ

There is another factor that must be considered in predicting the major and minor products for this type of substitution reaction. A close examination of the reaction mechanism shows that the intermediate leading to the formation of the product in which there is an α-substitution is more stable than the intermediate leading to the formation of the product where there is a β-substitution, as shown in Reaction (17-62).

(17-62)

Thus, the energy of activation leading to the less stable intermediate is greater than that leading to the more stable intermediate. As a result, in order to form the substitution product with the substitution in the β-position, additional energy must be supplied in order to overcome the higher energy of activation. On the other hand, to form the least stable product in which the substitution is in the α-position, a lower temperature is required. Varying the temperature of a reaction to have the reaction go through different pathways is known as thermodynamic and kinetic control of the reaction. This concept is illustrated in sulfonation of naphthalene shown Reactions (17-63) and (17-64).

(17-63)

(17-64)

For Reaction (17-63), the temperature is lower than that of the reaction in Reaction (17-64), hence the kinetic product is formed as the major product. On the other hand, for the reaction in Reaction (17-64), which is carried out at a higher temperature, the more stable less sterically hindered product is produced since there is enough energy to overcome a high activation barrier. This is a classic example of kinetic vs. thermodynamic control of reactions and is illustrated in the energy profile diagram shown in Figure 17.20.

For the substitution reaction in which the electrophile is not very bulky, steric stability of the product is not a major factor, but the stability on the resonance stabilized intermediate carbocation is important. Thus, at room temperature, the bromination of naphthalene readily occurs as shown in Reaction (17-65).

(17-65)

The intermediate formed in the first step of the mechanism is the more stable intermediate as shown in Reaction (17-66).

(17-66)

Figure 17.20 Energy profile for the electrophilic substitution of naphthalene under different reaction temperatures (black represents thermodynamic control and red represents kinetic control).

Problem 17.11

i. Give the organic products for the reaction of naphthalene with each of the following reagents.

1. Cl₂/FeCl₃

2. Br₂/FeBr₃

3. CH₃COCl/AlCl₃

ii. Shown below are the structures of two polycyclic aromatic compounds. Give the expected organic product for the reaction of each with Br₂ in carbon tetrachloride as the solvent at an elevated temperature.