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

Aromaticity and Aromatic Substitution Reactions
17.10 Electrophilic Substitution Reactions of Pyrrole

Pyrrole is a five-member ring aromatic compound, but unlike benzene it has a dipole moment greater than zero. To fully appreciate the difference, an examination of the dipole moment and its direction must be carried out. For pyrrole, the dipole moment is 1.81 debyes, but the direction is toward the aromatic ring as shown in Figure 17.21.

As a result, the aromatic ring is electron rich and will prefer to react with electrophiles and undergo electrophilic aromatic substitution similar to that of benzene. For pyrrole, however, there are two possible positions on the aromatic ring to which the electrophile can bond and the intermediates generated are shown in Reaction (17-67).

(17-67)

Based on these resonance structures, addition of the electrophile to position 2 produces a more stable intermediate, compared to the addition of the electrophile to position 3. Thus, the major product for the nitration of pyrrole, which is given in Reaction (17-68), is the product with the substitution in position #2 and not position #3.

Figure 17.21 Direction of the dipole moment of pyrrole.

(17-68)

Problem 17.12

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

1. Br₃/FeBr₃

2. CH₃CH₂Cl/AlCl₃

3. CH₃CH₂COCl/AlCl₃