Question

Polycarbonates (Section 29.5C) are also formed by using a nucleophilic aromatic substitution route (Section 22.3B) involving aromatic difluoro monomers and carbonate ion. Propose a mechanism for this reaction.

Short answer

Question: Propose a mechanism for the formation of polycarbonates from aromatic difluoro monomers and carbonate ion using nucleophilic aromatic substitution (NAS). Answer: The mechanism for the formation of polycarbonates involves the following steps: 1. The nucleophile, carbonate ion (CO3^2-), attacks one of the electrophilic carbon atoms of the aromatic difluoro monomer (ArF2), forming a Meisenheimer complex. 2. One of the fluorine atoms is eliminated, and the aromaticity of the ring is regained. 3. Another carbonate ion attacks the other electrophilic carbon atom, forming a second Meisenheimer complex. 4. The second fluorine atom is eliminated, and the aromatic ring regains its aromaticity. The polycarbonate product is formed, with two carbonate ions bridged between the two electrophilic carbon atoms of the aromatic ring.

Step-by-step solution

Identify the nucleophile and the aromatic compound in the reaction

The carbonate ion (CO3^2-) is the nucleophile in this reaction, while the aromatic difluoro monomer (ArF2) is the aromatic compound undergoing nucleophilic substitution.

Formation of the Meisenheimer complex

The nucleophilic carbonate ion attacks one of the electrophilic carbon atoms of the aromatic difluoro monomer, and the negatively charged intermediate called the Meisenheimer complex is formed. This step involves the transition from the aromatic to a non-aromatic species.

Fluoride ion elimination

In the next step, one of the fluorine atoms attached to the carbon atom where the nucleophile attacked is eliminated as a fluoride ion (F^-), regenerating the aromaticity of the aromatic ring.

Formation of a second Meisenheimer complex

Now, another carbonate ion attacks the other electrophilic carbon atom of the difluoro monomer, forming a second Meisenheimer complex. This step involves the disruption of aromaticity again.

Final fluoride ion elimination and formation of polycarbonate

Finally, the second fluorine atom is eliminated as a fluoride ion, and the aromatic ring regains its aromaticity. The two carbonate ions are now bridged between the two electrophilic carbon atoms of the aromatic ring, forming the polycarbonate product.