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: Describe the mechanism for the formation of polycarbonates through nucleophilic aromatic substitution using aromatic difluoro monomers and the carbonate ion. Answer: The formation of polycarbonates proceeds as follows: 1. The nucleophilic carbonate ion attacks the electrophilic carbon atom in the aromatic difluoro, creating a new C-O bond and generating a negative charge on the ortho carbon. 2. The negative charge is distributed across the aromatic ring, and one of the fluorine atoms leaves as a fluoride ion, forming an intermediate species with one C-O bond and another fluorine attached to the ring. 3. A second carbonate ion attacks the intermediate at the carbon attached to the remaining fluorine atom, creating another C-O bond and a new negative charge on the same carbon. 4. The negative charge is distributed again, and the second fluorine atom leaves as a fluoride ion, completing the formation of a polycarbonate unit with two C-O bonds. 5. The newly formed polycarbonate further reacts with other difluoro monomers in a similar manner, resulting in the growth of the polymer chain until all monomers have reacted to form the final polycarbonate product.

Step-by-step solution

Identify the key species and functional groups

In this mechanism, we have the carbonate ion (CO3^2-) as the nucleophile, an aromatic difluoro monomer, and the product––a polycarbonate.

Carbonate ion attacks the aromatic ring

The nucleophilic carbonate ion approaches the electrophilic carbon atom in the aromatic difluoro in a concerted manner, creating a new C-O bond. This temporarily generates a negative charge on the ortho carbon, which is stabilized by the nearby aromatic ring.

Formation of the intermediate and loss of a leaving group

The negative charge on the ortho carbon is distributed across the aromatic ring, and one of the fluorine atoms leaves as a fluoride ion, which returns the aromatic character of the ring. This results in an intermediate species with one of C-O bond formed and another fluorine (F) attached to the ring.

Second attack of the carbonate ion on the intermediate

A second carbonate ion attacks the intermediate at the carbon attached to the remaining fluorine atom. This creates another C-O bond, with a new negative charge on the same carbon.

Loss of the second leaving group and formation of the polycarbonate

Just like in step 3, the negative charge on the carbon atom is distributed across the aromatic ring, and the second fluorine atom leaves as a fluoride ion, regaining the aromatic character of the ring. This completes the formation of the polycarbonate unit, with two C-O bonds formed between the carbonate ion and the aromatic ring.

Polymerization of monomers to form the polycarbonate

The newly formed polycarbonate repeats the same nucleophilic aromatic substitution mechanism with other difluoro monomers, leading to the growth of the polymer chain. This chain growth process continues until all monomers have reacted to form the final polycarbonate product. By following the above proposed mechanism through nucleophilic aromatic substitution, the reaction between aromatic difluoro monomers and carbonate ion leads to the formation of polycarbonates, which are widely used in various applications due to their high strength and durability.