Question

Propose a mechanism for the formation of this polyphenylurea. To simplify your presentation of the mechanism, consider the reaction of one \(-\mathrm{NCO}\) group with one \(-\mathrm{NH}_{2}\) group.

Short answer

Question: Explain, in brief, the mechanism for the formation of a polyphenylurea. Answer: The formation of polyphenylurea involves the reaction between isocyanate ("-NCO") and amine ("-NH2") groups through a nucleophilic attack that forms a zwitterion intermediate. The rearrangement of charges results in the creation of a urea linkage. This mechanism can be repeated multiple times, leading to the formation of the polyphenylurea molecule with multiple urea linkages.

Step-by-step solution

Identify the groups reacting

In this exercise, we will consider the reaction between an isocyanate ("-NCO") group and an amine ("-NH2") group to intuit a possible mechanism for the polyphenylurea formation.

Formation of a zwitterion intermediate

The first step in the reaction is the nucleophilic attack of the nitrogen atom from the amine ("-NH2") group on the carbon atom from the isocyanate ("-NCO") group. This will break the carbon-oxygen double bond and form a zwitterion intermediate. The zwitterion has a positive charge on the nitrogen from the amine group and a negative charge on the oxygen from the isocyanate group.

Rearrangement of charges and formation of urea linkage

In the next step, the negatively charged oxygen will donate its lone pair of electrons back to the positively charged nitrogen, thus forming a new nitrogen-carbon double bond and removing the positive charge. At the same time, the nitrogen from the isocyanate group will bond with a hydrogen from the amine group, causing the positive charge to transfer to the nitrogen of the isocyanate group. This results in a urea linkage and a neutral molecule.

Formation of polyurea linkage through multiple reactions

The reaction mechanism we described above involves the formation of a single urea linkage between an isocyanate and an amine group. In reality, a polyphenylurea molecule would consist of multiple urea linkages formed via the same mechanism. To create such a polymer, additional isocyanate and amine groups would react, continuously propagating the chain growth and forming a polyphenylurea molecule. In summary, the reaction between an isocyanate ("-NCO") group and an amine ("-NH2") group proceeds through a zwitterion intermediate, leading to the formation of a urea linkage. This mechanism can be repeated multiple times with multiple reacting groups, ultimately forming a polyphenylurea molecule.