Question

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

Short answer

Question: Describe the mechanism for the formation of a polyphenylurea when an isocyanate and an amine group react. Answer: The formation of polyphenylurea involves the following steps: 1. Identify the reactants: an isocyanate group (-NCO) and an amine group (\(-\mathrm{NH}_{2}\)). 2. Analyze the reaction mechanism: the nucleophilic nitrogen in the amine group attacks the electrophilic carbon of the isocyanate, forming a new C-N bond. 3. Rearrangement to form urea linkage: the negatively charged oxygen attacks the positively charged nitrogen in an intramolecular nucleophilic attack, forming the urea linkage. 4. Proton transfer: a proton is transferred from the amine nitrogen to the urea oxygen, leading to a neutral urea linkage. This process can be repeated to form the polyphenylurea.

Step-by-step solution

Identify the reactants

The reaction involves an isocyanate group (-NCO) and an amine group (\(-\mathrm{NH}_{2}\)). These groups are found in two different molecules, where one molecule has the isocyanate group and the other has the amine group.

Analyze the reaction mechanism

In this step, the nucleophilic attack of the amine group on the electrophilic carbon atom of the isocyanate occurs. The nucleophilic nitrogen in the amine group donates a pair of electrons to form a bond with the electrophilic carbon in the isocyanate, resulting in a new C-N bond. At the same time, the carbon-nitrogen double bond (C=N) in the isocyanate becomes a carbon-nitrogen single bond.

Rearrangement to form urea linkage

Now we have an intermediate structure where the nitrogen atom in the isocyanate has a single bond to the previously double-bonded oxygen and a new-formed single bond to the electrophilic carbon. The nitrogen atom in the isocyanate group is now positively charged, and the oxygen atom is negatively charged as it has gained an electron pair previously shared with the carbon atom. The negatively charged oxygen then attacks the positively charged nitrogen atom in what is known as intramolecular nucleophilic attack, forming the urea linkage.

Proton transfer

In the final step, there is a proton transfer from the amine nitrogen to the newly formed urea oxygen. This gives rise to a neutral urea linkage, and completes the formation of the polyphenylurea. These steps can be repeated for each addition of an isocyanate and amine group, leading to the formation of the desired polyphenylurea.